Note: Descriptions are shown in the official language in which they were submitted.
~0~07S0
Cross Reference to Related Applications
Packard Instrur.lent Company, Inc.
Serial No. 985,925 (Robert E. Cavanaugh), filed
March 22, 1967, now Patent No. 822,215
Serial No. 13,234 (Arthur Karmen and Lyle E. Packard),
filed February 23, 1968.
~ Serial No. 117,892 (Robert E. Cavanaugh), filed
: July 9, 1971.
Background of the Invention
The present invention relates in general to liquid
scintillation spectral analysis of test samples containing unknown
isotopes disposed in a liquid scintillator and, more particularly,
to spectral analysis equipment and techniques which may be wholly
automatic in operation, which virtually eliminate the effects of
quenching as a source of error in determining true activity levels
of test samples, and which readily permit direct display of such
true activity levels, either visually or in printed form, in units
of decay events per minute (dpm). In its principal aspects, the
invention is concerned with an improved data processing system,
as well as with improved methods and apparatus for simulating a
controlled quench condition for all test samples, irrespective of
whether the latter are actually unquenched or quenched and
l irrespective of the degree to which such samples may actually be
: quenched, whereby the effective quench condition of the sample
, (viz., the amount of actual quench internally of any given sample
~; . plus the simulated quench condition superimposed thereon
~ externally of the sample) converges to a known selected value for
.'j which instrument counting efficiency is known with a high degree
of accuracy.
Modern apparatus for detecting and measuring radioactivity
:;l
~` has reached an unusually high state of development with systems
:, .,
currently available which offer unusual sensitivity to low energy
: radiation, as well as various options of full automation, semi-
; automation, or the more economical manual operating version. In
.~ ~
,,
:~
.
~040750
a relatively few years, great strides have been made towards
improving the preciseness and accuracy of counting efficiency in
compliance with the very stringent requirements of users of this
highly technical and sophisticated equipment. However, certain
problems have continued to plague both the manufacturers and users
; of such equipment. A particularly prevalent and vexing problem
has been the error introduced into computations of true sample
activity levels because of a phenomenon commonly encountered with
liquid scintillation samples known as "quenching". Stated very
simply, this phenomenon results in attenuation of light
scintillations within the sarnples, thus significantly affecting
the statistical accuracy of the equipment wllich determines
activity levels based upon the number and energy of such light
scintillations, the latter being counted over known units of time
and being proportional in energy to the energy of the decay events
which produce them.
Many efforts have heretofore been made to minimize and,
preferably, to eliminate, the errors which result from the quench
phenomenon, some of which have completely failed and others of
which have met with varying degrees of success and acceptance --
however, notwithstanding all such efforts, the problem has
remained a source of error, which ofttimes amounts to significant
- error in the computation of true activity levels.
; One principal effort that has heretofore been made
~ towards minimizing the quench problem has been that of development
; of various constituents which make up the sample and which are as
free of quench characteristics as possible. Such constituents
include, without limitation thereto, scintillation substances,
solvents, and the rnaterial from which the light transmissive
sample vial is made. However, perfect light transmitters
completely devoid of quench characteristics are simply not
; available, and even if they were, the problem would remain since
--3--
,
~040750
the test specimen itself may, and often will, contain quench
materials such, for example, as blood or urine, which tend to
attenuate the light because of their color. Moreover, unless the
i detection system is maintained in a completely enclosed
atmosphere of an inert gas such as argon, quench can occur simply
because of the presence of air.
Faced with the seeming impossibility of eliminating the
quench phenomenon as a source of error, numerous efforts have
been made to cope with the problem by providing methods and
apparatus for compensating for such errors. Typical systems
which are currently employed and which have found great acceptance
today by people employing this sophisticated equipment include
systems in which an external standard source which emits highly
penetrating radiations is placed adjacent the sample in the
detection chamber during a portion only of its overall counting
cycle. Light scintillations occurring in the sample are then
counted during at least two discrete intervals during one of
which the scintillations are created only by the isotope in the
sample and during the other of which the scintillations are
created by the composite effect of the isotope and the external
standard. Suitable electronic equipment is provided for
separating the pulses from the two sources on the basis of their
different energy levels and, therefore, those which are counted
l primarily from the external standard provide a fairly accurate
indication of the degree of quenching present in the sample since
-:,
the counting efficiency for such external standard is known or
can be readily ascertained by use of an unquenched standard
: ~
sample. Typical systems of this type are described in detail in
Lyle E. Packard U.S. Patent No. 3,188,468, issued June 8, 1965,
as well as in the aforesaid Canadian Patent No. 822,215
(Cavanaugh).
:
~ Both of the aforementioned prior systems are of the type
~0~0750
which are commonly referred to as "external standardization"
systems and both represent basic and significant improvements over
earlier known systems described therein, such as "internal
standardization" and "channels ratio" systems. In effect,
however, all of these prior systems have had certain aspects
which are common to one another, a principal one of which is that
the measured quench correlation parameter (e.g., "net external
standard count", "external standard ratio", "channels ratio",
etc.) generally provides an indication of the degree of quench
present in the sample, which indication must then be compared
with a previously prepared quench correlation curve in order to
determine the counting efficiency. Once knowing the counting
efficiency, the counts per minute (cpm) r.~easured for the isotope
being analyzed can be divided by counting efficiency to determine
activity level in decay events per minute (dpm). Unfortunately,
however, the quench correlation curve itself differs widely from
instrument to instrument, from isotope to isotope, from channel to
channel, with sample volume, and with other variable conditions.
Consequently, the preparation of each one of which has been time
consuming, expensive, and subject to numerous human errors.
Moreover, once the curves are prepared, it is necessary that the
measured quench correlation data be compared with them in order
to determine counting efficiency, thus introducing even further
.:
danger of human error.
Even ~ore significant, however, has been the fact that
while such a correlation curve can be prepared, it is only as
accurate as the number of points which actually define the curve.
~ .
~t has been established that such points simply do not fall on a
straight line, or even on a smoothly curved line--quite to the
contrary, the points will be non-uniformly distributed in an
unpredictable random pattern which only generally defines the
correlation curve. Consequently, even when the technician uses
104~75(~
extreme c~re in his computations, he has been forced ~o
ex~rapolate or interpolate between known points and, since the
extrapolated or interpolated data can vary significantly from the
actual data, the computed efficiency can still vary greatly from
actual efficiency with maximum errors on the order of up to ten
per cent (10%) and average errors on the order of up to two per
cent (2%) being common, dependent upon the number of differently
quenched standard samples selected to prepare the quench
correlation curve.
Errors of the foregoing magnitude are simply not
acceptable to highly trained technical personnel who use this
generaltype of equipment. Indeed, such errors are highly
objectionable, and the more so in view of the high state of
sophistication that the overall art has reached. However, thesé
errors have heretofore been tolerated only because the prior
systems briefly described above, and described in considerably
! greater detail in the aforesaid Packard Patent No. 3,188,468,
`~ havë represented the best available solutions to the problem
up until the present date.
Continued efforts have, however, been made towards
providing a more satisfactory solution to the problem. Por
example, it has been suggested that true activity level for a
sample can be computed simply by dividing the measured variable
quench correlation parameter (e.g., "external standard ratio",
net external standard count", "channels ratio", etc.) into the
measured value in counts per minute (cpm) for the sample
undergoing analysis. This suggestion, however, is not
satisfactory for many reasons. First, it assumes that the quench
correlation curve is a straight line, which it is not. Secondly,
i~ fails to take into account the non-uniform random distribution
of points which define such a curve. Therefore, even were the
curve a straight line or substantially a straight line, errors of
the same general magnitude as described above would still be
experienced. The fact that the quench correlation curve is not a
straight line actually adds to the magnitude of such errors with
the result that errors on the order of up to twenty-five per cent
(25%) can be, and have been,experienced.
It has also been proposed that the problem can be
resolved by adjusting in any of various known manners, overall
system gain so as to restore the measurable quench correlation
parameter to a value indicative of an unquenched sample, and
thereafter, analyzing the sample as if it were unquenched. Again,
however, such a proposed "solution" is no solution at all since
the gain correlation curves do not coincide with nor follow the
quench correlation curves and, consequently, the magnitude of
error can be and often will be, even greater than that experienced
,
with the interpolation/extrapolation techniques referred to above.
It is a general aim of the present invention to provide
an improved data processing system which overcomes the foregoing
:; disadvantages and which is characterized by its reliability and
rapidity of operation. In this connection, it is an object of the
invention to provide improved radioactivity spectrometry methods
`,!
"
and apparatus which permit the determination of activity levels
for test samples having any of a wide range of different quench
characteristics with virtually unlimited accuracy, yet wherein this
is accomplished by the utilization of quench correlation data based
upon the measurement of only a relatively few, and indeed, in some
instances, only one, known standards or standard. While not so
limited in its application, the invention will find especially
advantageous use when the measured variable parameter of such
quench correlation data takes the form of net external
standardization ratios, although it can also be used in connection
with other measurable variable parameters which also provide an
indication of the degree of quenching such, merely by way of
lQ~07S0
example, as channels ratios, or net external standard counts.
As a consequence of attaining the foregoing general
objective of the invention, it has been found that ancillary
; benefits achieved are that the danger of human error in both the
preparation and reading of quench correlation data is greatly
reduced; the versatility of such data is significantly increased;
and the time and cost required to prepare such data are held to a
minimum.
In another of its important aspects, it is an object of
the invention to provide improved methods and apparatus suitable
for use in radioactivity spectrometry applications which permit
highly accurate determination of sample activity levels in terms
of decay events per minute (dpm), and where such information can
be printed or read directly without requiring the technician to
perform close and tedious comparisons of detected or measured
data with quench correlation curves.
It is a related object of the invention to provide
improved spectro~etry methods and apparatus which will greatly
facilitate and speed up the quantitative determination of activity
levels in terms of decay events per minute (dpm), yet wherein this
is not only accomplished without sacrificing accuracy of the
computations but, to the contrary, wherein the measurements
;~ are, on an average, considerably more accurate than has heretofore
been feasible on a commercial basis.
; It is one of the principal objectives of the present
invention to provide novel spectrometry methods and apparatus for
determining activity levels of test samples wherein provision is
made for automatically compensating for errors attributable to
quenching so as to permit direct display of activity levels in
units of decay events per minute (dpm), either visual display or
printed display, all irrespective of the amount of quenching that
may be present in any given sample.
; .. .
.
:
~:040750
Another general aim of the present invention is the
provision of novel and improved methods and apparatus suitable for
; use in computing true activity levels, corrected for quench
errors, of test samples in units of decay events per minute (dpm)
which are characterized by their accuracy and by their complete
elimination of any need to visually or automatically interpolate
or extrapolate between known points on a quench correlation curve,
which interpolation or extrapolation has heretofore been essential
and which has inherently introduced significant errors into
activity level computations.
A further object of the invention is the provision of
novel methods and apparatus for producing simulated quench
conditions for each test sample undergoing examination so as to
bring the effective quench level of the sample (the effective
quench level consisting of the actual internal quench level of the
sample plus the superimposed external simulated quench level) to
a known and selectable level for which counting efficiency is
accurately known, yet wherein this is accomplished without any
noticeable loss in statistical counting accuracy.
It is a more specific object of the invention to provide
novel methods and apparatus for statistically modulating signals
:.,
representative of decay events in each test sample so as to
simulate the presence of a quenching agent in the sample, and for
adjusting the modulating signals so as to cause the effective
.,:i
quench level of the sample to converge upon a particular
preselected quench level for which the counting efficiency is
known with a high degree of accuracy. In this connection, it is
an object of the invention to simulate quenching for all test
samples, whether the latter are quenched or unquenched, and
` irrespective of the degree of quenching that may be present, so
as to bring the particular variable quench correlation parameter
being measured (e.g., external standard ratio; net external
_g_
` 1040750
standard count; channels ratio; etc.) from its actual value for
any given sample to a particular preselected simulated value where
counting efficiency is known with a high degree of accuracy and
which is at or somewhat below the value for such parameter for the
most quenched sample that would normally be encountered by the
technician.
It is a more detailed object of the invention to provide
novel methods and apparatus for modulating the photon energy given
off by the scintillating medium in the region between the
scintillator and the photon-to-electron energy transducer, and
thus to simulate a quench condition for the sample equivalent to
that produced by the presence of an actual quenching agent in the
sample itself, which simulated quench condition is automatically
controlled so as to adjust the effective quench level for any
given sample (irrespective of its actual quench level) to a
preselected point where counting efficiency is known with a high
degree of accuracy.
An ancillary object of the invention is the provision
of novel methods and apparatus for establishing quench correlation
data rapidly, economically, and with a high degree of accuracy,
yet without requiring any advance preparation of standards having
known and different quench conditions, and without requiring
repetitive manipulation of such standards either manually or
automatically, and wherein an unlimited number of simulated quench
conditions can be produced using only a single unquenched standard.
Other objects and advantages of the invention will
become apparent as the following description proceeds, taken in
conjunction with the accompanying drawings, in which:
FIGURE 1 is a fragmentary side elevational view, partly
in section, depicting an exemplary radiation detection chamber and
elevator mechanism suitable for processing samples in accordance
with the present invention, the apparatus here being depicted with
--10--
~0407SO
the elevator mechanism in the down or "sample loaded!' position
with the sample to be analyzed interposed between a pair of light
transducers;
FIG. 2 is an enlarged fragmentary side elevational view,
partly in section and partly in diagrammatic form, here showing a
conventional apparatus for selectively positioning and
recirculating external standard radioactive source material
between a first position remote from and shielded from the
detection cnamber and a second position adjacent a sample disposed
in the detection chamber;
FIG. 3 is a fragmentary schematic wiring diagram of the
control components utilized for positioning standard radioactive
source material in a selectable one of two positions in accordance
with the particular cycle of operation determined by the mode
program control, the latter being depicted partially in block and
partially in diagrammatic form;
FIG. 4 is a schematic wiring diagram, partly in block
form, here depicting an exemplary control system for operating
the elevator mechanism shown in FIGS. 1 and 2 together with a
conventional electrical system which accepts, counts and records
the output of the radiation detector while at the same time
providing certain control inputs in accordance with the present
invention for the exemplary computer program control shown in
FIG. 13;
FIG. 5 is an elevational view of a typical master control
panel suitable for use with apparatus embodying the features of
the present invention and capable of carrying out the methods of
the present invention;
FIG. 6 is an elevational view, partly in diagrammatic
form, of an exemplary auxiliary control panel by which the
technician may selectively dial in predetermined efficiency
values, or numerical representations thereof, for one or more
--11--
.;,., . ~ .
~04~750
isotopes in different pulse height analyzing channels so as to
enable direct display of isotope activity level in units of decay
events per minute (dpm) in accordance with the present invention;
FIG. 6a is an enlarged fragmentary elevational view of
one of the BCD (binary coded decimal) decade dial switches shown
in FIG. 6;
FIG. 7a is a graphic representation of typical pulse
height spectra characteristic of two beta emitting isotopes of
different energy levels and illustrating particularly the
principle of "balance point" operation for the lower energy
isotope as well as the effect of ~quenching~ on both the lower
energy isotope and the higher energy isotope;
FIG. 7b is a graphic representation of typical pulse
height spectra similar to FIG. 7a but here indicating the
abscissa, which is scaled in units of pulse height, on an
attenuated scale so as to shift the pulse height spectra
downwardly and to the left with the higher energy isotope being
illustrated in "balance point" within its pulse heigllt analyzing
channel;
FIG. ~ is a graphic representation of a typical smooth
quench correlation curve (in which the abscissa is scaled in units
of net external standard ratio as.the measured variable quench
correlation parameter indicative of quenching), which smooth curve
is commonly used for determining the counting efficiency of a
liquid scintillation counting system and which has heretofore led
to significant statistical errors in computations of true activity
levels for isotopes in test samples, here emphasizing particularly
the non-uniform random distribution of actual measured points
which define such a curve;
FIG. 8a is a graphic representation similar to FIG. 8
with the continuous smooth curve deleted and emphasizing
~articularly specific designated intercept points for which
-12-
- . . . ,, ~' :
.. . . . ..
'' ' ,.. .' ' . :, ,
104~750
counting efficiency is known with a high degree of accuracy;
FIG. 9 is a graphic representation of a slightly
different type of quench correlation curve which is somewhat
:; similar to the curve shown in FIG. 8 but in which the abscissa is
scaled in units of net external standard count as the measured
variable quench correlation parameter indicative of the degree of
quenching;
FIG. 10 is a graphic representation similar to FIGS. 8
and 9 but in which the abscissa is scaled in units of normalized
channels ratio as the measured variable quench correlation
parameter which is indicative of the degree of quenching;
~ FIG. 11 is an enlarged elevational view, partly in
I section and partly in diagrammatic and block form, somewhat
~:~ similar to FIG. 1 but with the nousing depicted in phantom and
certain parts removed for purposes of clarity, here illustrating a
~ radiation detection chamber made in accordance with one exemplary
.:l form of the invention and which is suitable for simulating
".'"'I
~`j controllable quench conditions for test samples disposed in the
1 chamber;
: 20 PIG. 12 is a perspective view on a reduced scale,
partially in diagrammatic and block form, depicting the same
general organization of components as is shown in FIG. 11, but
here illustrating particularly a slightly different type of
. .
- detection chamber suitable for simulating quench conditions in
~I test samples in accordance with a modified form of the invention;
: FIG. 13 is a block diagram of an exemplary data
processing system suitable for use in conjunction with the present
invention, here illustrating particularly in block form a
conventional time-shared digital computer together with the
appropriate computer inputs and outputs;
~: FIG. 14 is a block diagram of exemplary control circuit~y
suitable for use in conjunction witll the present invention for
10407S0
.. . .. .
providing incremental changes in the quench simulation modulating
signal so as to cause the effective quench level of the sample to
converge upon a selectable predetermined quench level where
counting efficiency is known with a high degree of accuracy;
FIG. 15 is a schematic wiring diagram of an exemplary
arbitrary function digital-to-analog converter whicn is suitable
for converting the output signals from the circuit of FIG. 14 to
a useable control signal for modulating simulated quench
conditions; and,
FIG. 15a is a schematic wiring diagram illustrating
details of one of the illustrative transistor circuits shown in
FIG. 15.
While the present invention is susceptible of various
modifications and alternative forms, specific embodiments thereof
have been shown by way of example in the drawings and will herein
be described in detail. It should be understood, however, that it
is not intended to limit the invention to the particular forms
disclosed, but, on the contrary, the intention is to cover all
modifications, equivalents and al~ernatives falling within the
spirit and scope of the invention as expressed in the appended
claims.
- The Environment of the Invention
`~ Before discussing the present invention in detail, it
may be helpful to first briefly consider the prior art background
or environment within which the present invention finds especially
advantageous application. In radioactivity measurements, it is
the ultimate objective of most technicians to determine the
activity level of isotopes which may be either singly present or
present in multiple combinations within test samples, such
activity level being expressed generally in units of decay events
per unit time, e.g., decay events per minute (dpm). Thus, the
quantity of a particular isotope present in a test sample is, in
-14-
. .- . , ~ .
~04~750
general, proportional to the rate of decay events produced by that
isotope, such rate being termed the "activity level" of the
source. As a generalization, the decay events or radiation
emanations from a radioactive source are, for purposes of
measurement or counting, converted into light scintillations
which are generally proportional in photon energy to the energy of
the decay event which caused them. The light scintillations are
then generally converted into corresponding voltage or current
pulses which are generally proportional to both the light
scintillation and the decay event which caused the particular
pulse. Such pulses are then discriminated on the basis of their
amplitude, and then counted. The pulses may be counted for a
predetermined time (termed "preset time" operation) or,
~,~ alternatively, a predetermined number of pulses can be counted and
;~
the time required to reach such predetermined count measured
~termed "preset count~ operation). Generally, the ratio of
counted pulses to the elapsed time period is indicative of the
i activity level of the sample.
(a) General Organization and Operation
of an Exemplary Sample Processing
Apparatus
Referring now to FIGS. l and 2 cojointly, there has been
illustrated an exemplary automatic sample processing apparatus,
generally indicated at 20, which is intended to transfer a
plurality of samples one at a time in seriatim order to and from
a detection station. To this end, the exemplary apparatus 20
includes an elevator and detector mechanism, generally indicated
at 21, which is positioned beneath a support table 22 (FIG. 1) on
which one or more sample vials 24 are positioned both prior to
and subsequent to transfer to a counting or detection station. As
the ensuing description proceeds, those skilled in the art will
appreciate that the particular means employed for conveying
samples 24 to and from a point of alinement with the elevator
--15--
~407S0
mechanism 21 is immaterial to the present invention. Thus, it
will be understood that the samples 24 may be carried in rotatable
trays in the manner described in greater detail in both the
aforesaid Packard Patent No. 3,188,468 and Packard et al. U.S.
Patent No. 3,257,561, issued June 21, 1966 and assigned to the
assignee of the present invention. Alternatively,a plurality of
successive sample vials may be conveyed to a point of alinement
with the elevator mechanism 21 by means of an endless conveyor
having separate supports for the various vials. And, of course,
it will be understood that successive sample vials may be manually
placed on and removed from elevator apparatus 21. Moreover, while
there has herein been illustrated and will be described a power
driven elevator mechanism 21 for conveying successive samples 24
into and out of a detection chamber, the elevator need not be
automatic and could take the form of a manually operated elevator
of the type illustrated in one of the forms of the invention
disclosed and claimed in Robert E. Olson U.S. Patent ilo.3,198,948,
issued August 3, 1965 and assigned to the assignee of the present
invention. Indeed, the present invention will also find
20 advantageous use in more rudimentary forms of radiation detection
devices which are completely manual in operation and which do not
employ any type of elevator, the samples being manually positioned
within and removed from the detection chamber.
However, to facilitate an understanding of the present
invention, the general organization and operation of the elevator
and detector mechanism 21 will be briefly described hereinbelow.
Those interested in a more complete opexational and structural
description of the mechanism 21 are referred to the aforesaid
Packard and Olson patents.
Referring to Fig. 1, it will be noted that the elevator
and detector mechanism 21 includes a base assembly 25 which houses
a pair of light transducers, for example, photomultipliers PMT#l
-16-
"
~04~750
..
and PMT#2 disposed on opposite sides of a vertical elevator shaft
26. Mounted within the elevatQr shaft 26 is an elevator 28 having
a platform 29 at its upper end for supporting one of the
radioactive test samples 24 and transporting the sample downwardly
into the elevator shaft where it is alined between the
photomultipliers PMT#l and PMT#2. Each sample 24 may simply
comprise a light transmissive vial or other suitable light
transmissive container within which is placed a liquid
scintillator and a radioactive isotope or isotopes to be
measured. Thus, as the isotope or isotopes undergo decay events,
light scintillations are produced in the light scintillator, and
~, such scintillations are then detected by the photomultipliers
which produce electrical output signals in the form of voltage or
current pulses corresponding to each light scintillation detected.
At the completion of the counting cycle the elevator 28 is
returned upwardly so as to eject the sample 24 from the elevator
and detector mechanism 21. A shutter mechanism 30 is mounted on
', the upper end of the base assembly 25 for the purpose of
;~ preventing erroneous output signals from the photomultipliers
, 20 resulting from environmental light. At the same time, the base
assembly 25 is formed of suitable shielding material such, for
example, as lead, which serves to minimize the amount of
environmental ionizing radiation causing light flashes in either
the scintillation medium or the photomultipliers.
In order to insure that the shutter mechanism 30 is
opened and closed in timed relationship with vertical movement of
the elevator 28, the two devices are interconnected and actuated
by a common reversible drive motor Ml (Figs. 1 and 4). While not
illustrated in detail, the shutter mechanism 30 comprises a
plurality of movable shutter blades which are interleavea with
a plurality of fixed shutter blades, the latter having apertures
therein alined with the elevator shaft 26 and with an aperture 31
~040750
formed in the tablelike support 22. The arrangement is such that
when the movable hlades are pivoted about a pivot point (not
shown), they swing between limit positions to selectively open and
close the upper end of the elevator shaft 26.
To effect such pivotal blade movement, the movable
shutter blades are rigidly secured to a stub shaft ~not shown) in
a manner more fully described in the aforesaid Olson patent.
Suffice it to say that the stub shaft is rigidly secured to the
upper end of a generally flat, depending shutter actuating shaft
32 (Fig. 2) having a twisted portion 34 intermediate its ends.
The lower end of the actuating shaft 32 is received within a
tubular drive shaft 35 (Fig. 2) the latter being coupled adjacent
its lower end to the elevator 28 by means of a brac]~et 36. A pair
of dowel pins (not shown) or similar cam means extend transversely
through the tubular drive shaft 35 in closely spaced surrounding
relation to the shutter actuating shaft 32.
The arrangement is such that as the drive shaft 35
starts to move vertically upward, force is transmitted through the
bracket 36 and the ele~ator 28, thus driving the latter upwardly
to unload the sample 24. Just prior to the time that the sample
'` 24 reaches the shutter mechanism 30, the dowel pins or similar
cam means traverse the twisted portion 34 of the shutter actuating
shaft 32, rotating the latter about its own vertical axis and
pivoting the vable blades of the shutter mechanism out of the
path of vertical movement of the elevator 28. During a sample
loading cycle, the dowel pins or similar cam means serve to cam
the shutter actuating shaft 32 in the opposite direction
immediately after the new sample 24 passes through the alined
apertures in the shutter mechanism and the table, thus swinging
the movable blades to the closed position shown in Fig. 1.
To effect vertical movement of the drive shaft 35 and
the elevator 28 for the purpose of introducing samples 24 into
'' :.~ . ' - ~ '
104~)750
and ejecting such samples out of the elevator sllaft 26, the drive
shaft 35 is drivingly coupled to a conventional reversible motor
Ml (Figs. 1 and 4). The particular means employed for coupling
$ the motor to the drive shaft may vary and have not been
illustrated or described in detail. Those interested in a more
complete description are referred to the aforesaid Packard and
Olson patents. It should suffice to state for the purpose of
an unders~anding of the present invention that the motor ~l is
coupled to the drive shaft 35 in the exemplary apparatus by means
of cables diagram~atically indicated at 38 in Fig. 4. The
arrangement is such that when the motor Ml is driven in one
direction, the cables 38 are paid in and out so as to raise the
I elevator mechanism 21. Conversely, when the motor is driven in
`' the opposite direction, the cables are paid in and out in the
opposite direction, thus lowering the elevator mechanism 21.
The energizing circuit for the motor Ml includes a lower
i limit switch LSl (Figs. 2 and 4) which is mounted on the frame of
the sample handling apparatus 20 in a position to have its
actuator LSl depressed by a laterally projecting flange 39
mounted on the lower end of the elevator when the latter is in a
down position with the sample 24 carried thereon alined between
the photomultipliers PMT#l, PMT#2. Depression of the actuator
LSla serves to de-energize the motor Ml and the apparatus is then
ready for a counting cycle. A second limit switch LS2, included
in a second energizing circuit for the rnotor Ml, is mounted on
the frame of the apparatus 20 in position to have its actuator
LS2a depressed by the flange 39 when the elevator arrives at its
uppermost limit position with the sample 24 carried thereon
having been ejected from the elevator shaft 26. Thus, the limit
switch LS2 serves to de-energize the motor Ml when the elevator
reaches its uppermost limit position~
Referring now to Figs. 1, 2 and 4 conjointly, a brief
:~;
--19--
~ .
~040750
description of a typical "sample unload" and "sample load" cycle
of operation will be set forth. Assuming that the exemplary
elevator mechanism 28 is in its down position and that the sample
vial 24 positioned in the detection chamber has undergone a
complete counting operation for determining the activity level of
the radioactive source therein, the technician is now ready to
remove the particular sample 24 from the detection chamber
between the photomultipliers PMT#l, PMTX2 and to substitute
therefor a new sample 24. Considering for the moment, a semi-
10 automatic operating cycle, it is merely necessary for thetechnician to press the "UNLOAD" button on the left hand portion
of the master control panel (Fig. 5). When this is done, an
energizing circuit will be completed from the terminal L2 of a
suitable a-c source (not shown) through any suitable circuit (not
shown) in the Mode Program Control 40, and from thence through
the "Change Sample" terminal 41, the normally closed "RUN"
contacts of the upper limit switch LS2, and through the "UNLOAD"
terminal of the elevator motor ~1, the latter being coupled to the
terminal Ll of the a-c source. Under these conditions, the
20 motor Ml will be energized and will start to rotate so as to
raise the drive shaft 35 and the elevator 28. As the elevator 28
;~ starts upwardly, the flange 39 which is integral with the bracket
36 will release the actuator LSla of the lower limit switch LSl,
thus permitting the latter to return to its normal condition with
the "RUN" contacts closed and the "STOP" contacts open. As the
elevator 28 approaches its upper limit position (the shutter
mechanisms 30 having been opened by coaction of the shutter
actuating shaft 32 and the drive shaft 35), the flange 39 engages
the actuator LS2 of the upper limit switch LS2, thus shifting
the latter to open its normally closed "RUN" contacts and close
its normally open "STOP" contacts. When this occurs, the
eleyator motor ~1 is de-energized and the sample vial 24 is in the
.
--20--
.
. .
:,
.: :
"sample ejected" position.
During a semi-automatic operating cycle, the technician
simply replaces the ejected sample vial with a new sa~ple vial and
then depresses the "LOAD" r.~ode selector switch shown in the left
hand portion of Fig. 5. On the other hand, in a completely
automatic cycle of operation, closure of the "STOP" contacts of
the upper limit switch LS2 could, through suitable circuitry not
shown but described in greater detail in the aforesaid Packard
et al. patent No. 3,257,561, cause energization of the indexing
mechanism for automatically moving the next sample into place.
In either case, when the new sample is in place an energizing
circuit is completed from the terminal L2 of the a-c source
through the Mode Program Control 40, its "Sample Changed"
; terminal 42, the normally closed "RUN" contacts of the lower limit
switch LSl, and tllen through the "LOAD" terminal of the elevator
motor back to the a-c terminal Ll. The motor now runs in the
opposite direction to again return the elevator 28 to its
lowermost position. At the same time, the shutter mechanism 30
~; ! iS closed as the drive shaft 35 moves downwardly and the cam
means therein traverses the twisted portion 34 of the shutter
actuating shaft 32. When the elevator 28 reaches its lowermost
limit position, the cam actuator or flange 39 again engages and
depresses the actuator LSla of the lower limit switch LSl, thus
breaking the "RUN" contacts and making the "S~OP" contacts
thereof. The motor Ml is again de-energized and the apparatus is
now ready for another count cycle. Closure of the "STOP" contacts
of the lower limit switch LSl is effective to create a control
signal from the terminal Ll of the a-c source to the "Elevator
Down" terminal 43 of the Mode Program Control 40, thus signaling
the latter that t}-e apparatus is in condition for automatic
initiation of the next counting cycle.
(b) Automatic External Standardization
'~
.;
-21-
,.
:, :
~040750
As stated above, the aforesaid Packard patent No.
3,188,468 discloses and claims various forms of procedures and
apparatus for automatically subjecting successive samples to two
separate counting cycles, during one of which the sample to be
counted is exposed to a known quantity per unit time of radiant
energy emanating from either an internal or an external standard.
Generally stated, external standardization techniques are based
upon a phenomenon known as "Compton Scatter", a phenomenon
wherein the interactions that occur between penetrating radiation
and electrons that comprise part of the test sample, produce
electrons in the liquid scintillator having an energy spectrum
similar in shape to that produced by a beta emitter. The
"Compton Scatter" phenomenon is well known and need not be
described in detail. Those interested in such a detailed
description are referred to the aforesaid Canadian Patent No.
822,21S ~Cavanaugh). Briefly, however, and with reference to
Fig. 2, it will be observed that a standard source of
penetrating radiation, here a compound source generally indicated
at 44, has been illustrated at a position located exterior of and
; 20 in proximity to the test sa~ple 24 disposed in the detection
chamber. As is characteristic of gamma emitters, or emitters of
~; similar penetrating radiations, the source 44 will undergo a
plurality of decay events in a given period of time, such decay
events resulting in the emission of gamma rays in diverse
directions. Certain of such gamma radiations will be directed
towards, into or through the sample vial 24 disposed in the
detection chamber, thus resulting in interactions between the
gamma radiation and matter within the liquid sample, thereby
~ causing excitation of electrons and producing a light flash
;~ therein. Under some circumstances, the energy of the gamma
radiation may be totally absorbed, although more often the energy
of the impinging gamma radiation is only partially absorbed. In
'~ ~
:
~ -22-
;: .:
.:
:: ~ , . - - :.
104V7S0
the latter eVent, a photon will veer off randomly, in accordance
with the principle of conservation o~ momentum, at a reduced
energy until a second "Compton interaction" occurs. Since the
photon is at a reduced energy, the chances of producing a second
"Compton interaction" are increased. If and when the photon
interacts with matter within the sample vial a second time, the
energy of such photon will again be either totally or partially
absorbed, thus producing electrons and creating a second light
scintillation in the vial 24. The net result of the foregoing
is that "Compton interactions" occurring in the sample 24 will
produce an energy spectrum which is highly related by physical
laws to that produced by a beta emitter. Consequently, if the
isotope disposed in the sample 24 happens to have an energy
spectrum that is highly related by physical laws to the energy
spectrum produced by the "Compton interactions", then it is
possible to determine the true activity level of the isotope
regardless of the degree of quenching, changes in line voltage,
or instrument drift, since the effect of these variables would
be the same on both the isotope and the standard. This has
conventionally been done by either arithmetical computations or
by comparison with previously prepared sets of calibration curves.
As shown in FIG. 2, there has been illustrated an
exemplary apparatus for pneumatically shifting the compound
external standard radioactive source material 44 into and away
from proximity to the sample vial 24 disposed in the detection
chamber. Those interested in the specific details of this system
:.
are referred to the aforesaid Canadian Patent No. 822,215.
However, in order to facilitate an understanding of the present
; invention, this prior automatic standard positioning system will
` be briefly described below.
-~ Referring to FIG. 2, it will be observed that the
~ standard radioactive source material 44 is positioned within a
~"
~ -23-
-
~,.
10407S0
generally vertically extending conduit 45 which terminates at its
upper end adjacent the detection chamber within which the sample
vial 24 is positioned. The upper end of the conduit 45
terminates in a fixed stop 46 and is coupled to atmospheric
pressure through a suitable transverse conduit 48. The lower end
of the conduit 45 projects into a shielded housing 49 which is
rigidly secured to the frame of the apparatus 20, there being an
annular stop 50 formed in the lower end of the conduit 45 within
the housing 49. As here shown, the lower end of the conduit 45
is coupled directly to a pair of control valves 51, 52, the
valves being respectively coupled to the pressure and vacuum
sides of a conventional fluid pump which may simply take the
form of a pneumatic pump P. In the illustrative apparatus, the
pressure valve 51 is controlled by means of a solenoid Sl having
terminals Tl, T2, while the control valve 52 is actuated by
means of a solenoid S3 having terminals T3, T4.
The arrangement is such that when the solenoid Sl is
energized, the conduit 45 is coupled directly to the high
pressure side of the pump P through the valve 51. Under these
conditions, the compound standard source 44, which is confined
within the conduit 45, is blown or urged upwardly within the
conduit 45 until it engages the fixed stop 46 at the upper end of
the conduit. Preferably, the solenoid Sl is only energized
; momentarily to provide a pulse of fluid pressure and, therefore,
provision is made for magnetically holding the compound source
;,~
material 44 in the position shown in FIG. 2 adjacent the sample
..,
~` vial 24. To this end, a steel ball 54 or other suitable
magnetically attractable material is disposed within the conduit
45 immediately beneath the compound source 44. An annular
torroidal magnet 55 is disposed near the upper end of the conduit
45 in surrounding relation thereto, the magnet being positioned
::`
generally at or near the upper edge of the elevator platform 29.
-24-
':~
':. ' ', :
,: ~ .
, , :
1041)7S0
Thus, when the $olenoid Sl is deenergized, tlle source material
44 and the steel ball 54 will tend to fall downwardly through the
conduit 45 until the steel ball is magnetically attracted by the
magnet 55, thus precisely positioning the compound source
material. When the technician wishes to remove the compound
source material, it is merely necessary to momentarily energize
the solenoid S2, thus coupling the conduit 45 directly to the
vacuum or low pressure side of the pump P through the control
valve 52. When this occurs, a vacuum is drawn in the conduit 45
and such vacuum, together with atmospheric pressure exerted
; through conduit 48 serves to drive the compound source material
44 and the steel ball 54 downwardly through the conduit until the
train engages the annular fixed stop 50 within the shielded
housing 49.
Turning now to FIG. 3, there will be desaribed typical
`'I
operating cycles for both the Automatic Standardization OFF and
ON modes. Thus, assuming first that the technician desires
merely to count a particular sample without subjecting the sample
to external radiation, it is merely necessary to condition the
automatic standardization control switch 56 (FIGS. 3 and 5) in
the OFF or OUT positions. Under these conditions, when the
elevator 28 reaches the down position and a signal is imposed
upon the "Elevator Down" terminal 43, such signal is effective
through suitable circuitry in the Mode Program Control 40 to
immediately create a control signal at the "Start Count"
terminal 58, thereby initiating a counting cycle in a manner to
be described in greater detail below. Upon completion of the
counting cycle, a control signal will appear at the "Count
Complete" terminal 59 for the Mode Program Control 40, which
signal will be transmitted directly to the "Start Print" terminal
60 for the purpose of commencing a print cycle for the display
portion of the apparatus. Upon completion o the prin~ cycle, a
-25-
104~)750
control signal appears at the "End Print" terminal 61 which is
transmitted directly to the ~'Chan~e Sample" terminal 41 for
purposes of energizing the elevator motor Ml through its "UNLOAD"
terminal in the manner previously described so as to eject the
test sample 24 from the detection chamber.
- Assuming next that the technician wishes to count a
sample in an Automatic Standardization operating mode, it is
merely necessary that the control switch 56 be conditioned in the
ON or standard IN condition. When this occurs, and the elevator
28 reaches its lowermost position with the new sample, the
signal presented on the "Elevator Down" terminal 43 is conveyed
through a differentiating device 62 to the "Insert Source"
terminal 64 of the Mode Program Control. The signal pulse which
is presented at the terminal 64 is applied directly to the "set"
section S of a monostable flip flop 65 to cause the latter to
switch from its "reset" to its "set" condition.
Since flip flops of the bistable and monostable variety
are well known to those skilled in the art, they will not be
described herein in detail. Rather, the flip flops have been
illustrated symbolically as having a "set" section S and a
"reset" section R with a junction therebetween. It will be
understood that when a signal is presented at the junction of a
bistable flip flop the latter will shift from one side to the
other. Similarly, when an input signal or pulse is applied to
the S section of a monostable flip flop, the latter will be
~ momentarily "set", thus producing a predetermined output signal
; from the S section. After a time delay dependent upon the
~ characteristics of the monostable flip flop, the latter will
`i
automatically return to its "reset" state.
Keeping the foregoing characteristics of conventional
flip flops in mind, it will be observed upon reference to FIG. 3
that momentary switching of the flip flop 65 to the "set" state
.'~ .
10407S0
will complete an energizing circuit for the solenoid Sl through
the normally closed contacts Rla controlled by a relay Rl, thus
energizing the control valve 51 and coupling the conduit 45 to
the high pressure side of the pump P. At the same time, switching
of the monostable flip flop 65 to the "set" state will also
complete a momentary energizing circuit for the pump P, thus
pressurizing the conduit 45 and shifting the compound source
material 44 from the position shown in FIG. 3 to the position
shown in FIG. 2.
At the same time that the control signal is applied to
the "Insert Source" terminal 64, it is also conveyed to the
"Source In" terminal 66 of the Mode Program Control 40 through a
conventional time delay device 68 which provides a sufficient
delay to insure that the compound source material has shifted
from its shielded position in housing 49 to its effective
position adjacent the sample 24. The control signal presented
at the "Source In" terminal 66 is then conveyed directly to the
Start Count" terminal 58 to initiate a first counting cycle for
the sample 24 while the external standard source material 44 is
in proximate relation to the vial 24. Upon completion of the
counting cycle, a signal is presented at the "Count Complete"
terminal 59 of the Mode Program Control 40, which signal is
passed through the "ON~ contacts of the automatic standardization
control switch 56 to the junction of a bistable flip flop 68 to
cause the latter to switch from its "reset" to its "set"
condition. When this occurs, a control signal is passed from the
S section of the bistable flip flop 68 through a differentiating
device 69 to the "Retract Source" terminal 70 of the Mode
Program Control 40. The signal presented at the terminal 70
completes an energizing circuit for the relay Rl causing the
normally closed contacts Rla controlled thereby to open and
closing the normally open contacts Rlb controlled by the relay.
-27-
104~750
When this occurs, momentary energizing circuits are
simultaneously completed through the now closed contacts Rlb for
the solenoid S2 and the pump P, thus energizing the latter and
- shifting the control valve 52 to a position where the conduit 45
is coupled directly to the vacuum side of the pump, thereby
drawing the compound source material 44 from its position
adjacent the sample vial 24 in the detection chamber back into
its shielded position in the housing 49. At the same time that
the control signal is presented on the "Retract SourceU terminal
70, it is also conveyed to the "Source Retracted" terminal 71 of
the Mode Program Control 40 through a conventional time delay 72
to insure that the compound source has been retracted. The
signal presented at the "Source Retracted" terminal 71 is then
conveyed directly to the "Start Count" terminal 58 of the Mode
Program Control 40 to initiate a second counting cycle for the
sample 24, this time with the external standard source removed.
At the completion of the second counting cycle, a control signal
is presented at the "Count Complete" terminal 59 and passed
directly through the ON switch contacts for the automatic
standardization control switch 56 to the junction of the bistable
flip flop 68, this time causing the latter to switch from its
"set" to its "reset" condition. Switching of the flip flop 68
to its "reset" condition establishes a control signal level
which, after differentiation in a differentiating device 74 is
applied as a control pulse at the "Start Print" terminal 60, thus
;~ again initiating a display cycle identical to that previously
described, at the end of which a control signal is presented at
the ~Change Sample" terminal 41 which is effective to complete
an energizing circuit for the elevator Motor Ml through its
"UNLOAD" terminal, thereby ejecting the sample vial 24 from the
detection chamber after it has undergone two successive counting
cycles, one with the external standard source material 44 in
'~
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104V7S0
proximity to the vial and one in which the external standard
source material 44 is remote from the vial and within the
shielded housing 49.
(c) Programming Logic
Since procedures and equipment embodying the features
~ of the present invention will normally be used with an associated
`~; programming control circuit, a typical programming system,
~ generally indicated at 75 (FIG. 4), will be briefly described
,
hereinbelow. To this end, it will be observed that after a
sample 24 has been properly positioned between the detector
i;
photomultipliers PMT#l, PMT#2, a signal is presented on the
: .
"Start Count" terminal 58 in the manner previously described
indicating that the new sample 24 is loaded and that the next
counting cycle should be started. The control signal presented
at the terminal 58 is then passed over a line 76 to the START
terminals of a timer 78 and a start-stop control 79. At the
same time a control signal is passed from the Mode Program
Control 40 through a "Select Time" terminal 80 directly to the
timer 78, which signal serves to reset the timer to its zero
time condition while at the same time selecting the increment of
time for the timer 78 to operate during the ensuing counting
cycle. The signal passed through the start-stop control 79 from
the "Start Count" terminal 58 provides one of two control input
signals which are necessary to open gates 81a, 81b, 81c, 81d and
81e, such gates being respectively associated with five pulse
height analyzing channels. During the predetermined time
interval measured off by the timer 78, voltage or current pulses
produced by the photomultipliers PMT#l, PMT#2, aresimultaneously
passed to a coincidence logic 82, an output signal is passed
from the coincidence logic 82 directly to the gates 81a-81e,
thus providing the second control input signal necessary to open
the gates. It should be noted at this point that the gates
'.
-29-
104V750
81a-81e are of the type which are normally closed to prevent the
passage of pulses therethrough and which are opened only by
virtue of the simultaneous presence of control input signals
from the coincidence network 82 and the start-stop control 79.
The pulses from the photomultipliers PMT#l, PMT#2
which are passed to the summing amplifier 84 are algebraically
added to provide a single output pulse representative of the sum
of the two input pulses, which single output pulse is
simultaneously presented to the input terminals of variable gain
controls 85a-85e which are respectively associated with the five
pulse height analyzing channels. The output signals from the
variable gain controls 85a-85e are, in turn, respectively passed
to the input terminals of five pulse height analyzers 86a-86e.
Those skilled in the art will appreciate that the pulse height
analyzers 86a-86e comprise suitable discriminator circuits ~not
shown) which may be selectively adjusted to permit passage of
only a selected amplitude band of pulses therethrough. ~50reover,
the input pulses to the five pulse height analyzing channels may
be differentiall~- amplified by means of the variable gain controls
85a-85e in a manner described in greater detail in Lyle E.
Packard U.S. Patent No. 3,114,835, issued December 17, 1963 and
assigned to the assignee of the present invention. Thus, those
pulses in each of the five pulse height analyzing channels which
exceed in amplitude the base discriminator level for the
respective analyzers 86a-86e but do not exceed the maximum
discriminator level are passed from the pulse height analyzers
to the input of the gates 81a-81e. Assuming that coincidence
signals have been detected by the coincidence network 82, such
pulses are passed through the now open gates directly to the
inputs of respective scalers 88a-88e or other suitable counting
devices. In a completely automatic system of the type to be
described herein, the scalers 88a-88e respectively provide
.
104V7S0
output signals Sl-S5 which are passed to the data processing
portion of the system.
At the end of the timed period provided by the timer
78, the latter supplies a signal over lines 89 and 90 to
respectively indicate to the Mode Program Control 40 that the
count has been comp1eted and to cause the start-stop control 79
to close the gates 81a-81e. Thus, referring to FIG. 4 it will
be observed that the line 90 which is coupled to the timer is
coupled directly to the "Count Complete" terminal 59 for the
Mode Program Control 40. ~t this time, the timer 78 provides a
timing signal Tl indica~ive of the length of the time period and
a "Count Complete" signal which are also conveyed to the data
processing system to be subsequently described in the same
manner as the output signals Sl-S5 from the scalers 88a-88e.
Finally, at the time that the timer 78 times out and provides a
"Stop" signal for the start-stop control 79, the latter, after a
sufficient delay to insure that the data recorded in the scalers
88a-88e has been read out by the data processing portion of the
system, provides a reset signal for the scalers over line 91,
which signal is effective to reset all of the scalers to their
zero state.
Because the system shown diagrammatically in FIG. 4 may
take any of a variety of forms known to those skilled in the art,
it need not be illustrated or described in greater detail. It
will, however, be understood that the "counts" recorded by the
scalers 88a-88e and provided to the data processing portion of
the system as signals Sl-S5 will include responses to background
radiation which produces scintillation flashes in the liquid
scintillator and which is received from extraneous sources, such
background responses being in addition to the responses to
; radiation from the sample being measured. However, this
"background count" can be first measured with no sample, or
-31-
r~ ~
1040750
a sample of known radioactiye strength in the detector. Such
background count would, of course, include any negligible counts
that are created by the external standard source 44 when the
latter is in its shielded housing 49. The background count can
then be subtracted from each sample reading to arrive at an
indication of the sample's radioac~ive strength. To accomplish
this, the apparatus is provided with three sets of four thumb-
wheel type dial-in switches which are located on the master
control panel (FIG. 5) and are designated as BGl, BG2, and BG3.
Thus, after the technician has determined what the background
count is for each of the three data channels -- viz., the
channels including scalers 88a, 88b and 88c -- he need only dial
such readings into the system and they will thereafter provide
input signals BGl, BG2 and BG3 for the data processing portion
of the system which can then be subtracted from gross counts to
provide an indication of net counts per minute.
Besides counting the number of responses by the
photomultipliers in a predetermined time interval (preset time
operation), the time period required for the generation of a
predetermined number of responses (preset count operation) may
be measured and recorded, as is well known, in which event the
Mode Program Control 40 would provide a suitable signal over the
"Select Time" terminal 80 to cause the timer 78 to generate a
"Stop" signal for the start-stop control 79 at the time that the
preset count is reached.
(d) Spectral Distributions and Factors
Affecting Pulse Height Spectra
`~ It is well known that beta emitting isotopes produce
decay events which individually involve energies spread over a
very wide range or spectrum. Each isotope has its own
characteristic spectrum with a known maximum energy, such
spectrum including a few decay events of near zero energy, a
. .
-32-
: , .
; 1040750
few decay eventS of maximum energy, and a majority of decay
events having energies in the region between the upper and lower
limits. ~etween these limits, the spectrum rises to a peak and
then falls. Since the li~ht transducers or photomultipliers
PMT#l, PMT#2 produce pulses which are substantially proportional
in amplitude to thè energies of the corresponding decay events,
the pulse height spectrum will, for a given gain of the photo-
multiplier, correspond to the energy spectrum of decay events.
A characteristic pulse height spectrum for a typical low energy
beta emitter such as tritium- (3iI) is graphically represented by
the spectral curve 92 shown in FIG. 7a. A similar characteristic
pulse height spectrum for a typical higher energy beta emitter
such as carbon-14 (14C) is graphically represented by the
spectral curve 94 shown in FIG. 7b. It will, of course, be
understood that the area under the curves 92, 94 is representative
of the total number of output pulses from the photomultipliers,
and is, therefore, proportional to the total number of decay
events occurring in the test sample in a given unit of time, say,
one minute.
Considering, for the moment, the spectral curve 92
shown in FIG. 7a diagrammatically representative of the spectrum
. . .
for tritium (3H), it should be noted that such curve is here
representative of the distribution of voltage or current pulse
heights (either at the outputs of photomultiplier tubes or
subsequently in the amplification circuits). Thus, for purposes
of discussion the abscissa of the graph shown in FIG. 7a may be
considered as volts as a measure of pulse height, while the
ordinate is expressed in counts per unit time, or counts per
minute (cpm). Fox an actual curve, the numerical values of
pulse height would depend upon the gain settingS of the
:::
photomultiplier Andfor subsequent amplification stages (e.g.,
~` the amplifiers 84 and 85a-85e shown in FIG. 4), while the
.
~ ~ -33-
.. . ,, , :- .
.: . . . .
.- . .: . . ..
, !
104~)7S0
counts per minute scale would ~epend upon the activity level of
the sample.
; Let it be assumed for the moment that a sample
containing a tritium isot~pe is to be analyzed and the technician
wishes to record pulses representative of decay events occurring
in the sample in the channel including pulse height analyzer 86a
and scaler ~8a (FIG. 4). In this event, the technician would
adjust the two discriminators which define the pulse height
analyzer 86a to establish a base level A and a maximum level B
(shown diagrammatically in FIG. 7a) for the pulses that are to
be counted. In other words, the technician would adjust the
pulse height analyzer 86a so that any pulses which did not
exceed level A in amplitude would be rejected, while any pulses
which exceeded level B would also be re~ected. Thus, the only
pulses which would be counted would be those falling between the
:
levels A and B and, thus, the AB discriminators define what is
commonly referred to in the art as an "AB windown. In order to
operate the spectrometer at, or near, optimum counting
conditions, it is necessary to adjust the pulse height analyzer
86a (FIG. 4) so that counting efficiency in the channel
including scaler 88a (i.e., the ratio of counts observed on the
scaler 88a to the number of decay events occurring in the test
sample 24) is high while the number of background counts are
:::"
low -- preferably the AB window should be adjusted so that the
ratio E2/B (where E is counting efficiency and B is background
noise) is maximized. To achieve this desirable objective the
AB window (FIG. 7a) of the pulse height analyzer 86a should be
wide, but not so wide that the number of background pulses
included in the window are great in comparison with the number
of pulses xesulting from decay eVents in the test sample.
Moreoyer, in order that the counting efficiency be as high as
possible for a given window width, the AB window should embrace
, .
-34-
'
10407S0
the peak pQrtion of the pulse height spectrum 50, or as nearly
so as possible. At the same time, however, it is essential that
the level A be somewhat above the threshold level T for the
electronic components of the equipment while the level B
discriminator must be below the saturation point level SP for
such equipment.
Considering FIG. 7b, it will be observed that the above
discussion is also applicable to the setting of the equipment for
the higher energy carbon-14 isotope (14C) represented by the
spectral curve 94. In this instance, however, since carbon-14
is considerably more energetic than is tritium, it is necessary
to attenuate the pulse height scale in order to count with
optimum efficiency. Thus, and assuming that the technician
wishes to count the carbon isotope in the channel including
pulse height analyzer 86b, the technician would first adjust the
variable gain control 86b so as to shift the carbon spectrum
downwardly to the solid line position shown by curve 94 in FIG.
7b. Comparing the curve 94 for carbon-14 in FIG. 7b with the
curve 94 for carbon-14 in FIG. 7a, it will be noted that when
the pulse height analyzer 86a is adjusted for optimum counting
of the tritium isotope in the AB window, it is virtually
impossible to establish good counting conditions for the carbon
isotope. The reason for this is simply that in order to separate
the two pulses the CD window defined by the discriminators which
form pulse height analyzer ~6b must be set close together and
close to the saturation point of the equipment. Otherwise, there
will be a significant contribution of counts in the CD window by
the lower energy tritium isotope. However, by adjusting the
variable gain control 85b, it is possible to produce conditions
similar to that diagrammatically represented in FIG. 7b where
satisfactory counting conditions can be established in the CD
window for carbon-14 and wherein counts contributed by the lower
-35-
'' : ' :. .
- ... ; ,,; . .
~ 1040750
energy tritium isotope are excluded.
A real problem often encountered in liquid
scintillation spectro~etry results from the phenomenon generally
known as "quenching", a phenomenon which causes the pulse height
spectrum represent~tive of a glven isotope to vary from that
which would normally be observed when no quenching occurs. When
the test sample 24 is prepared, a solvent for the scintillation
medium is selected which is transparent and which has maximum
light transmitting characteristics. The vial which contains the
sample is also carefully selected to insure that it will not
impede the transmission of light photons to the photomultiplier.
However, the substance containing the radioactive material to be
assayed often has relatively poor light-transmitting
; characteristics. Merely by way of example, if the radioactive
isotope is contained within a blood or urine sample, the test
sample will be red or yellow in color rather than clear. Such
red or yellow coloring of the test sample impedes the
transmission of light from the scintillation flashes to the
.
photomultipliers PMT#l, PMT#2 so that the latter do not detect
the same number of light photons as they would otherwise have
detected had the test sample 24 been colorless. Stated another
:
way, the light produced in the scintillation medium by a given
^ decay event is attenuated in its passage to the photomultipliers
with a consequent attenuation of the output pulses from the
photomultipliers. Moreover, since certain of the lower energy
~;~ decay events produce only a few light photons, the effect of
i
light attenuation in the test sample will, in some instances,
prevent a sufficient number of light photons from reaching the
photosensitive cathode so tilat no detectable responses in the
photomultipliexs ~MT#l, P~T#2 are produced. The foregoing
phenomenon is commonly refexred to as 'Icolor quenching" and can
be represented graphically as shown in FIG. 7a by the spectral
;
-36-
~' ~ ' .
104~750
curves 92i! 92ii and 92iii which respectiyely represent
progressively increased quench conditions for the unquenched
tritium isotope represented by the curve 92. Thus, while the
particular isotope being tested would, in the absence of
quenching, produce a spectrum such as shown at 92, in the
presence of such color quenching, the entire spectrum would shift
downwardly (or to the left as shown at 92i in FIG. 7a) because of
light attenuation in the sample 24 and, as the degree of
quenching is increased, the curve will progressively shift
downward to positions represented by the curves 92ii or 92iii.
There is still another source of quenching error which
introduces problems into liquid scintillation spectrometry
techniques. This latter source of error is commonly referred to
as "chemical quenching", and results from the presence of
certain substances in the test sample which, irrespective of
color, interfere with the conversion of radiation energy into
light energy. Such substances cause a portion of the radiation
energy to be dissipated as heat rather than producing light
photons in the scintillation medium. The presence of chemical
quenching can in some instances involving relatively low energy
decay events, prevent generation of a sufficient number of light
photons to trigger the photomultiplier. It will be apparent,
however, that chemical quenching will produce an effect similar
to that produced by color quenching; i.e., the pulse height
spectrum will be shifted to the left. And, of course, in certain
instances, the total effect may be cumulative -- that is, the
test sample may be subject to both chemical and color quenching.
Referring to FIG. 7b, there have been diagrammatically
illustrated spectral curves 94i, 94ii and 94iii which are
respectively representatiVe of pxo~xessively increased quench
conditions for the higher energy isotope carbon-14. As here
illustrated, it will be observed that the effect of quenching
-37-
~0407S0
with the higller energy isotoPe is quite similar to that for the
lower energy isotoPe -- vis., quencl~ing causes the curve to shift
downwardly to the left because of the attenuation of light pulses.
A second factor important to optimum counting
conditions is that of "balance point" operation, an operation
wherein the spectrometer is adjusted so that it is relatively
insensitive to slight shifts in the pulse height spectrum due to
drift or changes in the system gain. Thus, referring to FIG. 7a,
it will be observed that the effect of quenching which causes the
spectral curve to shift from the position 92 to the position 92i
is that some pulses are lost from the AB window as the peak of
the curve moves to the left. At the same time, however, some
pulses are regained because of the shift. Assuming that the
shift is slight, it will be observed that substantially the same
number of pulses will be regained as are lost, and this is due
to the fact that the peak of the curve is centered within the
window. I~owever, as the curve shifts more and more to the left
(perhaps due to progressively greater quenching), more and more
pulses will be lost and fewer will be regained. Indeed, it will
be observed from FIG. 7a that by the time the curve has been
quenched to the position shown at 92 , the only change is a net
loss of pulses and any additional quenching will not produce any
recaptured pulses. However, operation of a spectrometer with the
pulse height window adjusted to coincide approximately with the
peak of the spectral curve is desirable since minor unavoidable
drifts in system gain and minor quench effects, will cause the
small loss of pulses from the spectrum shift to be balanced by a
corresponding small gain of pulses. The balancing effect occurs
whether the spectrum shifts slightly to the left or to the
right, although it Will be ap~reciated that shifts due to
quench~ng will be downward shifts -- that is, to the left, while
shifts due to changes in gain can be in either direction. From
-38-
104U750
the foregoing explanation it will be apparent t;lat lf the
counting window (i.e., the AB window in FIG. 7a or the CD window
in FIG. 7b) is not adjusted for balance point operation, shifts
in the spectrum could result in appreciable erroneous changes in
the counting efficiency and tlle measured count rate. Therefore,
it is desirable to operate in balance point operation whenever
~; possible. Referring to FIG. 7b, it will be appreciated that with
the higher energy carbon isotope, the effect of quenching not
only causes the curve to shift to the left, but it also causes
the spectral curve to peak considerably lligiler while the peak
itself does not move significantly to the left. In this instanc~
however, it has been found that the crossover oetween curves of
varying degrees of quench occurs somewhat to the right of the
peak of the curve. Consequently, balance point operation can be
achieved by adjusting the CD window so that the approximate
crossover point of the curves coincides with the ~idpoint of the
window.
(e) Quantitative Determination of
Quenching and True Sample
Activity Levels by External
Standardization Techniques
Those skilled in the art will appreciate that external
standardization techniques are well known for their ability to
provide a quantitative indication of the degree of quenching.
Such techniques are described in detail in the aforesaid Packard
Patent No. 3,18S,468 and the aforesaid Canadian Patent No. 822,215
(Cavanaugh). Consequently, it should not be necessary to describe
this system in detail. Suffice it to say that the output pulses
from the summing amplifier 84 (FIG. 4) which are passed to the
channels respectively containing the scalers 88d and 88e are
adjusted in gain and discriminated so as to produce counts in
the scalers 88d and 88e which are primarily representative of
-39-
10407S0
decay events occurring in the external standard source. For
example, as thorou~hly descrlbed in the aforesaid Bristol
application, the pulse heiyht analyzer 86d could have its base
discriminator define the lower level of a G-to-infinity window
(not shown) while the corresponding pulse height analyzer 86e
could have its base level set to define an H-to-infinity window
(not shown) where the level H is greater than the level G. In
other words, in the G-to-infinity window all pulses w~ich exceed
in amplitude the G level would be counted in the scaler 88d,
whereas only those pulses which exceed a higher amplitude level
H would be counted in the scaler 88e. For practical purposes,
it is preferable that the G-to-infinity and H-to-infinity windows
be so adjusted that approximately two times as many counts will
be recorded in the scaler 88d as in the scaler 88e when dealing
with an external standard and an unquenched sample. Again,
however, when quench occurs the spectral curve (not shown) for
the external standard will shift downwardly and to the left in
precisely the same manner as the spectral curves 92, 94 shown in
FIGS. 7a and 7b. As a consequence of quenching, therefore, there
will be fewer counts recorded in the G-to-infinity and H-to-
infinity windows, thus changing the ratio of counts in the two
windows. Since the ratio of counts received by each of the two
infinity channels will no longer be the same, it provides a
versatile procedure for quantitatively determining the amount of
quenching.
Referring next to FIG. 8, there has been illustrated a
conventional quench correlation curve for determining counting
efficiencies for a typical beta emitting isotope. In this case,
the quench correlation curVe is for tritium (3H) and the
ordinate is~ therefoxe~ scaled in units o~ percentage of counting
:
~ efficiency for tritium, while the abscissa is scaled in units of
.,,
a measurable quench correlation parameter, in this case, net
: '
-40-
~, .
~ ~ .
r
104075~0
external standard ratio (vis., the ratio of counts recorded in
the G-to-infinity and H-to-infinity windows).
In order to prepare a calibration curve such as is
depicted in FIG. 8, the technician has heretofore normally
prepared a series of samples of known activity for each different
isotope that may be of interest. In the exemplary case, where
the isotope of interest is tritium, the technician might, for
example, prepare a series of 10 samples each of which include
the same amount of tritium activity (e.g., each of the ten
samples might include 100,000 dpm of tritium activity). The
technician will next add to each of the samples precisely the
same amount of liquid scintillator medium, for example, 15.75 ml
of liquid scintillator medium per sample vial. The technician
then adds varying quantities of suitable quench material to
different ones of the ten samples. For example, the first
sample in the series of ten will usually be the unquenched
sample and, consequently, no quench material is added to that
sample. The second sample in the series will have a small amount
of quench material added -- perhaps on the order of approximately
15 microliters of quench material. The third sample will have a
greater quantity of quench material such, for example, as
approximately 30 microliters, while each succeeding sample in the
series will have successively greater quantities of quench
material inserted therein. The net result of this advance
preparation is the formation of a series of ten differently
quenched samples each of which has approximately the same volume
and each of which possesses the same activity.
Once the series of differently quenched samples has
,,
-~ been prepared in the manner described above, the technician will
then successively inse~t each sa~mple in the series into the
apparatus 20 (FIG~. 1 and 2) where the actiVity level of the
sample is measured. Referring next to FIG. 4, let it be
-41-
.. , :. - , .
.:
.
.. ~ : - , . :
: ~040750
assumed that the apparatus there depicted has been adjusted so
as to permit counting of the isotope undergoing test (here
tritium) in the channel containing scaler 88a while the channels
containing scalers 88d and 88e are preset so as to permit
counting of an external standard (i.e., emissions emanating
from the standard source material 44 shown in FIG. 2).
With the apparatus 20 and programming logic 75 (FIG.4)
adjusted in the aforegoing manner, the technician may first
insert the unquenched sample into the detection chamber of the
~; 10 apparatus and initiate a counting cycle for a predetermined time
interval -- say, one minute. During that one minute period, the
standard source material 44 is shifted into proximity with the
sample vial then disposed in the detection chamber, for example,
by means of the apparatus shown in FIGS. 2 and 3 as previously
described. During the oourse of this first one-minute counting
cycle, counts will be recorded in the scalers 88d and 88e which
can readily be converted into a count ratio for the two channels,
which ratio can be arbitrarily set at 1.0 for an unquenched
sample. Thus, since the particular first sample being counted
; 20 is the unquenched sample, the apparatus will display a ratio of
1Ø The standard source is then retracted in the manner
previously described and the unquenched sample is subjected to a
second counting cycle for a period of, say, one minute. During
the second counting cycle, a certain number of counts will be
recorded in the channel including scaler 88a which are, under
the illustrative conditions, representative of counts emanating
.:
from the tritium isotope which, in this instance, has a known
~; activity level of 100,000 dpm. Let it be assumed further that
;~ ~
~ the scaler 88a reflects a recorded count of 50,000 counts during
.:
the one=minute cQunting period. The technician then knows that
for that particular sample he has counted with an efficiency of
fifty per cent (50~). Under these conditions, the technician
1~4~750 l,
is now able to plot the $irst point requlred to form the quench
` correlation curve generally depicted at 95 in FIG. 8, such point
-. being represented at 96.
: The foregoing procedure is then repeated for the second
. sample in the series which, in this instance, contains
`. approximately 15 microliters of quench material. It will be
found that since the second sample is quenched slightly, the
counting efficiency recorded in the tritium counting channel will
be somewhat less than the counting efficiency determined for the
first unquenched sample, and in the illustrative instance, the
second sample may show a counting efficiency of on the order of
34%. Similarly, during the automatic standardization portion of
the counting period for the second sample when it is exposed to
radiations emanating from external standard source material 44,
it will be found that the ratio of counts recorded in the
automatic standardization channels will drop slightly -- for
example, to approximately .80. Thus, the efficiency of 34~ and
the ratio of .80 detexmine a second point 97 on the calibration
or correlation curve 95. The foregoing procedure is then
repeated for each of the remaining eight samples that were
prepare~ and the information recorded during the two counting
~ periods for each of the samples is then entered onto the graph
:~ shown in FIG. 8. When all ten samples have been counted, the
curve 95 may be drawn in.
While the quench correlation curve 95 shown in FIG. 8
~. has, for exemplary purposes, been shown as a smooth curve, as a
:~ practical matter the various points which define it do not fall
precisely on the curve. Quite to the contrary, it has been
found that the points which define the quench correlation curve
95 fall in an un~xedictable rando.m distribution with some points
being on the curve as indicated at 96 and 97, some points being
somewhat above the curve as indicated at 98, 99 and 100 by way of
-43-
. ' . ~ ,
" ~ . .
` ~040750
example, and other points being somewhat below the curve as
; indicated at 101! 102, 103 byi way of example. Unfortunately,
this random distribution of pulses can present serious problems
and can lead to significant statistical errors in true activity
level computations. This may be readily demonstrated by
reference to the quench correlation curve 95 shown in FIG. 8 and
the point 98 which is shown somewhat above that curve. Thus, let
it be assumed that the technician prepared the curve 95 based
upon the 10 samples which were described above and which did not
include a standard sample having a quench level that produced an
external standardization ratio of between .8 and lØ Under
these conditions, the technician would have no way of knowing
that the true efficiency for any given sample having a quench
level between the points represented by the ratios .8 and 1.0
was not accurately represented by the smooth curve 95. Thus, if
it is assumed that the technician inserted an unknown sample
containing a tritium isotope into the apparatus 20 an~ first
computed an external standardization ratio of .87 and then a
count in the tritium counting channel of 40,000 counts per
minute, it will be apparent that when he refers to the
calibration curve 95, he will b~ extrapolation and interpolation
calculate that a sample which produces an external standardiza-
tion ratio of .87 should have a counting efficiency of
.,
approximately 39% and, therefore, an activity level of 102, 564
dpm. However, as is made quite evident by the mea~ured point
98, sucn a sample will not have a counting efficiency of 39%
. . ~
but, rather, it will have a significantly higner counting
efficiency of on the order of 43% indicative of a true activity
level, corrected for quenching, of 93,023 dpm. Hence even using
the greatest o~ care with his calculations, the technician will
arrive at a resultant activity level which is inerror by
9,541 dpm or more than ten per cent (10%).
-44-
: ~041~7SO
T~UE QUENCH COM~NSA~ON BY SIMULATED
QUENCH~¢ ~CC~D~N~ T~ THE~PRESENT INVEI~T20N
Thus far, the environment of the invention has been
described in connecti~n with apparatus and procedures for
determining sample activity levels by comparison of detected
variable quench correlation parameters (for example, net external
standard ratios) with a previously prepared quench correlation
curve which, unfortunately, is simply not accurate between the
points which actually define the curve and which have previously
been determined by actual measurements of differently quenched
samples having the same known activity levels. As is well known,
quenching is a phenomenon which affects counting statistics --
that is, it either causes the photon energy to be attenuated to
the point where it cannot produce a response at the photo-
multiplier cathode, or, alternatively, the number of photons is
reduced sufficiently that fewer electrons are emitted from the
photosensltive cathode of the photomultiplier. In both cases,
counting statistics are altered. If one attempts to compensate
for this error simply by adjusting system gain, the net effect is
to either increase or decrease the amplitude of all the pulses
caused by electron emission from the photosensitive cathode
without in any way reducing the photon energy arriving at the
light transducer; without affecting the number of electrons
.... .
emitted at the photomultiplier cathode or collected at the first
dynode; and, therefore, without affecting counting statistics.
.~
; In accordance with the present invention, provision is
made for taking advantage of the fact that certain points which
define the quench correlation curve are known precisely -- vis.,
these points w~ich are based upon prior measurements of
;~ diffexently quenched standa~d samples having the same known
activity le~el -- and, conse~uently, of the fact that counting
efficiencies corresponding to anY preselected one of such known
`:
-45-
.. - .. :; . . ..
.
. .
10407S0
points can be divided into the nu~ber of counts per minute (cpm)
recorded for any particular isQtope to ascertain true activity
levels in decay events per minute (dpm). To this end, provision
is made for creating a controlled condition by which counting
statistics are affected and thus simulating a quench condition
for each unknown sample sufficient to shift the detected data
representative of each sample along the quench correlation curve
until it coincides with a particular preselected one of the
points for which counting efficiency is accurately known, which
preselected ~oint represents an effective quench level at or
somewhat below the actual true quench level for the most
quenched sample that the technician will normally encounter.
Thereafter, all test samples, irrespective of their true
internal quench level, will have superimposed on their true
quench condition a si~ulated quench condition sufficient to
lower counting statistics to a point corresponding to the
preselected effective quench level.
For this purpose, and as best illustrated in FIG. ~a,
let it be assumed that the technician has prepared a series of
eleven standard samples each containing a known amount of
,:
tritium (e.g., 100,000 dpm). Let it further be assumed that the
first of these samples is unquenched and, when subjected to two
counting cycles as described above, produces a net external
, .
standard ratio of 1.0 and a measured counting efficiency of 50%.
Under these conditions the first point Pl can be plotted on the
graph shown in FIG. 8a. Let it next be assumed that the
technician adds to the second sample of the series a sufficient
amount of quenching agent to reduce the net external standard
ratio to 0,9 during the first or automatic standardization
portion of the c~unting cycle for that sample. If during the
second portion of the counting cycle only 42,000 counts are
recorded, the technician will know that for any sample having a
-46-
.
~0407S0
net external standard ratio of .9, he will have a counting
efficiency of 42% in that ~articular counting channel -- thus,
the second point P2 can be plotted. In like manner, the
technician then proceeds to count each of the remaining nine (9)
standard samples, adding to each one progressively increasing
amounts of quenching agent so as to reduce the net external
standard ratio for each to 0.8, 0.7, 0.6 . . . 0.0 respectively,
and then determines the counting efficiency for each sample at
its particular known automatic standardization ratio. In this
manner, the remaining points P3-Pll can be graphically plotted
as shown in FIG. 8a. At the completion of this procedure, the
technician will have a quench correlation curve represented by
the actual known points Pl-Pll, which points may or may not, but
probably will not~ lie on a smooth continuous curve. Based upon
the foregoing, the technician will know that for the particular
isotope -- here tritium (3H) -- in the particular counting
channel -- for example, the AB channel including scaler 88a
(FIG. 4) -- the counting efficiency will be as set forth simply
by way of example in Table I below.
. .
E~nal _ _
Ratio 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
~; 3fficul~7 .500 .426 .338 .271 .199 .148 .102 ~ - .023 .007
:
TABLE
It will be appreciated that the efficiencies set forth
in Table I correspond to the number of counts contributed in the
AB window of FIG. 7a by a tritium isotope under progressively
increased quench conditions, such number of counts representing
only a percentage of the total decay events of the tritium
isotope.
Haying completed the foregoing, the technician will
now prepare a second correlation curve (not shown) for the
-47-
:
. ~, , ,
~0407S0
carbQn-14 isotope in precisely the same manner. However, in
this instance the technician rnust take into account the fact that
carbon-14 will not only produce counts in the CD window (i.e.,
the pulse height analyzlng channel containing scaler 88b), but
it will also produce some counts in the AB window as is made
clearly evident upon inspection of FIG. 7a. Consequently, for
each net external standard ratio from 1.0 through 0.0, the
technician will be able to record two carbon-14 counting
efficiencies--one ~f which is the carbon-14 efficiency in the CD
window and the other of which will be the carbon-14 efficiency
in the AB window. Based upon the foregoing, the technician will
know that for the particular carbon-14 isotope, the counting
efficiencies in the AB and CD windows will be as set forth below
in Table II simply by way of example.
3~ctern~ _ _ _ _
_ 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
~ficiency .156 .189 .235 .295 .363 .399 .475 .S99 .491 .470 .301
~: 14C __ _ _ _
Efficiency .799 .765 .741 .669 .586 .492 .388 .251 .169 .077 .021
20 ~ window _
TABLE II
In keeping with the present invention, provision is
made for artificially simulating a controllable quench condition
:
for each unl;nown sample, preferably by controllably modulating
the quantum of light energy that reaches the photosensitive
cathode of the photomultiplier, thereby altering the counting
statistics so as to superimpose upon the actual quench
characteristics of any given sample a simulated quench condition.
In this manner, it is possible to cause the variable quench
correlation paramçter, for example, net external standard ratio,
to shift towards a particular preselected one of the known
points Pl-Pll (FIG. 8a) while simultaneously changing counting
-48-
.: . .. . . . .
~0407SO
statistics in a ~anner indistinguislable fro,m that resulting
from true quench, within a s~m,ple. To this end, and in
accordance with one exem,plary form of the invention best
illustrated in FIG. 11, there is provided a detection apparatus
120 generally similar in its arrangement of components to the
elevator and detection mechanism 21 previously described in
connection with FIG. 1. Thus, the detection apparatus 120
includes a base assembly 25t which houses a pair of photo-
multipliers PM,T#l, PMT#2 disposed on opposite sides of an
elevator shaft (not shown here for purposes of clarity). As in
the FIG. 1 construction, the elevator and detection mechanism
21 ! includes a yertically reciprocable elevator 28 driven by an
elevator motor Ml and having a platform 29 at its upper end for
supporting a sample vial 24 intermediate the photosensitive
cathodes of the photo~ultipliers PMT#l, PMT#2.
,~ However, in keeping with one aspect of the present
, invention, the photomultipliers are not fixedly mounted within
the base assembly 25' but, rather, they are mounted therein so
as to permit controlled simultaneous movement of the two
photomultipliers towards or away from the vertical center line
passing through the detection chamber. In this way, it is
possible to controllably vary the radial distance r from the
center line to the photosensitive cathodes of the photomultipliers
simultaneously and by the same controlled incremental change,
' ' thereby creating simulated quenching by reduction of the light
input in terms of the number of photons which reach the cathodes
from scintillations occurring within the sample vial 24 in
; accordance with the following equation:
N ~ l(c)
2 [I]
r
~ where N is thç number of photons and "c" is a small number.
-49-
1040~S0
To ~ccomplish thi$! the photomultipliers PMT#l, PMT#2
are respectively mounted on vertically depending brackets 121,
122, the latter respectively having coaxially alined, non-
rotatable, internal oppositely threaded bearing sl2eves 124, 125
rigidly mounted therein. A horizontally disposed actuating
shaft 126 passes through the alined bearing sleeves 121, 122
and is supported at its left hand end (as viewed in FIG. 11) in
a bearing assembly 128 mounted on the base assembly 25', and
near its right hand end in a similar bearing assembly 129 also
10 rigid with the base assembly 25~. TAe left end of the shaft 126
is provided with a left hand threaded portion 130 threadably
engaged with the bearing asse~bly 124 and brac];et 121 supporting
photomultiplier pr~T#l. The right hand portion of the shaft 126
; is provided with a right hand threaded portion 131 which is
threadably engayed with the bearing sleeve 125 and bracket 122
supporting the photomultiplier PI~T#2. The two brackets 121, 122
and hence the photomultiplier tubes, are maintained free for
:i
controlled movement towards and away from one another along a
common axis while being prevented from rotating about the axis of
the actuating shaft 126 by proyision of a longitudinally extend-
ing track 132 fixedly mounted on the base assembly 25' and
slidably engaged in complementally shaped grooves 134 formed in
the brackets 121, 122.
For the purpose of rotating the actuating shaft 126 in
controlled incremerts, ana hence moving the photomultiplier tubes
in controlled linear increments, the right hand end of the
actuating sha~t projects slightly beyond its bearing support 129
; and is drivingly coupled to a gear train 135, 136 (here shown
only in diagrammatic form, it being understood that the particular
means employed for driving the shaft 12G could take any of well
known conventional forms and could, ~f desired, be mounted within
the base assembly 25'), As here shown, the gear 136 is coupled
-50-
104~7S0
to the drive silaft 138 of a conventional reversible servo motor
139 whicn receives its input from a ~ervo motor driver 140, the
latter being provided with a control signal derived from
terminal 240 (FIGS. 11 and 15). In keeping with the present
invention and ln a manner to be subsequently described, the
control signal presented at terminal 240 is adjusted so as to
permit the photomultipliers PMT#l, P.~T#2 to move away from one
another sufflciently far that the light energy reaching their
cathodes is reduced by a factor required to simulate quenching
to the preselected effective quench level. Tlle apparatus shown
in FIG. 11 might, if desired, be provided with a hand crank or
the like, illustrated in phantom at 141, for purposes of manual
operation and/or calibration, thereby enabling manual rotation
of the actuating shaft 126 to vary the radial dimension r.
In carrying out the present invention, provision is
made for subjecting each of n successive samples to repetitive
countin~ cycles during the first several of which the sample is
counted twice for a short time interval -- say, for example, once
for ten seconds with the external standard 44 adjacent the vial
24 and once for ten seconds with the external standard 44 remote
from the vial. During each of these counting cycles the data
measured is transmitted to the computational portion of the
system (see FIG. 13) which automatically determines the net
external standard ratio upon the completion of the exemplary 20-
second counting cycle. That computed counting ratio is then
compared with the particular preselected one of the eleven
predetermined ratios for which counting efficiencies are accurately
known (see FIG. 8a) and provision is then made for increasing the
potential level at terminal 240 to simulate quenching so as to
cause tile computed net external standardization ratio to converge
upon the lower preselected point~ ~hus, let it be assumed,
merely by way of exa~ple, that the tecinician knows that for a
~ . ~ . ..
.
10407S0
particular group of samples the most quenched sample will produce
a net external standard r~tio (unaffected by si~ulated quenchiny)
of at least 0.450. Under these asswmed condi,ions he might elect
~o establish a preselected fixed external standardization ratio
of 0.400 as.the ~articular point to which he wishes to converge.
In other words, all samples in th~t ~roup, irrespective of their
actual quench levels, will be subjected to simulated quenching
in accordance with the invention so as to create an effective
quench level for each sample of substantially 0.400. Assuming
further that for any given sam~le 24 in that group the first
computed ratio happens to be 0.850, then provision would be made
for increasing the potential level at terminal 240 (FIGS. 11 and
. . . .
13) so as to simulate quencning and cause the ratio to converge
upon the preselectea fixed ratio of 0.400, at which fixed ratio
counting efficiency is accurately know~l (see, e.g., FIG. 8a)
Once the potential level has been increased, the foregoing
procedure is repeated and a new ra~io is computed which, in this
instance, may be 0.459. As a result of the second computed ratio,
a small additional voltage increment is provided at the terminal
240, the foregoing steps ~re again repeated,. and a new or third
net-external standard computed ratio is determined which may,
for example, by 0.409. As the ensuin~ description proceeds, it
will become apparent to those skilled in the art that more or
fewer than the exemplary tnree counting cycles discussed above
could be utilized to cause the simulated quench characteristics
. . - .
to converge upon the selectea fixed ratio of 0.400. However, with
the exemplary forrl of the invention it is believed that three
: such computations are generally sufficient to achieve close
; enough convergence that.any error introduced into subsequent
calculations is negligible. Thus, naving converged to the proper
fixed ratio, the apparatus is now conditioned for a data counting.
cycle during which the true activity level of the isotope or
-52-
1040750
iSotoPes within the test sam~le may be accurately co~puted in
units of decay events per/mlnute (dpm).
~ eferring now to FI~. 13, there has been illustrated in
diagrammatic and block form a typical computational system which
may ~e utilized in carrying out the present invention. As there
shown, the computational system includes Multiplex Gating
Circuits and a Computer Program Control, designated generally at
150, into which are fed the signals from the scaler outputs Sl-
S5 (from FIG. 4) numerically representing the count held in each
respective scaler, the signal Tl numerically representative of
the time period (f~om FIG, 4), the signals Pl, P2 and P3 (from
FIG. 3) indicative of the particular program or routine to be
followed in ensuing computations, and the signals BGl-BG3
numerically representing the background count for the data
counting channels dialed in on switches BGl-BG3 (FIG. 5). The
output from the ~lultiplex Gating Circuits and Computer Program
Control 150 is fed directly to a time-shared digital computer 151
together with suitable command instructions for effecting
addition, subtraction, multiplication or division operations.
~he computer output is in turn fed directly to an Answer Register
152 which provides an answer signal AN, the latter being fed
back into the Multiplex Gating Circuits and Computer Program
Control 150 as well as to the input of four normally closed gates
154, 155, 156 and 157. As here shown, the output from gate 154
is transmitted directly to a storage register 15~ which provides,
upon demand, an input signal SRA for the Computer Program
Control 150. Similarly, the gates 155 and 156 respectively
provide input signals for storage registers 159 and 160 which,
upon demand, respectively provide control input signals SRB and
SRC for the Com~uter Pxogram Control 150. The gate 157 provides
an input si-~nal to a conventional printing device 161 which, upon
completion of a printing cycle, provides an "end of print" signal
-53-
10407S0
EP which is c~nveyed to the terminal 61 shown in FIG. 3. Finally,
the computational system illustrated diagFammatically in FIG. 13
is provided with a clock s~urce 162 of any conventional type
which provides clock pulses to a timing signal generator 164,
the latter being capable of initiating time signals tl through
tn for initiating operation of the various programmed cycles and
opening the various gates 154-157 at prescribed time intervals.
-~ In order to facilitate an understanding of the present
invention, there will hereinbelow be described an exemplary
counting cycle for a typical unknown sample which is known to
contain two different isotopes -- say, for example, tritium
(3H) and carbon-14 (14C). Thus, the particular sample 24 will be
placed upon the elevator platform 29 by the technician, either
manually or automatically, and the elevator 28 will start its
-
downward movement,all as previously described in detail. Wllen
the elevator 28 reaches its lowermos1- position, closure of the
"STOP" contacts of the lower limit switch LSl will serve to
impress a control signal upon the "Elevator Down" terminal 43
(FIGS. 2 and 3~ which signal is then transmitted through the "ON~
` 20 contacts of the automatic standardization control mode selector
switch 56 and through the differentiating device 62 (FIG. 3) to
the "Insert Source" terminal 64, thereby causing the compound
source material 44 to shift from its shielded housing 49 into a
position adjacent the sample vial. As previously stated, after a
sufficient time delay determined by the device 68, a "Source In"
signal is impressed upon terminal 66 which then causes a signal
to be presented upon the "Start Count" terminal 58. At this time
there is presented on the "Select Time" terminal 80 of the Mode
Program Control 40 suitable signals which serVe to reset the
timer 78 to its zero time cQndition and Which indicate to the
timer the preselected length of time of the next counting period--
here, for example, ten seconds--after which the timer is to
-54-
1040750
transmit a "~to~" signal. The ~articular circuitxy for
establishing such signals at the "Select Time" ter~nal 80 are
well known and have not been described herein. Suffice it to
say that such circuitry would normally be internal of the Mode
Program Control 40.
Under the foregoing conditions and when control signals
are simultaneously presented on the terminals 58 and 80, the
timer 78 will start to time a ten-second counting interval while
the start-stop control 79 will provide control input signals for
the gates 81a-81e~ thereby enabling the latter to be opened upon
the presence of coincident input signals at the photomultipliers
PMT#l, P~T#2. During the next ten seconds, the output signals
from the summing amplifier 84 are simultaneously presented to the
five pulse height anal~zing channels in each of which the signal
gain is controlled and the pulses are discriminated. When
coincident signals are detected, the network 82 provides the
second input signal for the gates 81a-81e which serves to open
the latter, and those pulses passing through the windows defined
by the pulse height analyzers 86a-86e are passed directly to the
respective scalers 88a-88e.
Upon completion of the ten-second counting period, the
timer 78 tF~G. 4) provides a "Stop" signal for the start-stop
;~ control 79 which, in turn, serves to close the gates 81a-31e, thus
preventing any further accumulation of counts in the scalers 88a-
88e. At this time, the timer 78 provides a signal at the "Count
Complete" terminal 59 which is routed through the No~ contacts
of both the Automatic Standardization Control Mode Selector
Switch 56 and the "ON" contacts of an Automatic DP~ Control ~lode
Selector Switch 110 (FIG. 3), t~e latter having been turned to
the "O~" state by the technician prior to initiation of the DPM
countin~ cycle. The ~'Count Complete~' signal is thus conveyed
directl~ to the input of a conventional seven-stage ring counter
-55-
104~7S0
165, thus dxiving the first stage thereof from its "1" state to
"0" and its second st~ge $rom ~o!~ to the "1" state, all other
stages thereof being in the !~0!~ state. It will be observed that
when the second stage of the counter 165 shifts to its "1!' state
a signal is passed through the differentiating device 69 to both
~ the "Retract Source" terminal 70 and a terminal Pl (FIGS. 3 and
-- 13), the latter of which signifies ~or the Computer Program
Control 150 that the first count is complete and the
com~utational portion of the system should follow Program No. 1.
~ Assu~ing for the sake of discussion that the clock
;; source 162 produces recurring ~ulses at a frequency of 500 kHz,
and that the timing signal generator 164 operates to produce
sequenced output signals tl, t2 . . . t500 in response to five
hundred successive inPut pulses, the successive full cycles of
the generator will each transpire in successive .001 second
intervals. During each such cycle, the ti~ing signals tl, t2,
t3 . . . will occur during successive 2~ sec. intervals, with
each signal existing for 1 ~ sec. and being spaced in time from
the next signal for an interval of 1 ~ sec. These timing rates
and intervals are more than sufficient for the Computer Program
Control 150 to cause the computer 151 to execute the necessary
computations with the numbers represented by input signals on the
various sets of input terminals shown at the left in FIG. 13,
the result of each computation immediately appearing in the
Answer Register and being represented on the trunk conductors AN.
Program No. 1
When an initiating "1" level signal on terminal Pl
(FIG. 3) is sent to the same terminal in FIG. 13, the Computer
Program Control 150 is conditioned to execute a predetermined
sequence ~f operati~ns, here termed Program No. 1. First, the
number represented by signals at S4 (and corresponding to the
counts stored in the scaler 88d) is applied to computer input
-56-
.
~04~)7S0
terminal Il, signals representing zero are applied to input
terminal I2, and an enabling signal is applied to the "add" or "+"
control terminal. This may occur, for example, in response to
the timing signal tl which next appears after the enabling signal
appears at terminal Pl. The computer adds the number from
scaler 8~d to zero, and stores the answer (in BCD notation) in
the Answer Register 152. After this, and say in response to
timing signal t2, the gate 154 is opened to transfer the
contents of the Answer Register into storage register 15~, so
that the nun~er of gross counts held in scaler 88d now are
numerically signalled at SRA.
Program No. 1 continues, in response to timing signals
t3 and t4, by a similar se~uence of steps to transfer t}le
signals from scaler 88e on lines S5 through the computer 151
to the Answer Register 152 and thence into storage register 159,
so that tile gross counts held in scaler 88e are numerically
signalled at SRB. At this point, the gross counts recorded
in scalers 88d and 88e (which are primarily produced by the
external standard source material 44 during the first ten-
second counting period) are respectively stored in the storageregisters 158, 159 in binary coded decimal notation, and
Program ~o. 1 is complete.
The apparatus is now ready to commence its second
ten-second counting period, this time with the external standard
source material 44 retracted. To this end, it will be recalled
; from the above discussion that upon completion of the first
ten-second counting period the ring counter 165 was stepped so
as to drive its second stage to the 'Il'' state, thus imposing a
control signal at the "Retract Source" terminal 70. That signal
is thereafter effective to cause energization of the relay Rl,
thereby causing the compound external standard source 44 to be
retracted in the manner previously described. During such
-57-
. ~ .
~0~7so
retraction, Program No~ 1 as described above is being carried
out by the computational system shown in FIG. 13. At the same
time, the signal pulse presented at terminal 70 is passed
through the time deIay device 72 to the "Source Retracted"
terminal 71 and thence to the "Start Count" terminal 58, thus
signifying to the timer 78 that a second counting cycle should
be initiated. It should be noted at this time that the time
delay devices 68, 72 which have previously been described
are selected such that they not only provide a sufficient delay
to insure proper positioning of the compound external standard
source 44 before a new count is initiated but, also, the time
delay devices provlde a sufficient delay to insure that the
computational system shown generally in FIG. 13 has performed
the various routines demanded of it subsequent to the previous
count cycle and which are herein designated at Program Nos. 1,
2 and 3.
At this point, the apparatus 75 depicted in FIG. 4
is ready to repeat a second ten-second counting cycle, the ten-
second time interval again having been selected by the Mode
Program Control 40 and passed to the timer 78 through the
"Select Time" terminal 80. Since simultaneous signals are now
present on both terminals 58 and 80, the apparatus now starts
through a second ten-second counting period during which the
external standard source material 44 is retracted and again
disposed within the shielded housing 49 (FIG. 3). Moreover,
it should be noted that in response to the preceding "stop"
signal transmitted to the start-stop control 79 from the timer
78, the start-stop control, after a suitable time delay, pro-
vides a reset signal over line 91 which is effective to restore
all of the scalers 88a-88e to their "zero" count state. The
second ten-second counting cycle now proceeds and counts are
`'
-58-
:,
:
~750
again accumulated in thè scalers-88a-88e, although in this in-
; stance the compound source material 44 does not contribute sig-
nificantly to the accumulated counts.
:
Program No~ 2
Upon completion of the ten-second counting interval,
the timer 78 again times out and the gates 81a-81e are again
closed in precisely the same manner as described above. The
accumulated counts in scalers 88d and 88e are again respectively
presented as binary coded decimal inputs S4 and S5 respectively
to the input of the ~lultiplex Gating Circuits and Computer
Program Control 150. At the same time, a signal is passed from
the timer 78 to the "Count Complete" terminal 58, thereby provi-
ding an input signal for the ring counter 165 (FIG.3) which
shifts the latter so that its second stage is restored to the
"zero" state and its third stage is driven to the "1" state.
When this occurs, a control input signal is provided at one
terminal of a conventional "AND" gate 219 (FIG. 3) for a reason
which will become readily apparent. At the same time, when the
third stage of the counter 165 is driven to the "1" state it
provides a signal which is differentiated by a device 218 and
then transmitted to a terminal P2 (FIGS. 3 and 13), thus serving
to command the computational system of FIG. 13 to thereafter
follow Program No. 2. The computational portion of the system
is now prepared to compute its first net external standard ratio
indicative of the quench level of the unknown test sample.
In order to compute the net external standard ratio,
the computational portion of the system now steps through its
second routine--viz., Program No~ 2. To this end, and in res-
;~ ponse to the timing signal tl which next appears after the en-
abLing signal appears at terminal P2, the binary coded decimal
data stored in storage register 158 numerically representative
:
_ _
~ -59
.
,
1~40750
of gross external standard counts during the'first counting
period, and the hinary coded decimal data numerically represen-
ted by S4 during the second counting period are respectively
transferred to the input terminals 11 and 12 for the time-
shared digital computer 151, together with a control signal
indicating that the input 12 should be'subtracted from 11.
Those skilled in the art will appreciate that the number of
time increments required to perform this subtraction operation
will vary dependent upon the particular type of computer em-
ployed. Assuming, however, that four time increments are
required to perform this subtraction step, then at time interval
t5 the answer will appear in the Answer Register 152 and be
presented at the gates 154-157. At time instant t6, the gate
156 is clocked open and the binary coded decimal information
in the Answer Register 152 is transferred into the storage
register 160 for storage therein. This quantity now represents
the net external standard counts accumulated in the scaler 88d.
.,,
'~ At time instant t7, the binary coded decimal data
stored in storage register 159 and applied to the input of the
Computer Control 150 as quantity SRB, and the binary coded
decimal representation S5 of counts accumulated in scaler 88e
during the second ten-second counting period, are respectively
transferred to the terminals 11 and 12 Of the time-shared
digital computer 151, together with another command signal in-
dicating that the computer 151 should perform a subtraction
operation in which the counts accumulated during the second
counting period are subtracted from those accumulated during
the first counting period to arrive at net external standard-
ization counts accumulated in scaler 88e during the preceding
counting cycle. Assuming that this subtraction operation again
..:.
requires four timing increments, the answer will appear in the
Answer Register at time'interval tll.
-60-
~ . .
~.o4075
The answer now in the Answer Register 152 represen-
tative of net counts in s:caler'88e'is now, transmitted in binary
coded decimal form to the'"~W" input of the Computer Program
Control 150 and, at time interval tl:2 it is transmitted to the
input terminal 11 of the computer 151. At that same time in- !
terval tl2, the data stored in storage register 160 represen-
! tative of net external standard counts in scaler 88e is trans-
mitted from the storage register to the computer input SRC and
rom thence to the terminal 12 of the time-shared digital com-
I lO puter together with'a command signal indicating that the quan-
:l, tity on terminal 12 should be divided into the quantity on
¦ terminal 11. Assuming that thi's divisional operation requires
,'', eight time increments, the resulting quotient which is repre-
~:1 sentative of the net external standardization ratio R will
appear in the Answer Register 152 at time interval t22, where it
will again be fed to the ''AN'I input terminal of the Computer
~:¦ Program Control 150. At time interval t23 the Computer Program
I Control 150 provides a ~Iratio ready" signal RR at terminal 166.
I Consistent with the assumptions previously made, let
i 20 it again be assumed that the computed ratio calculated during
the preceeding counting cycle is 0.850, which quantity appears
in the Answer Register 152 in binary coded decimal form. The
actual computed value of the net external standard ratio is
now used in accordance with the present invention for causing
: a controlled voltage level or other suitable signal at terminal
.. ~ 240 (FI~S. 11 and 15) so as to simulate a quench condition for
the sample 24.
,' In order that the computed net external standard
ratio can serve to control the value'of the modulating signal
presented at terminal 240 for energizing the servo motor 139,
^., and thus controllably vary the radial distance r between the
,. pho'tosensitive cathodes of the'photomultipliers PMT#l, PMT#2
''
,. -61-
: . .- . . . . . .
. ,. . ~ . . . .
:: .
:: 11)40750
and th.e.verti.cal centerline through. the detection changer, the
three least significant digits. are utilized to seIect and ad-
just the voltage level. As best illustrated by reference to
FIG~ 14, it will be observed that the four digits appearing in
the Answer Register have been arbitrarily designated as digits
"a" through "d" from the most significant to the least signi-
ficant number. Thus, in the exemplary instance the "a" digit
. is zero while the "b", "c" and "d" digits are respectively "8",
: "S" and "0". Under these conditions, at time instant t23 when
the "ratio ready" signal appears at terminal 166 (FIGS. 13, 14),
such signal is transmitted directly to the set section S of a
bistable flip-flop 168 (FIG. 14), thus switching the S section
of the flip-flop to the 'Il'' state and providing a control sig-
nal which is effective to open the gate 169. When the gate 169
::~. is opened, pulses from a conventional pulse source or clock 170
are transmitted through the gate and appear on the output line
171 thereof. These pulses are transmitted directly to a decade
counter 172 which, upon receipt of ten input pulses, provides
an output pulse which is conveyed over line 174 to the reset
section R of the flip-flop 168 so as to switch the latter to its
"reset'l state wherein the S section of the flip-flop is again
returned to its "0" state and the gate 169 is, therefore, closed.
$hus, the decade counter 172 serves to insure that for each
"ratio ready" signal presented at the terminal 166, only ten
~: pulses from the pulse source 170 pass through the gate 169.
Each of the ten pulses passing through the gate 169
are simultaneously presented to three normally open gates which
have here been designated as the "b", "c", and "d" gates. Con-
sidering first the "b" gate, it will be observed that the ten
. pulses are passed to the input of this normally open gate with
the output from the "b" gate being transmitted in two directions
over lines 178 and 179. Those puIses transmitted over the line
-62-
, .: ' : ~
1040750
178 are conveyed to the input of a BCD decade counter 180
which provides input signals to a comparison network 181, the
latter serving to compare the state of the counter 180 with
the binary coded decimal notation stored in the "b" digit of
the Answer Register. Thus, in this case when the eighth input
pulse passes through the "b" gate and into the counter 180, the
comparison network 181 will detect equality between its two
sets of inputs and it will, therefore, provide an output signal
effective to close the "b" gate and preclude the passage of any
more of the ten pulses therethrough. Consequently, only eight
of the ten pulses are allowed to pass over the line 179. These
eight pulses are transmitted to an "OR" gate 182 and, from there,
over line 173 through a normally open gate 175 to the input of
a binary coded decimal decade counter 184. Since only eight
pulses are passed to the counter 184, the output of the counter
in binary coded decimal form is representative of the numeral
"8". Here such output is transmitted to a conventional BCD-
to-decimal decoder 185, thus causing the output terminal b.8
thereof to be raised in potential.
The operation of the "c" and "d" gates is substantially
identical to that described above for the "b" gate. Thus,
pulses are passed through the "c" and "d" gates to respective
different decade counters 186, 188 for comparison with the bin-
.,
ary coded decimal data in the "c" and "d" digits of the Answer
Register 152 by means of comparison networks 189, 187, respec-
tively. When either of these comparison networks detects con-
; ditions of equality between their respective sets of inputs,
,.~,
control signals are provided for the "c" and "d" gates to close
the latter. In this case, since the "c" digit in the Answer
Register is "5", equality is detected by the comparison net-
work 189 when the fifth pulse arrives at counter 186. The
comparator 189 then transmits a control signal to close the
-63-
.
1040750
"c" gate, thus preventing any additional pulses from passing
over line 190 to the counter 186 or line 191 to: an "OR" gate
192. Since the "c" digit is "5", five pulses pass through
the "c" gate and the "OR" gate 192, and thence to the input of
a BCD decade counter 194. When the counter 194 reaches the
"5" state, it transmits a signal to a decoder 195, raising the
c.5 terminal of the latter in potential.
Similarly, since the binary coded decimal notations
: stored in the "d" digit of the Answer Register is representa-
tive of "0", equality is immediately detected by the comparison
network 187 and, therefore, the "d" gate remains closed, thus
preventing any pulses from passing to either the BCD counter
188 or the BCD counter 196. Consequently, the counter 196 re-
mains at zero and the d.0 terminal of its decoder 198 is raised
in potential.
It will thus be observed that the net external
standard ratio (here 0.850) computed at time instant t22 and
presented in the Answer Register 152 in binary coded decimal
notation has, by the circuitry depicted in FIG. 14 and des-
cribed above, been converted into digital notation and storedin the respective decoders 185, 195, 198.
Turning next to FIG. 15, there has been illustrated
~.,
`.l an exemplary arbitrary function digital-to-analog converter
199 which is suitable for converting the digital information
in the decoders 185, 195 and 198 of FI~. 14 into increments of
voltage for presentation at an output terminal 240, the par-
ticular potential level being determined by the value of the
, ....... . .
; computed net external standard ratio, here 0.850. As here
shown, the exemplary converter 199 includes three voltage
programmers generally indicated at 204, 205, and 206, such
~ voltage programmers being connected in parallel and being
: respectively coupled to the outputs from the decoders 185,
-64-
.
195 and 198 associated with the "bl', "c" and "d" digits. As
here shown, the voltage programmer 204 includes transistors
Ql-Q9; the programmer 205 includes transistors Q10-Q18; and the
programmer 206 includes transistors Ql9-Q27.
Referring momentarily to FIG. 15a, there has been
schematically illustrated a typical wiring circuit for any one
of the transistors Ql through Q27 shown in FIG. 15. Thus, it
will be observed that the transistors are each conventional PNP
type junctions having a collector connected to a regulated
voltage source E and an emitter connected to ground. The base
B is normally maintained at ground potential and, hence, the
transistors are normally "OFF"~ However, when the potential
level of the base B is raised, this serves to create a forward
positive bias which turns the transistors "ON" and establishes
a main current path through the collector-emitter circuit.
Keeping the foregoing characteristics of a typical
transistor circuit in mind, it will be appreciated from the
foregoing that since the b.8 terminal of the decoder 185 (FIG.
14) has been raised in potential, and since such terminal is
connected to the base of transistor Q2 (FIG.15), the latter
transistor will be turned ON, thus completing a current path
from the regulated voltage source E through the resistor Rb 8'
transistor Q2 and resistor RL to ground, thus creating a voltage
~`l drop across the resistor RL and establishing a potential level
at point x which is a function of the value of the resistances
:!
R and R
Similarly, the particular states of the decoders 195
` and 198 associated with the "cl' and "d" digits also serve to
select a particular resistance value in the voltage programmers
- 205, 206 which values together with the value established by
- programmer 204 (all three programmers being in parallel) de-
termine the potential level at terminal 240. Thus, since the
,.~
;~ -65-
.
.
. ' ' ~ '. ~ ' ' ' ' .
~040750
c..5 terminal of decoder 195 has been raised in potential, tran-
sistor Q14 is turned ON, th.e.re~y completing a current path from
the voltage source E, through the resistor Rc 5, the ON tran-
sistor Q14, and resistor RL to ground. The voltage drop across
resistor ~ produced by this completed circuit adds to the
potential level at point x. In the case of voltage programmer
206, since the terminal d.0 associated with decoder 198 has been
raised in potential, the current path includes an infinite re-
.
sistance, thereby effectively precluding the programmer 206 from
contributing to the potential lev*l at terminal 240. The totalvoltage drop across th.e load resistor RL created by the various
current paths described above creates a potential level at
point x which is then passed to an amplifier here illustrated
at 210, the output of which is presented at terminal 240 as the
modulating signal for establishing simulated quench through
controlled movement of the photomultipliers PMT#l, PMT#2 (FIG.
11) .
The value of the potential level at terminal 240
.~ should be sufficient to simulate quenching so as to cause the
previously calculated net external standard ratio of 0.850 to
~ converge towards 0.400. It should be noted here that those
: skilled in the art will appreciate that the particular incre-
........... mental potential change established by the foregoing procedure
is a function of the resistance values inserted into the circuit
by selectively turning either one transistor in each programmer,
or no transistor in one or more of such programmers, ON. More-
.,;
. over, the most significant contribution is that provided by
programmer 204 (the programmer associated with the most signi-
-;
~. ficant of the "b", "c" and "d" digits. Therefore, in keeping
,
~: with the invention, it is contemplated that the technician will
,:
.~ alter the resistance values, at least those in the programmer
204, in accordance with the particular fixed ratio to which he
-66-
10407S0
eIects to converge. This might be~done, merely by way of
example, by estabLishing pres'et resiætor boards, printed cir-
cuits, or *he'like, for each different possible fixed external
standard ratio, ~hus enabling the'technician to select and in-
stall the proper board or boards at the time he selects the
ratio to which he will converge.
The overall apparatus is now ready for a second cycle
of operation during which a new net external standard ratio
will be computed and again compared with the desired fixed
external standard ratio to which the apparatus is now condition- '
ed to converge -- here, 0.400. In order to permit such continu-
ed operation, when the decade counter 172 (FIG. 14) receives
its tenth input pulse from the course 170 and passes a control
signal over line 174 to the reset section R of the flip-flop
168, it also passes a control signal over lines 215, 216. The
signal transmitted via line 215 is impressed upon the reset
terminals for the BCD decade counters 180, 186, 188 associated
with'the "b", "c" and "d" digits, thus serving to reset each of
these counters to zero. Of course, prior to the time that these
~'~ 20 counters are reset to zero, the decoders 185, 195, and 198 will
.: .,i
have already been conditioned so as to raise selective ones of
~' their output terminals in potential and thus establish the
~.;
~- potential level transmitted through the terminal 240 to the
:.
servo motor drive 140. However, it should also be noted that
~' the ~CD decade counters 184, 194 and 196 are not reset at this
. . ,
' time. The signal transmitted from the decade counter 172 over
'' line 216 is also impressed upon a "cycle complete" terminal
CY--C.
... .
In order that the second complete counting cycle may
be commenced, and as best illustrated by reference to FIGS. 3
and 4 conjointly, it will be recalled that at the time the timer
78 timed out after the second ten-second count period with the
-67-
' ' : '' . ',,
~040750
external standard source in the retracted position, there was
provided a "count complete" signal which was again impressed
upon the "count complete" terminal 59 for the Mode Program
Control. It will also be recalled that such signal was trans-
mitted through the "ON" contacts for each of the Automatic
Standardization Control Mode Selector Switch 56 and the Auto-
matic DPM Control Mode Selector Switch 110 so as to return the
second stage of the counter 165 (FIG. 3) to the "0" state and
raise the third stage thereof to the "1" state. Finally, it
will be recalled that the change in state of the third stage
of the ring counter 165 to the "1" condition also created one
of two required input signals for an "AND" gate 219. It will
now be observed that the second required input signal for the
"AND" gate 219 is derived from the "cycle complete" terminal
CY-C (FIGS. 3 and 16) upon completion of the first counting
cycle during which a controlled voltage level was established
at terminal 240. Thus, when the first counting cycle has
been completed, an output signal will be derived from the "AND"
gate 219 which is conveyed through differentiating device 62
and impressed upon the "Insert Source" terminal 64. The com-
pound external standard source 44 will again be inserted and
., .
a third ten-second counting program initiated. Upon comple-
tion of the third counting oPeration, a "count complete"
signal will be again generated by the timer 78 and applied on
the terminal 59, which signal will be ~irected to the input of
the ring counter 165 (FIG. 3) so as to return the third stage
thereof to the "0" state and to set the fourth stage thereof
in the "1" state. This will again create a control signal at
terminal Pl (FIGS. 3 and 15) by which the Computer Program
Control 150 will be instructed to again follow Program No. 1
during which the gross external standard counts accumulated
in the scalers 88d and 88e will be respectively stored in
-68-
10407S0
storage registers 158 and 159. It should be kept.in mind that
storage registers 158 and 159 will, at thi~ time, still CQntain
the information stored the.rein during the first complete operating
cycle. However, these storage registers, together with storage
register 160, are of the conventional type which do not require
that they be reset but, to the contrary, wherein any input thereto
will cause material already stored therein to be erased.
When the fourth stage of the ring counter 165 is driven
to the "1" state, the external standard source 44 will again be
retracted by. virtue of the fact that the signal passed through
differentiating device 69 is also presented on the "Retract Source"
terminal 70. This will initi.ate a fourth ten-second counting cycle
precisely the same as the second ten-second counting cycle during
which counts will again be accumulated in the various scalers
with the external standard source in its retracted position within
: its shielded housing 49. Upon completion of this count, another
~ input signal will be provided to the ring counter 165 from the
timer 78 through the IlCount Complete" terminal 59, thus restoring
the fourth stage of the counter to its "0" state and driving the
:,
` 20 fifth stage of the counter to the "1" state. ~7hen this occurs, a
signal is passed through differentiating device 218 to the terminal
........ P2 to signify to the computer program control that Program No. 2
.. should again be followed. As was the case upon the completion of
the second ten-second counting cycle, Program No. 2 will cause the
computational portions of the system to again compute a net
external standard ratio in precisely the same manner as discussed
above. However, in this instance the net ratio will be considerably
: lower in view of the simulated quenching produced by movement of
the photomultipliers PMT#l, PMT#2 away from the centerline of the
detection chamber.
-69-
:
,
1040750
Again, to be consistent with the assumptions made
previously, let it be assumed that the second computed ratio is
0.459. Thus, the "a", "b", "c" and "d" digits in the Answer
Register 152 (FIG. 14) will be "0", "4", "5" and "9" respectively.
When Program No. 2 has been completed and the second net ratio
computed, the "ratio ready" signal impressed upon terminal 166
(FIG. 14) will again drive the flip-flop 168 to its set condition,
thus opening the gate 169 and permitting ten more pulses to pass
therethrough in precisely the same manner as described before.
In this instance, however, the "b" digit is no longer at an "8"
level, but, rather, it has converged to the "4" level which here
represents the preselected value for the tenths digit ("b") of the
computed ratio. In other words, during the first convergence
cycle the contribution of the voltage programmer 204 has been
sufficient to bring the tenths digit ("b"~ to the selected value
and hence, there is no need to change the now existing state of
that particular programmer. Consequently~ there is provided a
' second comparison network 176 which receives its input signals from
the "b" digit in the Answer Register 152 and from a binary coded
i 20 decimal switch 177 (FIG. 14) similar to that shown in FIG. 6a.
The arrangement is such that when the technician makes his initial
decision to converge to a fixed ratio of 0.400, he simply sets
the switch 177 to ".4", thus providing one input to the comparator
176. When the computed ratio reaches 0.4 nn, whether on the first
convergence as here, or on a later convergence, the comparator 176
will detect equality between its two sets of input signals, thus
generating a control signal wl~ich is effective to close the
normally open gate 175 in line 173. Under these conditions, no
pulses are transmitted to the BC~ counter 184 from either the "b"
gate or the counter 194. Conse~uently, the decoder 185 remains in
a state wherein its output terminal b.8 is raised in potential. It
-70-
~0407SO
should be noted that this occurs despite the fact that the "b" gate
will not be closed by comparator 181 until after it passes four
pulses to the OR gate 182.
Considering the "d" digit next, and skipping for the
moment the "c" digit, it will be noted that in this instance, the
value of the digit is "9". Consequently, nine pulses will be
permitted to pass through the "d" gate and into both the decade
counter 188 and the decade counter 196. When the decade counter
1~ provides a binary coded decimal output corresponding to the
numeral "9~, the comparator 187 will again detect equality between
its two sets of input signals, thus closing the "d" gate. It
will also be recalled that during the first cycle of operation
the BCD decade counter 196 associated with the "e" digit did not
receive any input pulses and, therefore, remained in the zero
state. Consequently, the nine pulses which are now delivered to
the c~unter 196 will drive the latter to a condition representative
of the numeral "9" in binary coded decimal form, thus raising the
terminal d.9 of the decoder 198 to a positive level.
.,:
Considering now the ~'c" digit, it will first be
app-eciated that since the "c" digit in the Answer Register 152
is equal to the numeral "5" in binary coded decimal notation, five
pulses will be transmitted through the "c" gate before the
comparison network 189 closes such gate. These pulses will
.
also be transmitted to the "OR" gate 192 and from thence into
the BCD decade counter 194 associated with the "c" digit.
However, since the counter 194 was not reset at the completion
of the first external standarization ratio computation cycle,
it is still in the "5" state. Therefore, the first four pulses
of the five ne~ pulses will step the counter 194 to the "9"
state. The fifth and last pulse will cause the counter 194 to
return to its "zero" state, thus raising the potential ~evel for
output terminal c.0 of decoder 195 and producing a "carry" signal
-71-
.:
10407S0
which is trans~itted to the "OR" gate 182 and, normally, ~rom
such gate over line 173 through the normally open gate 175 to
counter 184 associated with the "b" digit. In this instance
however, the gate 175 has been closed since comparator 176 has
detected that the tenths di~it (nb") is already at its desired
level. Those skilled in the art will appreciate that the "OR"
gate 182 is o a conventional type and will include suitable means
internally thereof which will insure that all pulses received
from either counter 194 or the "b" gate are counted; i.e., that they
are received without coincidence. Consequently, the BCD decade
counter 184 will normally receive pulses from either or both of
the "b" gate and the counter 194 until such time that comparator
~ 176 causes the gate 175 to cloæe.
; Referring now to FIG. 15, it will be appreciated that
with the decoders 185, 195 and 19~ respectively in the "0.8", ".0"
and ".9" states, the transistors Q2 and Q19 will be turned O~l.
This time, however, no transistor will be turned ON in the
. voltage programmer 205 since the terminal c.0 of the decoder 195
has been raised in potential, thus introducing an in4inite
resistance into that particular portion of the circuit. Under
; these conditions, a small additional increment of potential
` will appear at terminal 240, its value now being determined by
the resistancesl Rb ~, Rd g and RL. As a result of this additional
increment of potential, the photomultipliers (FIG. 11) will be
moved slightly further apart by servo motor 139, thereby creating
additional simulated quench which will tend to cause the net
external standard ratio to converge even closer to the selected
fixed ratio of 0.400. The apparatus is now ready for a third
counting cycle.
It will be recalled that upon comPletion of the ~ourth
ten-second counting cycle, a signal was presented at the "count
complete" terminal 59 (FIG. 3) which caused the ring counter 165
-72-
,
~040750
to step to a position wherein the fifth stage thereof was at the
"1" state. Since this stage is still at the "1" state, one
control input signal or an AND gate 220 is provided. Upon
completion of the second cycle when the decade counter 172 (FI5.
14) has received its tenth pulse, a second "cycle complete"
signal CY-C is generated which provides the second input signal
for AND gate 220. By the time that such signal is generated, the
second increment of potential will have been transmitted to the
servo motor driver 140 (FIG. 11). Therefore, when the AND gate
220 receives control signals from both the "cycle complete"
terminal CY-C and the fifth stage of the ring counter 165, another
control signal is transmitted through differentiating device 62
to the "Insert Source" terminal 64, and the foregoing cycle o'
.,:
` operation is again repeated. Thus, there will be a fi'th ten-
second count with the compound standard source material 44
adjacent the sample 24 and, upon completion of that count, another
signal will be presented at the "Count Complete" terminal 59
which will cause the ring counter 165 to again step, thereby
;~ restoring the fifth stage o' the counter to its "0" state and
driving the sixth stage thereof to its "1" state.
Under these conditions, another control signal will be
:.
impressed upon terminal Pl (FIGS. 3 and 13) by which the Computer
Program Control 150 will be instructed to again follow Program
No. l during which gross external standard counts accumulated in
the scalers 88d, 88e will be stored in the storage registers
158, 159 respectively. At the same time, a signal will be
impressed upon the "Retract Source" terminal 70 to effect
retraction of the compound standard source 44 and the sixth ten-
second counting interval will be commenced. Upon co~pletion o'
the sixth ten-second counting interval, another "complete count"
signal will be generated and impressed upon the terminal 59,
thus causing the ring counter 165 to be stepped to its seventh
-73-
1040750
stage, driving such stage to the '1' state and restoring the
sixth stage thereof to the "0" state. Under these conditions,
another sianal will be presented upon the terminal P2 (FIGS. 3
and 13) to cause the computer to again step through its Progran
No. 2 routine for the purpose of computing a third automatic
external standardization ratio. Assuming that that r.~tio turns
out to be 0.409, a slight additional increment of potential
will be generated in precisely the manner described above, thereby
moving the photomultipliers even farther apart and establishinq a
still greater simulated quench condition. It has been .ound that,
: as a practical matter, any deviation between the selected fixed
ratios (here, 0.400) and the computed ratio following the third
voltage adjustment can be ignored since it will not contribute
. any significant error to subsequent computations.
, At the same time, driving o' the ring counter 165 to a
: condition with .its.seventh stage in the "1" state will create one
input signal for an AND gate 221. ~ihen the decade counter 172 7 `
~FIG. 14) has counted the necessary ten pulses to establish the
third incremental change in the current flowing in the coils 105,
it will again produce a "cycle complete"signal at the terminal
CY-C, which signal will provide the second necessary input for
the AND gate 221. At this point, the AND gate 221 will provide
an output signal which is conveyed directly to the "Start Count"
terminal 58 so as to initiate a seventh counting cycle here
termed the "data" count. Up until this point in the operational
cycle, there have been a total of six ten-second counting
intervals, during only the first, third and fi'th o' which was the
sample subjected to radiation from the compound external standard
source 44, and during the second, fourth and sixth of which the
sample was shielded from the source. The purpose of these six
ten-second counting periods is to allow repetitive computations
of the external standard ratio under successive controlled
_7 ~._
,
~ .
~040750
conditions of simulated quench so as to enable sufficient voltage
or potential changes at terminal 240 (FIGS. 11 and 15) whereby
quenching can be simulated to cause tne computed external
standardization ratios indicative of the effective quench level
- for the particular test sample to converge towards the preselected
fixed external standardization ratio for which counting efficiency
is known with a high degree of accuracy -~ here, a ratio of 0.400.
In response to receipt of the third "cycle complete"
signal, the seventh or "data" counting cycle will be initiated in
the manner set forth above. In this instance, however, the
compound external standard source material 44 will remain retracted
and the Mode Program Control 40 will select the desired counting
period which~may, merely by way of example, be one minute. Upon
completion Or the one-minute time cycle established by the 2-lode
Program Control 40 and the timer 78 (FIS. 4~, the timer will
produce a "stop" signal on line 89, which signal will be transmitted
directly to the start-stop control 79 and to the "count complete"
terminal 59. Considering, for the moment, FIG. 3, it will be
observed that in this instance when the seventh or "data count"
has been completed, an input signal will be transmitted to the
ring counter 165 which will cause the latter to step so that its
first stage is drivento the "1" state and its seventh stage is
restored to its "0" state. Since the first stage of the counter
165 is set in the "1" state, a signal will be transmitted to a
terminal P3 (FIGS. 3 and 13) for the purpose of instructing the
Computer Program Control 150 that from this point on it is to
follow Program ~o. 3 during which time activity levels in units
of decay events per minute (dpm) will be computed for each of
the two isotopes ( H and 14C~ in the exemplary sample 24.
Finally, when the first stage of the ring counter 165 is driven
to its "1" state it will provide an output signal which is
passed through the differentiating device 74 (FIG. 3) to the
-75-
:. ' '.
.
104~)7S0
"Start ~rint" terminal 60 (~IG5. 3 and 13~, thereby enabling print
out device 161 (FIS. 13) so that data in the Answer Register 152
can be printed out on ti~ed demand in accordance with the
routine established by Program l~o. 3.
Before proceeding with a description of the routine
established by Program ~lo. 3 as carried out by the computational
portion of the system illustrated diagrar,~atically in FIG. 13,
it may be well to emphasize that during the entire seventh or
"data" counting cycle during which counts are accumulating in
scalers 88a, 88b (FIG. 4) numerically representative o the
activity levels of the two isotopes in the test sample (viz.,
tritium and carbon-14), current will continue to flow in the
,:,
coils 105 surrounding the photomultipliers. The value of such
current will be that determined upon completion of the third
current adjustment following the sixth ten-second counting
period. Therefore, during the entire "data" counting period
(i.e., the seventh count) the test sample will be quenched not
only by its true internal quench characteristics which produced
the original external standardization ratio of 0.850, but also
by the simulated quench created by movement of the photomulti-
pliers. Therefore, the effective quench level of the system
will be the composite values of the actual internal quench
characteristics of the sample 24 and the amount of simulated
quench and, in this case, the effective quench level of the
sample will produce a net external standardization ratio which
will, in all probability, be closer to the selected fixed ratio
of 0.400 than the last computed ratio of Q.409 sin~e it will be
understood that the final voltage adjustment will tend to cause
the simulated quench level to converge even closer to the
selected fixed level.
-76-
Program No. 3 ~0407~
Keeping the foregoing in mind, upon completion of the
exemplary one-minute counting period determined by the Mode
Program Control 40 and the timer 78 (FIG. 4), the "stop" si~nal
transmitted by the timer 78 to the start stop control 79 is
effective to close the gates 81a-81e. Consequently, no further
counts are accumulated in the scalers 88a-88e. At this point,
the timer 78 also transmits a signal TI to the Computer Program
Control 150 which is representative, in binary coded decimal
notation, of the length of the time period, in this case, one
; minute. Also at this time the numerical representations for the
accumulated counts in scalers 88a and 88b are transmitted to the
input terminals Sl, S2 respectively of the Computer Program
Control 150. Under the routine established by Rrogram ~Jo. 3,
the binary coded decimal signal Sl is transmitted to the terminal
I1 of the computer 151 at the first occurrence of timing signal
tl after presentation of a signal at terminal P3 which enables
the computer to follow Program No. 3. At the same time, signal
T1, which is numerically representative of the length of time
of the "data" count (here, one minute), is simultaneously
. ~
transmitted to the terminal I2 of the computer. A command
signal is then transmitted from the Computer Program Control 150
to the computer 151 signifying that the value of I2 should be
divided into the value of Il. This particular divisional
operation may, for example, require ten timing intervals,
whereupon at time instant tll there will be produced in the
Answer Register 152 an answer in binary coded decimal notation
which is here numerically representative of the gross counts
per minute for the AB channel including scaler 88a (hereinafter
referred to as Channel I). This data is then presented to the
input of the Computer Program Control 150 as a binary coded
decimal signal A~ numerically representative of gross counts
--77-
'` ~ ` .
.
~J04
per minute. At time interval tl2 the data present at the input
terminal AN to the Computer Program Control 150 is transmitted
to the terminal Il of the computer 151. At that same time
increment tl2, the binary coded decimal input B51 to the
computer Program Control representative of the "background
count" previously measured for Channel I (which background
count, it will be recalled, had been dialed into the system by
the technician with the finger dial switches BGl shown in FIG. 5)
is transmitted to the input terminal I2 for the computer 151
together with a command instruction indicating that the data
at terminal I2 should be subtracted from the data at terminal
Il, thus producing in the Answer Register 152 at a later time
interval (for example, at time t22) an answer in binary coded
decimal notation which is numerically representative of the
net counts per minute accumulated in Channel I for the two
isotopes, tritium (3H) and carbon-14 (14C). This data is then
stored in storage register 1~8 by virtue of a clock signal
which serves to open gate 154 at time interval t23.
Continuing with Program No. 3, at time interval t24
the binary coded decimal data represented at input terminal
S2 and the binary coded decimal data representative of the
! time signal TI are respectively transmitted to the input
; terminals Il, I2 of the computer 151 together with a command
signal ordering the computer 151 to divide the value at terminal
Il by the value at terminal I2, thus producing in the Answer
Register 152 at a subsequent time interval (say, for example,
time t39) a value which is numerically representative of the
gross counts per minute in the channel including scaler 88b.
As above, this channel will hereinafter be referred to for
simplicity as Channel II, it being understood that this is
the particular channel defined by the CD window of pulse
height analyzer 86b. The answer in Answer Register 152
-78-
. . .
~04~
numerically representative of ~ross counts per minute in Channel
II is then transmitted to the AN input or the computer 151. At
time interval t40, the binary coded decimal data respectively
presented at the computer inputs AN and B~2 (such data being
numerically representative of gross counts per minute and
background counts accumulated in Channel II) are respectively
transmitted to the inputs Il, I2 of the computer 151 together
with a command signal indicating that the background count at
terminal I2 is to be subtracted from the gross counts at
terminal Il. Assuming that this subtraction operation requires
,:,
five time increments, there will be produced in the Answer
:
Register 152 at time increment t45 an answer which is numerically
representative in binary coded decimal notation of the net counts
per minute in Channel II. At time interval t46, a clock signal
is provided to open gate 155, and the net counts per minute now
represented by the data in the Answer Register are transferred
through the gate 155 into the storage register 159.
The computational portion of the system is now
conditioned to calculate the true activity level for tritium
,:,
(3H) by solving the following equation:
~II]
:
Net CPM Net CPM
CH.I CH.II
DPM (3H) =
.I (3H~ ECH.I(3H ~CH.II ( C~
CH.I C J
'
Before proceeding with the operation of the compu-
. tational system shown in FIG. 13 to solve equation lII], it
will be well to here direct attention to FIGS. 6 and 6a in
: .
~:, conjunction with the tables of efficiency previously prepared
;:~. by the technician and set forth at pages 45 and 46 hereof.
~ Thus, it will be recalled that when the technician prepared
; the quench correlation curve 95 shown by way of example ~
::
-79-
- , '
: -
~0407SO
FIJ. 8a, he was also able to record in Table I the various
counting efficiency values for the isotope tritium (3H) as counted
Channel I. Referring to FI5S. 6 and 6a conjointly, it will be
observed that there is illustrated an auxiliary control panel which
here contains ten binary codea decimal switches 225 which are
arranged in a horizontal row comprising two groups of three switches
. .
and one group OL four switches. The arrangement is such that the
technician may readily dial into the control panel any set o'
three efficiency values set forth in Tables I and II corresponding
to a preselected one of the external standard ratios of 0.0 through
0.9 respectively. Thus, the three left hand switches would (in
the exemplary case where the technician has selected a fixed ratio
of 0.400) be respectively set to "1", "0" and "2" to signify a
counting efficiency for tritium in Channel I of .102. In like
manner, the three switches in the center group would be set to
the values "3", "8" and "8" to represent the counting efficiency
for carbon-14 in Channel II of .338 correspondin~ to an external
standard ratio of 0.4. Finally, the four right hand switches would
be set to the values "4", "7" and "5" to represent the counting
efficiency of .475 for carbon-14 in Channel I.
The efficiency values which have been dialed into the
auxiliary control panel shown in FIG. 6 are, of course, the very
accurate efficiency values which are based upon actual measurements
of differently quenched samples containing a known amount of
radioactivity for the fixed external standardization ratio of
0.4. If the technician were going to converge to any other
selected fixed ratio, it would merely be necessary to select the
proper efficiency values from his quench correlation curves. Thus,
since these actual efficiency values for the fixed external
standardization ratios have been previously dialed into the
equipment, it is now possible to solve equation [II] above.
-80-
: ,
- .. : , . .
~04~7SO
Thus, in keeping with this aspect of the invention, and
, continuing with Program No. 3, it will be observed that at time
interval t4,7 the numerical representation for net counts per
minute in Channel I can be transferred from the SRA input terminal
-. for the Computer Program Control 150 to the Il terminal for the
computer 151. Also at time'interval t47, the counting efficiency
, for tritium in Channel I is transferred from the X input terminal
for the Computer Program Control to the I2 input terminal for the
computer 151 together with a command signal for the computer to
divide the quantity on terminal I2 into the quantity on terminal
Il. It will be understood that the efficiency value at the X
input terminal is here derived from the dialed in efficiency
in the left hand group of switches shown in FIG. 6. Assuming
that *he foregoing divisional operation requires twenty time
increments, then at time interval t67 there will be,produced
in the Answer Register 152 an answer which is representative
of the net counts per minute in Channel I divided by the
efficiency of tritium in Channel I. At time interval t68, the
gate i56 (FIG. 13) is clocked open and the foregoing quantity is
stored in storage register 160. It will further be observed
upon comparison of the data stored in storage register 160
with equation lII] above that the value is here representative
of the left hand portion of the equation -- viz., net counts
, per minute in Channel I divided by the efficiency Oc tritium
: in Channel I.
`, For the purpose of computing the denominator of the
' right hand portion of equation [II], at time interval t69 the
numeral representations of the efficiency values for tritium in
Channel I and for carbon in Channel II (the latter value having
~ been derived from the setting o the middle group of switches.~
shown in FIG. 6) are respectively transferred from the input
- terminals X and Y for the Computer ~rogram Contr:ol 150 to the
-81-
~09075~
input terminals Il and I2 of the.computer 1.51 together with a
command si~nal instructing the computer to multiply these two
values together. Assuming tha.t the multiplication operation
requires twenty time increments, the answer will appear in
Answer Register 152 at time interval t 9. That answer is then
presented at the AN input terminal for the Computer ~rogram
: Control 150. At time interval tga the binary coded decimal data
at the AN input terminal and at the Z input terminal (the Z
terminal here being cou~led to the right hand group of dial
switches shown in FIG. 6) are respectively transferred to the
input terminals Il and I2 Or the computer 151 together with a
command signal instructing the computer to divide the value on
input terminal I2 into the value on input terminal Il, thus
producing in the Answer Register 152 at a subsequent time interval
(say, .for example, time interval tllo~ a numerical value
representative in binary coded decimal notation of the denominator
for the ri.~ht hand half of equation lII], which value is again
transmitted to the AN input terminal for the Computer Program
Control 150.
The apparatus is now prepared to solve for the right
hand half of equation [II]. To this end, at time interval tlll,
the binary coded decimal notations appearing at the S2B and
: the AN input terminals for the Computer Program Control 150
(the former being numerically representative of net counts per
~ minute in Channel II -- i.e., the numerator for the right hand
hal~ of equation [II], and the latter being numerically
; representative of the denominator~ are transferred to the input
terminals Il, I2 respectively, of the computer 151 together with
: a com~and instruction for the computer to divide the value on
input termi.nal I2 into the value on input terminal I , thus
providing in the Answer Register 152 at time interval tl31 a
value representative of the right hand portion Of equation [II].
-82-
'..;
1040750
This value is then presented at the AN input terminal or the
Computer Program Control 150. At time interval tl32, the binary
coded decimal data stored in storage register 160 is transmitted
from input terminal SRC for the Computer Program Control 150 to
the input terminal Il for the computer 151. Also, at time
interval tl32 the binary coded decimal input at the AN input
terminal for the Computer Program Control 150 is transmitted to
the terminal I2 for the computer 151 together with a command
instruction for the computer to subtract the value at terminal
I2 from the value at terminal I~ ssuming that this subtraction
operation requires four time increments, there will be produced
in the Answer Register 152 at time interval tl36 a numerical value
in binary coded decimal form which is equal to the true activity
level for tritium (3EI) in units of decay events per minute.
This value is then transmitted directly to the print out device
161 at time interval tl37 by virtue of a clock signal which
serves to open the gate 157. Since the print out device 161
has previously been enabled by means of the "Start Print" signal
appearing at terminal 60 (FIGS. 3 and 13) it will decode the
data and print out in any suitable form the data representative
of the true activity level of tritium in decay events per minute.
Let it be assumed for purposes of thîs discussion that fifty
timing steps are required to print out this data. Under this
assumption, print out of the activity level for tritium will be
completed at time interval tl 7.
The computational system is now in condition to
compute the true activity levels for the higher energy isotope
carbon-14 (14C) in units of decay events per minute in accordance
with the following equation:
:,.
Met CPM lII T
DP~ (14C) = ~ CH.II
~`` ECH II ~ C~
-83-
~.04~750
To this end, at time interval tl~ the value stored
in storage register 159 (here numerically representative of net
counts per minute in Channel II) is transferred from the SRB
input terminal for the Computer Program Control 150 to the I
input terminal for the computer 151. Also at time interval
tl88, the efficiency value for carbon in Channel II is transerred
from the Y input terminal for the Computer Program Control 150
to the I2 input terminal for the computer 151 together with a
command signal instructing the computer to divide the quantity at
terminal I2 into the quantity at terminal Il. Assuming that
such divisional operation requires twenty time increments, there
will be produced at time interval t208 an answer in the Answer
; Register 152 which is numerically representative of the true
activity level for carbon-14 in decay events per minute, such
value being in binary coded decimal notation. At time interval
t209, the gate 157 is again clocked open and the binary coded
- decimal information in the Answer Register 152 numerically
representative of carbon-14 activity level in decay events per
minute is trans~erred to the print out device 161. Since the
latter is still enabled because of the "Start Print" signal
presented at terminal 60 (FIGS. 3 and 13), it will immediately
print out in any suitable manner the true activity level of
carbbn-14 in decay events per minute. Assuming that the printing
operation requires fifty timing steps, it will be appreciated
that at time interval t259 print out will be completed for both
carbon-14 (14C~ and tritium (3H~ activity levels. At this
point the print out device 161 transmits an "End Print" signal
EP to the "End Print" terminal 61 of the Mode Program Control
40 (FIG. 3~, which signal is then transferred to the "Change
Sample" terminal 41 for the purpose of eneraizing the elevator
~ motor Ml through its UNLOAD terminal (see FIG. 4), and thus
-~ ejecting the sample. At the same time, and as best illustrated
: .
-84-
~0~97SO
in FIG. 14, the "End Print" signal EP from the print out device
161 is also conveyed to the reset terminals for the BCD decade
counters 184, 194, 196, thus effectively returning all of these
counters to their "zero" state in readiness for a counting cycle
for the next successive sample. Since these three counters are
returned to their "zero" states, it will be appreciated that their
- respective decoders 185, 195 and 198 will have their b.0, c.0
and d.0 terminals raised to a positive potential. Under these
conditions, infinite resistance values will be inserted into the
voltage programmers 204, 205, 206 and the potential level at
point x (FIG. 15) will now be determined by the current path
through resistors Rb o~ the now ON transistor Qll in voltage
programmer 200, resistance Rl, resistance Rc, the now ON transistor
Q49, and resistance RL.
The operating cycle for the system is now complete and
the apparatus is in condition to compute true activity levels
for the next sample introduced into the detection chamber.
A slightly modified elevator and detector assembly
120', also embodying the features of the present invention, has
been illustrated by way of example in FIG. 12. In this form of
the invention, advantage is taken of the fact that it has been
found that the quantum of light reaching the photosensitive
cathodes of the photomultipliers can be selectively decreased or
increased by interposing conventional variable iris lenses between
the sample 24 and the cathodes of the photomultipliers PMT#l,
P,~T#2, and thereafter adjusting the size of the opening through
` such lenses as a function of the single level presented on
terminal 240 (FIGS. 12 and 15). To accomplish this, the terminal
240 here presents its control signal input to a conventional
servo motor driver 140' which, in turn, provides regulated inputs
- to a reversible servo motor 139' so as to drive the output
shaft 138' of the latter through controlled increments of
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rotation. As here shown, the components 138', 139' and 140'
may be identical to the corresponding components 138, 139 and 140
previously described in connection with FI~. 11. In this instance,
however, the servo motor drive shaft 138' has keyed thereon, or
otherwise affixed thereto, a pair of spaced bevel gears 142, 143,
the latter being respectively meshed with bevel gears 144, 145.
In this instance, the bevel gears 144, 145 are respectively
coupled to actuating shafts 146, 147, the latter being respectively
coupled to a pair of conventional variable aperture iris type
lenses 148, 149. Tne arrangement is such that upon rotation
of the servo motor drive shaft 138', the actuating sha,ts
146, 147 respectively associated with the lenses 148, 149 are
rotationally driven so as to controllably vary the size of the
apertures in the lenses, and thus varying the amount of light
permitted to reach the photomultipliers so as to produce a
simulated quench condition.
Those skilled in the art will appreciate that the
variable aperture iris type lenses 148, 149 could readily be
replaced with conventional sets of polarized lenses (not shown)
for example, sets o~ the type having a stationary lens movable
with respect to the stationary lens. In this instance, the
actuating shafts 146, 147 (FIG. 12) could be utilized to effect
rotation of the movable lens so as to affect the quantum of
light reaching the photomultipliers PMT#l, MT#2, thereby
reducing counting statistics and creating a simulated quench
condition.
While the present invention has hereinabove been
; described in conjunction with various procedures and equipment
for simulating quench conditions for test samples having unknown
quench characteristics by means of repetitive computations of
net external standard ratio, those skilled in the art will
appreciate that other variable quench correlation parameters
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could be employed. Thus, for example, reerrin~ to ~IG. 11,
there has been illustrated a typical ~uench correlation curve
250 for a tritium isotope in which the variable quench
correlation parameter to be measured is net external standard
counts. That is, in this instance the abscissa of the curve is
scaled in units of net external standard counts in any selected
window -- for example, the G-to-infinity window which transmits
counts to scaler 88d (FIG. ~), while the ordinate is scaled in
tritium efficiency wherein the tritium window (i.e., the AB
window) in Channel I is a relatively wide window. BY "net
external standard counts" it is, of course, meant that the count
accumulated in scaler 88d when no external standard source is
present is to be subtracted from the count accumulated in such
scaler when an external standard source is present. In other
words, the net external standard count can be computed in
accordance with Program Nos. 1-3 hereinabove described. In this
instance, the curve 250 would be prepared in advance based upon
preselected fixed net external standard counts and the circuitry
shown by wav of example in FI~S. 14 and 15 would be effective
to simulate quenching until the net external standard count
computed converged to one of such preselected points where
counting effioiencv is known precisely.
Yet another form of quench correlation parameter has
been illustrated by way of example in FIG. 12. In this case,
. ~
~` there is graphically represented a quench correlation curve 251
based upon normalized channels ratio. In this particular
type of system the technician would set his channels so as to
compute channels ratio for the particular iSOtOpe being analyzed.
, ~,:. . .... .
In other words, assuming that the isotope is tritium, the windows
for Channel I and Challel III miaht be adiuste~ ~ th~t ~h~
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tritium spectrum produced counts in both scalers 88a and 88c
(FIG. 4). Thus, the ratio of counts in these two channels
(or for that matter, any two channels) would provide a variable
measurable parameter indicative of the degree of quenching.
Such parameter could again he arbitrarily set to 1.000 for an
unquenched sample. Use of the curve 251 would~be the same as the
curve 250 described above. In other words, the technician would
again create the curve 251 by selecting certain fixed points to
which he desired to converge.
It will be appreciated from the foregoing that there
~ has been disclosed herein a novel system for enabling simulated
I quench conditions to be imposed upon the true quench characteristics
of any given sample so as to create for the sample an effective
quench level for which counting efficiences are accurately known.
While the present invention has herein been descrihed
in connection with quench correlation curves defined by eleven
~11) preselected fixed ratio points, those skilled in the art
will appreciate that the particular selection of fixed points is
completely arbitrary. Thus, the technician can select any
desired number n of fixed points to converge to where such
number n may be two or more and will meet his particular
requirements. Moreover, the particular selected fixed ratio
points need not be spaced by multiples of tenths, nor do thev
need to be even numbers. Merely by way of example, it is entirely
; within the scope of the invention to arbitrarily select three
~;', fixed ratio points of 0.567, 0.782 and 0.789. Therefore, it
: .
~ ! will be understood that references to a plurality of fixed
;~ points in the claims appended hereto shall not be limited to
the exemplary forms of the invention described herein.
Also, it will be apparent from the disclosure herein
that the present invention will find equal application with dual
labeled samples, single labeled samples, intermixed samples, or
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samples that are multiply labeled with mere than two is~topes.
It will also be understood ~hat the present invention
; will find equally advantageous appli~ation with any desired
number of significant digits rather than simply the illustrative
four digits herein referred to. Th~s, it could be used with
systems having fewer or more digits. ~oreover, it is entirely
possible to arrange the system so that other than three voltage
adjustment cycles are provided. Indeed, even if the system
includes provision for normall~ following three adjustment cycles
as disclosed herein, it would be entirely possible to automatically
eliminate the second and/or third adjustment cycle if the first
and/or second voltage adjustment caused the computed ratio to
converge to within an acceptable tolerance range. For exam~le,
with the exemplary system shown herein, there might be provided
a comparison network (not shown) associated with either or both
of the "c~ and "d" digits in the Answer Register, the comparison
network or networks being arranged so that when they reached a
preselectedpoint they would transmit a control signal to the
Mode Program Control 40 (FIG. 3) which was effective to terminate
further ten-second counting cycles and to immediatel~ initiate
a "data"--count.
`- Finally, it will be understood that the various modified
; forms of the invention herein illustrated and described are all
directed to methods and equipment for simulating ~uenching for
any given sample, irrespective of its true internal quench
characteristics, by establishing a controllably variable
. . .
~ simulated quench modulating signal, the value of which is a
:;
~ function of a measured quench correlation parameter for the
.; ~
sample, and then employing such signal to affect counting
statistics by reducing the amount of light transmitted to the
light transducers. Consequently, it is within the scooe of the
;~ present invention that such signal can be utili2ed in other
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equivalent ways to produce the same result. For example, the
signal transmitted to terminal 240 (FIG. 15) might also be
employed to energize a suitable heater or the like (not shown)
which might be disposed in the elevator platform, thereby
creating incremental changes in the temperature of the sample
being counted so as to simulate quenching by changing the
efficiency of the scintillator, or by altering- through changes
in temperature the quench characteristics of other sample
constituents.
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