Note: Descriptions are shown in the official language in which they were submitted.
FTI 001 P2 1 3~3454
PHASE SHIFTED FEEDBAC~ ELECTROMETER
This is a divisional of Canadian Patent
Application Serial No. 564,266, filed April 15, 1988.
Background_of the Invention
In the past several years, there has been
increasing concern over the health hazard of exposure
to radon-222, a radioac~ive gas produced in the
uranium-238 natural decay series. This is in part due
to an improved understanding of the radiobiological
effects of radon, but more importantly to the recogni-
tion of an exposure hazard to the general population.The exposure hazard ~o uranium miners and mill employees
has been recognized for many years, but it has only
been recently that radon-222 and its radioactive progeny
have been found to pose a potential health hazard in
private dwellings. This is in part due to energy
conservation measures that lead to nearly airtight
structures with little outside air exchange. Radon
levels can build up in the~e closed structures by
diffusion from underlying rock and soil through cracks
2~ and pores in concrete floors and concrete block founda-
tion walls. Because radon is a gas and its decay
products are generally found as suspended particulat~s,
human exposure is primarily through inhalation. Frac-
tions of these radioactive species are retained in ~he
lungs and have the potential of pr~ducing lung cancer.
Present monitoring techniques used for radon
detection are either of the continuous type which
provide actlvity flux level information on a ~real-time~
basis or of ~he integrating type which provide do~e
information for a selected time period. Present con-
tinuous type monitors are generally based on gaq
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~TI 001 P2 -2-
proportional or scintillation techniques and are
sophisticated ins~ruments more suited to the laboratory
than for field or private home applications. The most
commonly used integrating type monitors use thermo-
luminescence detectors (TLD) or solid state nucleartrack detectors (SSNTD) Which are inexpensive passive
devices, but only give integrated dose information for
periods of from days to months. ~ecause radon levels
can vary an order-of-magnitude over a twenty-four hour
10 period, a home owner needs to have continuous monitoring
in order to provide economical heating and air condi-
tioning and at the same time control radon levels.
Moreover, a radon source tracking, ventilation control,
or remedial ac~ion analysis can only be accurately
carried out with continuous type detectors. Thus,
there is a need for portable, low cost, low power,
continuous detection instrumentation for moni~oring
airborne alpha radiation.
The use of ionization chambers as a means of
20 detecting nuclear radiation is an adaptation of a very
old art, going back to nineteenth-century work on
conduction in gases. The passage through a gas of
alpha radiation of MeV energies produce, through ion-
ization, approximately 10-14 coulomb of charges
25 before coming to rest. It is upon this effect that the
class of detection instruments called ionization
chambers are based. The ion pairs produced are
collected through the use of an electrostatic field
gradient imposed on the ionizing volume. If the field
30 gradient is great enough to result in collection at an
electrode before recombination, but insufficient to
cause secondary collision ioniza~ion, the chamber i8
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FTI 001 P2 -3-
said to be operating in the ion or linear region. The
linear region generally exists below approximately 100
VJcm. If the field gradient is between lO0 and 1000
V/cm, the chamber is said to be operating in the gas
proportional region where a controlled ~gain~ in charge
carriers is produced through collision ioniz~tion
processes, At field gradients greater than approxi-
mately 1000 V/cm, the ~Geiger~ region is reached where
every primary ionization results in an avalanche dis-
10 charge within the chamber.
The Geiger type counter is not suited forairborne radon detection because its sealed-tube dssign
precludes unobstructed sampling of the alpha radiation.
Gas proportional type counters are used for radon
15 measurements, but because this type of counter operates
with low electron attachment sample gas and high field
gradients, it requires costly and high energy consuming
power supplies, pumps and sample gas processing equip-
ment for its flow-through chamber.
Ion or electron chamber counters operating in
the linear or non-multiplication mode possess the
requisite low voltage and current characteristics and
are both simple in ~esign and can provide real-time
information. They can also be operated in either the
25 current or pulsed mode. However, when operated in the
current mode, both the ion and electron chamber suffer
from base-line drift and do not discriminate between
- different radiations. Moreover, current mode ion and
electron chambers are highly sensitive to charged
30 particles, such as smoke, and thus require special
pre-filtration of sample gases. The electron pulse,
sometimes referred to as the fast-pulse ~ode, can only
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FTI 001 P2 -4-
be applied in low electron attachmen~ gas environments
where electrons survive long enough to be collected and
counted. This precludes the use o~ the electron pulse
technique in normal air environments due to the high
electron attachment coefficient of oxygen and water
vapor. The ion pulse or slow-pulse mode is possible in
air environments, but because ion mobilities are
nominally a thousand times smaller than electron mobil-
ities, the pulses are long (>100 ms), irregularly
10 shaped and poorly suited for electronlc counting.
Thus, although the first pulse chamber~ used to detect
alpha radiation in ai~ were of the ion pulse type
(Greinacher 1927), the fast-pulse type is employed
almost exclusively in commercial instruments. Overhoff
15 (U.S. patent 4,262,203) describes a slow-pulse type
alpha monitor, but because ~he concept is based on a
flow-through ~hamber, relatively large volumes are
required for measurements in low level environmental
samples. The one liter chamber used by Overhoff
20 requires a high electrostatic potential of 500 volts to
maintain ~ield gradients suf~icient to overcome ion
recombination, Moreover, a multi-staged pulse recogni-
tion, shaping and amplification circuit is required to
produce countable output pulses~ Thus, this approach
25 has the ~ame high power consumption and cost disadvan-
tages as fast-pulse gas proportional counting.
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FTI 001 P2 -5-
Summary o~ the Invention
The above-noted parent application describes
a -~mall portable instrument for selectively detecting
airborne alpha radiation in a ~real time~ mode directly
in an air environment based on an ion pulse collection
and counting ~echnique.
The above application further describes a probe type
ion chamber design with sufficient active ~ample volume
to detect environmental concentration levels (0.1
10 pCi~l) of airborne radon radiation while requiring less
than 30 volt electrostatic potentials be~ween the
collecting electrodes.
The system further provides a means of
producing directly wi~hin the electrome~er stage,
15 amplified ion pulses suitably shaped and narrowed tfast
rise and short decay wi~h a full width at half maximum
of between 10 and 25 ms) for electronic counting.
Thus, eliminating the need for separate power consuming
delay, comparator, and correlation s~ages.
Also provided is an ion chamber
design capable of selectively sampling either radon
alpha activity or the total alpha activity in a gas
I mixture containing both radon and its alpha emitting
daughters. This is a particularly desirable Peature
25 because radiobiological efficacy i~ greater ~or the
alpha emitting daughters polonium-218 and poloniu~-214
than it is for radon-222 and knowledge of the re~ative
concentrations is required for accurate hazard assess-
ment.
There is described hereafter the unique combination of a
phase shifted, negative feedback, ~ield effect transis-
tor (FET~ electromete~ with an ultra, low capacitance,
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FTI 001 P2 -6-
open grid chamber design. The electrode spacing and
chamber volume are optimized to provide short ion
travel while maintaining a rela~ively large sampling
volume. Moreover, because this technique works at low
S field strengths (<10 V/cm) and accomplishes pulse
shaping and amplification directly within the electrom-
eter, low voltage, portable power supplies are possible
(<15 uA at 18 V).
Discrimination between radon and its alpha
10 emitting daughters is accomplished by biasing the
monitor cabinet negative with respect to the grid
electrode. Because the daughters are found in air as
either positively charged free ions or particulates,
they are swept ~rom the sensing volume and collected on
lS the cabinet wall and only radon alpha radiation is
de~ected.
An enhanced daughter detection mode is obtained
if the cabinet wall is biased positive with respect to
the grid. In this mode, an electrostatic potential-well
20 is established around the outside of the grid which
attracts and holds positively charged daughter particles
and ions at a favorable distance for detecting their
emitted alpha radiation.
If the cabinet wall and the grid electrode are
25 at the same potential, both radon and daughter alpha
radiation are detected. Best separations are accom-
plished if the distance between the grid electrode and
the cabinet wall is at least one alpha range for the
Aighest energy alpha, polonium-214. This distance is
30 found to be at least 7 cm -- the distance reguired to
stop a polonium-214 alpha particle in air.
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The invention of the present divisional relates to
a special circuit for use in the electrometer described
above. In particular it relates to an electrometer circuit
for amplifying short duration pulses including an FET having
a low capacitance input and output, feedback circuit
means connected between the input and output of said FET
for providing a negative feedback voltage to said input of
nearly 100%, and phase shift circuit means in said feedback
circuit means for delaying momentarily the application of said
feedback voltage, thereby to allow the output of said FET
to have a momentary power gain of sufficient magnitude and
pulse width to drive an external counter circuit.
Other objects and advantages of the invention
will be apparent from the following description, the
accompanying drawings and the appended claims.
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FTI 001 P2 -7-
Brief Description of the Drawin~s
Fig. 1 is a perspective view, partly broken
away, o~ a monitor for airborne alpha radiation con-
structed according to this invention.
Fig. 2 iS an elevational view of an ion pulse
collecting chamber.
Pig. 3 is an electrical schematic diagram of a
phase shifted feedback electrometer circuit.
Pig. 4 is a waveform diagram sho~ing the
operation of a 100% negative feedback electrometer.
Fig. 5 is a waveform diagram showing the
effect of phase shifting, or delayed application of the
negative feedback.
Fig. 6 is a waveform diagram showing a phase
shifted negative feedback circuit with diode clamping.
Description of the Preferred Embodiment
Referring now to the drawings which illustrate
a preferred e~bodiment of the invention, the monitor
shown in Fig. 1 is housed in a 6~H x lO~W x 7~D metal
instrument cabinet 10 with a horizontal divider 12
separating a lower electronic compartmen~ 14 from an
upper ion chamber enclosure 16. The metal cabinet
serves as an electromagnetic radiation shield for a
sensitive ion probe 20, but has screened openings 30
and 31 for allowin~ free ~low of air to the probe 20.
The exterior of the cabinet 10 is also provided
with a commercially available six digit LCD counter and
display device 35, a battery test button 37, a battery
test beeper 40 for indicating when new batteries are
3n needed, and a radon/enhanced/total alpha switch 45.
The interior of the cabinet 10 includes an electrometer
package 50 ~shown in ~ig. 31 and a plurality of
batteries 55.
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FTI 001 P2 -8-
As shown in Fig. 2, the ion pulse collecting
chamber or probe 20 includes a highly open grid cylin-
drical wall 21 which serves as one electrode, a center
rod 22 which serves as the other electrode of the ion
S eollection chamber and a high resistance insulator 23.
The unique features of this chamber are: (1) the elec-
trode spacing 24 is set, according to t~e field use~,
so that any ion created within the chamber will have no
greater than 50 msec transit time before being
lQ collected, and (2) the open grid reduces the chamber
capacitance and at the same time increases the effective
chamber volu~e by counting a por~ion of the alpha
particles that originate within one alpha range distance
25 outside the chamber wall 21. The effective chamber
boundary is defined in Fig. 2 by the dashed line 26.
A typical coaxial cylindrical chamber of one
inch diameter and two inches long with an 80% open area
outer wall has a capacitance o~ less than 1 pf and
results in a calibration factor of 0.3 cpm per pCi/l
radon-222. Although this chamber only has a geometrical
volume of 26 cm3, its effective volume for radon-222,
pol~nium-218 and polonium-214 are 82, 95 and 141 cm3,
respectively.
In its simplest configuration, the electrometer
circuit shown in Fig. 3 is comprised of a high power
gain (typically 109) field effect transistor Tl, an
input resistor Rl, high gain transistors T2 and T3 and
biasing/load resistors R2, R3 and R4. This type of one
hundred percent negative feedback has been employed
since the time of vacuum-tube electrometers to i~prove
linearity and stability.
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~TI 001 P2 -9-
Even more important to the present application,
however, is the reduction in the effective capaci~ance
of the input through the relationship Ce~f - C/~l~G),
where G is the current gain of the feedback amplifier
and C is the actual capacitance of the input. Thus,
with typical feedback circuit gains of 105, the input
capacitance becomes insignificant. Because the voltage
gain of this ~ype of circuit is essentially unity and
the pulse width is dependent on the time-of-flight of
the ions, further amplification and pulse ~haping is
required before the signals are suitable for nuclear
counting
The present invention overcomes these diffi-
culties, especially limiting in portable applications
damanding low power drain, through the use of a phase
shifted feedback technique. Circuit element~ C2, R7
and Dl provide the RC network to produce this phase
shift function. The remainder of the circuit elements,
R8, R9, C3, D2 and T4, provide a transistor switch for
incrementing a commercially available large scale
integrated circuit (LSI) counter 35 with liquid crystal
display (LCD).
The differences between a conventional negative
feedback electrometer and the phase shifted feedback
electrometer, as well as the operating principles of
the latter, can be seen by examining Figs. 4, 5 and 6.
The dotted curve in Fig. 4 represents the voltage-time
relationship for an ion chamber electrometer with no
feedback and an input RC time constant much greater
than the collec~ion times of the the positive and
negative ions, t~ and t~, respectively. The solid
curves in Fig. 4 represent the collecticn electrode
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FTI 001 P2 -10-
voltage, Vl, and output voltage, V3, for the same ion
chamber and load resi~tor as above, but with a high
percent of negative ~eedback. It is seen that the
output pulse rather faith~ully ~ollows the time-of-
flight collection time of the ions. However, withpractical chamber capacitances ~>1 pf) and input resis-
tor values (<1012 ohms), output pulse heights and
widths are in the millivolt and 50 to 100 msec range,
respectively. Therefore, further pulse shaping and
amplification must be carried out before these signals
are suitable for input to a nuclear counter.
The negative going pulse for Vl in Fig. 4 is
due to circuit non-idealities such as FET gate capaci-
tance, gate-to-drain leakage current, and a collection
of other control current losses around the feedback
circuit. The greater this negative voltage, the lower
the feedback and the longer ~he signal pulse tail. To
hold this control voltage to a minimum, the ~ET, load
resistor and other components of the feedback circuit
must be designed with ultra-low capacitances and current
leakage. By phase shifting the feedback, all the above
enumerated problems are either eliminated or greatly
reduced and the output signal pulse is of sufficient
amplitude and narrowed width to directly drive a nuclear
counter.
The principles of the phase shifted feedback
can be understood by examining Fig. 5. The voltage at
the collecting electrode, Vl, the intermediate phase
~hifted voltage, V2, and the output voltagel V3, are
all shown as functions of time. As in ~ig. 4, the
dotted curve 11 represents the voltage on the collection
electrode of the same ch~mber without feedback~ With
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PTI 001 P2 1 3~ 4 54
phase shifted feedback, however, Vl increases negatively
along the non-feedback curve until a ~control point- is
reached at time, tc. At this time, the voltage drop
across Rl is sufficient to main~ain a current through
Rl equal to the ion collec~ion curren~. At tc, the
output voltage, V3t peaks and starts down while V2
continues to rise until V2 equals V3, and then V~ falls
of f also.
If no damping is provided, the signals will
~ring-, as shown in Fig. 5. A diode, Dl, is thus
placed across R7 to clamp the circuit and minimize the
undershoot of V3, as illustrated in Fig. 6. This
clamping of ~he tail of the pulse rearms the electrom-
eter to be ready to accept another pulse so that dead
time is reduced. The circuit must be ~tuned~ so that
the time constant, (R7xC2~, is large enough to give the
desired ampli~ication of V3, but small enough to operate
within the ion collection times, t+ and t-.
The feedback phase shift circuit has the
effect of amplifying the output pulse by leading edge
overshoot and results in an increase in the output of
more than an order-of-magnitude in signal-to-noise.
The phase shifted feedback circuit tends to discriminate
against all noise frequeneies with rate of rise times
different from the ion pulses the circuit was designed
for. ~dditional high frequency filtering is provided
by the filter network formed by R~ and Cl. The follow-
ing values are typical for these components and lead to
output pulses of over 200 mV from the unphase shifted
and over 1.5 V from phase shifted circuits, ~hen count-
ing 5 MeV alpha particles.
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PTI 001 P2 -12-
Rl loll ohm
R2 1 meg ohm
R-~ 120 K ohm
R4 1 meg ohm
R5 lO x ohm
R6 220 K ohm
R7 470 K ohm
R8 100 K ohm
R9 330 K ohm
Tl MFE 823
T2 MPS 6534
T3 MPS 6531
T4 MPS 6531
Cl 0.001 mfd
C~ 0.1 mfd
C3 0.1 mfd
Dl IN34
D2 INgl4
Bl, B2 18 V Battery
The circuit shown in Fig. 3, without the phase
shifted feedback, has a signal-to-noise ratio of
approximately 20 whereas the phase shifted version, as
shown, has a signal-to-noise of over 200. Because
pulse wi~ths are narrowed to approximately 10 msec with
25 the phase shifted feedback ~ircuit, pulse pile-up is
not a problem for count rates below 50 counts per :::
second.
The combination of the small, low capacitance
open grid chamber with the phase shifted feedback
30 electrometer leads to an ion pulse detector capable of
operating in an air environment, drawing only micro
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~TI 001 P2 -13-
amperes of current and ea~ily capable of det~cting
environmental levels (approximately 0.1 pCi/l) of radon
and/or its daughters. Wi~h batteries $n~talled, the
monitor weigh~ less ~han f$ve pound~ ~nd can easily be
carried by a handle on top of th~ cabinet. T~i~ counter
module i8 ~elf~contained ~nd operate~ for up to four
years on two 1.5 V alkal~ne ~N~ cell~. The remainder
of the power requirement~ ~re 8Uppl~ ed by readily and
convenien~ly obtainable 9 V transistor radio batterie~
housed in the lower compartment o~ the monitor enclo-
sure. secause the electrometer draw~ le~s than 15 uA
current, a standard carbon zinc ba~tery glves oYer two
years of continuous operation -- alkaline o~ other
greater capacity batteries will provide commensura~ely
longer service. secause there i~ essentially no current
drain on the biasing batterie , their service life in
this application is essentially the ~el -life o~ the
battery.
While the orm of appara~us herein described
constitutes a preferred embodiment of this invention,
it is to be understood ~hat the inven~ion is not limited
to this precise form of apparatus, and that changes may
be made therein without departing from the scope of the
invention which is defined in the appended cl~ims.
The embodiments of the invention in ~hich an
exclusive property or privilege is claimed are defined as
follows:
