Note: Descriptions are shown in the official language in which they were submitted.
~L~S~ 76
Background of the Invention. This invention
relates to apparatus for causing ion beam accelerator tubes to
generate neutrons and is particularly directed to apparatus of
such character which are capable of being used in the logging
of boreholes in the earth.
Pulsed neutron generators are known in the art,
for example, those shown in U.S. Patent NO. 3,309,522 to Arthur
H. Youmans et al and those illustrated in my U.S. Patent No.
3,787,686. It is also known that ~uch prior art pulsed neutron
generators will emit neutrons at a steady state rate if the high
voltage pulser is disabled~ It is also known that such sources
may exhibit pre-ignition if the pulsing requency is much lower
than 1000 cycles per second. Although this characteristic is
of little or no consequence while pulsing the generator at ~hat
rate or higher, it has been put to use in the "subtraction method"
of detecting gamma rays produced by inelastic scattering of fast
neutrons as descirbed in my afoxementioned Patent No. 3,787,686.
.
This tendency to pre-ignite or to go into a
steady state conduction presents a disadvantage whenever it is
desired to make logging runs which provide short half-life acti-
vation measurements, i.e., those which might occur several milli-
seconds after the termination of the short burst of fast neutrons
rom the source.
The present invention relates to a pulsed neu-
tron source, comprising: an ion beam accelerator having an anode
and a cathode and corona current shunting circuitry between the
anode and the cathode. The accelerator also has a static atmos-
phere substantially composed of a heavy isotope of hydrogen. Means
are prov ded to ionize the atmosphere and a target containing a
heavy isotope of hydrogen is arranged to receive atmosphere ions.
Means are also provided to vary the effective impedance of said
shunting circuitry.
l~S(~7~;
These and other objects, features and advantages
of the present invention will be more readily appreciated from the
following detailed specification and drawings, in which:
FIGs. 1 and 2 are sahematic diagrams of pulsed
neutron sources in the prior art;
FIG. 3 is a schematic diagram of a pul~ed neutron
source in accordance with the present invention;
FIG. 4 is a schematic diagram of an alternative
embodiment of a pulsed neutron source in accordance with ~he pres-
ent invention;
FIGo 5 is a schematic diagram of an alternativeembodiment of a pulsed neutron source in accordance with the pres-
ent invention;
FIG. 6 is a block diagram of an electronic circuit
in accordance with the present invention which utilizes one o
the pulsed neutron sources in accordance with the present invention;
! FIG. 7 is a timing diagram illustrating various
wave-forms found within the circuitry of FIG. 6 in accordance with
tile invention; and
; FIG. 8 is a block diagram of circuitry in accor-
dance with the present invention which is utilized at the earth's
surface as a part of a well logging operation.
Referring now to the drawings in more detail,
especially to FIG . 1, there is illustrated a pulsed neutron
source which is built in accordance with the prior artO For
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example, such a neutron source is fully described in the U.S.
Patent No. 3,309,522 which issued on March 14, 1967 to Arthur
~l. Youmans et al, and which is assigned to the assignee of
the present invention. ~n brief, the neutron source 10 of
FIG. 1 includes an accelerator tube 11 having an anode 12 and
a cathode 13 wherein the tube 11 contains an atmosphere of
either deuterium or tritium (or a mixture of both). The source
10 also includes a belt-driven electrostatic generator 14, such
as the well-known Van de Graaf hi~h voltage generator. A belt-
shaped target 15, generally formed of a thin strip of titanium,
and impregnated with either deuterium or tritium, or a mixture
of both, is formed on the inside of the neutron source 10 in a
manner to encircle the cathode 13 and anode 12. Between the
grounded target 15 and the cathode 13 are found one or more
electrodes, generally referred to as the suppressor rings 16.
A pulse generator 17 is connected to the suppressorrings 16 by way of coupling capacitor 18, the capacitor 18
preferably being large in capacitance relative to the inter-
electrode capacitance between the suppressor rings 16 and the
cathode 13. ~he pulse generator 17 is adapted to supply a
sequence of negative pulses through the capacitor 18 to the
suppressor rings 16 at a fixed preselected frequency, or at a
rate determined by control apparatus generally located at the
surface. Since the operation of the neutron source in
accordance with FIGo 1 is fully described in the aforementioned
U.S. Patent NoO 3,309,522 and also in my U.S. Patent No.
3,787,686 which issued on January 22, 1974, and which is also
assigned to the assignee of the present invention, further des-
cription of this prior art neutron source need not be given
here.
Referring now to FIG. 2, there is illustrated another
prior art neutron source which is also described in the afore-
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mentioned U.SO Patent No. 3,309,522, and also in my U.S. Patent
No. 3,~87,68~. With this particular neutron source, a pulse
generator 20 is provided which supplies negative pulses through
a coupling capacitor 1~ to ~he suppressor ring 16 but which
also supplies positive pulses to the electrode 21 to further
aid in the pulsing of the neutron source.
As fully explained in the aforementioned U.S. Patent
No. 3,787,686, the neutron sources o~ FIG.'s 1 and 2 have a
common characteristic, i.e., if the pulse repetition rate is
too low, the neutron source pre-ignites and goes into a con-
tinuous mode and is no longer operating as a pulsed neutron
source. While this is acceptable in practicing the so-called
"subtraction method" in accordance with my aforementioned
patent, such a consequence is undesirable when it is desired to
have a longer period of time between neutron pulses such as is
desirable, for example, in activation logging.
Referring now to FIG. 3, there is illustrated a pulsed
neutron source 30 in accordance with the present invention which
can be operated having a much longer duty cycle, i.e., a much
greater time between neutron pulses without pre-ignition. The
neutron source 30 is substantially identical to that o the
circuit of FIG. 2 except for having a resistor 31 shunted
between the anode 32 and the cathode 33. As with the other
neutron sources, the source 30 includes a corona point 34
(which is a sharp pointed electrode) and which is preferably
fixed to the inside surface of the source. Since the space
between the tip of the corona point 34 and the cathode 33 is
narrower than the space between the cathode and any other
grounded point of the source, all leakage flow between the
cathode and ground (except for the beam current) will be con-
centrated between the corona point 34 and the nearest surface
of the cathode 33.
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S~ 76
Since the corona point looks at the ca-thode and the
anode is connected to the hi~h voltage terminal of the Van de
Graaf generator, the corona current will thus flow through the
resistor 31. The value of the resistor 31 is determined by the
firing voltage of the ion source and the current available from
the Van de Graaf generator and is selected so that the potential
across the ion source due to the corona current through the
resistor 31 is below the firing voltage of the ion source.
Then, with all control elements adjusted for normal source
operation but with the pulser 35 disabled, all current from
the ~an de Graaf flows through the resistor 31 as corona
current. Since the ion source cannot produce ionization
current, there are no neutrons produced by the source~ However,
when a high voltage pulse of appropriate amplitude from the
pulser 35 is applied to the pulsing electrode 36, the voltage
of the anode 32 with respect to the cathode 33 increases until
the ion source conducts and a burst of neutrons is produced.
The resistor and the conduction of the ion source cause the
voltage across the ion source to decrease rapidly and the ion
source to extinguish. The system capacitances are again charged
by the Van de Graaf and the next high voltage pulse causes the
cycle to repeat. It should be appreciated that the system can
remain energi~ed for an almost indefinite period oE time before
a high voltage pulse is applied to cause a burst of neutrons to
resuLt. This is in sharp contrast to the prior art embodiments
of FIG.'s 1 and 2 wherein it is known that a failure to pulse
the system causes the source to pre-ignite and to thus commence
a continuous mode of operation.
With the source according to FIG. 3, the width of the
neutron burst is determined by the combined effects of the hig'n
voltage pulse shape and amplitude, ~an de Graaf charging current
and the value of the shunt resistor 31. The source 30 of FIG.3
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may be pulsed at a fre~uency as low as desired to as high as
perhaps 10,000 cycles per second. It should also be appreciated
that the pulsing fre~uency can be modulated at some lower fre-
quency to provide a cycle appropriate for almo,st any measure-
ment that one might wish to make.
Referring now to FIG. 4, there is illustrated an
alternative embodiment of the present invention wherein the
pulsed neutron source 40, illustrated diagrammatically, is
fabricated substantially identical to the prior art sources of
FIG.'s 1 and 2 except for the following differences. A positive
trigger pulse is coupled into the anode 41 by means of trans-
former coupling. This transformer coupling may be effected by
means of a core 42 which is preferably formed of a substance
which may be magnetized but which is substantially non-conduc-
tive, though under favorable circumstances, adequate coupling
may be effected without a magnetic core. The core 42, which
may form the support column for the upper and lower pulleys 43
and 44 of the Van de Graaf generator, also supports a primary
winding 45 of the transformer which is connected to receive
trigger pulses from any suitable pulse generator 46 and a
secondary winding 47 which is connected between the anode 41
and the cathode 48. A current limiting resistor 49 is connected
in series with the secondary coil 47.
In the operation of the source of FIG. ~, it ~hould
be appreciated that the shunt resistance of the secondary coil
47 and resistor 49 function much like the resistor 31 in FIG. 3
which allows the source 40 to remain off until pulses are
generated by the pulse generator 46.
Referring now to FIG. 5, there is illustrated an
alternative embodiment of the present invention wherein a
neutron source 50 is fabricated essentially like the prior art
sources of FIG.'s 1 and 2 except for the following differences.
The anode 51 is connected to the high voltage side of the Van
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de Graaf ~enerator 5~. A shunt circuit 53 provides a means of
shunting the anode 51 to the cathode 54. The shunt circuit 53
includes a corona regulator 55, for example, one of the ~ictor-
een GV 1 series (Victoreen Inskrument Co., Cle~eland, Ohio) of
appropriate value in series with the collector-emitter of trans-
istor 56 wherein one side of the corona regulator 55 is tied to
the anode 51 of the source 50 and the emitter of transistor 56
is tied to the cathode 54 of the source 50. The base of trans-
istor 56 is tied to one or more small area silicon solar cells
57, for example, the Centralab 58C (Centralab Electronics Div
ision, Globe Union Inc., Milwaukee, Wisconsin), which in turn
are connected through a current limiting resistor 58 to the
cathode S~. The corona regulator 55 is selected to have a
regulating voltage about 50 to 100 volts below the iring vol-
tage of the ion source. A light source 59 with a focusing lens
is situated within the tank opposite the solar cells 57. The
light source 59 is powered from an appropriate lamp power
source 60 which may be external to the tank. A pulsing source
61 for pulsing the source 50 may or may not be included as des-
ired. If used, the pulse 61 is connected to the pulsingelectrode 62 and also to the suppressor rings 63 as desired,
all as shown in the prior art configurations of FIGs. 1 and 2.
In the operation of the source of FIG. 5, i.f the light
source 59 is extinguished, there is no base current for the
transistor 56 generated by the solar cells 57 and the source
will operate substantially as shown in my aforementioned Patent
No. 3,787,686. If the light source 59 is activated, transistor
5~ iS caused to conduct by the base current generated by the
Sol~ cells 57 and the Van de Graaf current flows through the
c~ron~ regulator tube 55 and the transistor in the form of cor-
- ona current caused by the corona point 64. Thus, the ion
source will be disabled and no neutrons will be produced. Then,
when the light source is again extinguished, the source will
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~050~76
operate as thouyh there were no shun~. With a fast respond-
ing light source, the ion source is caused to generate short
bursts of ions which produce corres~ondingly short burs-ts of
neutrons. However, in conjunction with the pulser 61, the
source may be operated in the normal manner when short bursts
of neutrons at a periodic rate are required and the shunt trans-
istor 56 operated when longer on off periods are desired.
Additionally, the two can be operated in conjunction, i.e., the
pulser 61 used in conjunction with the lamp power source 60, to
produce intervals where no neutrons are produced interspersed
with intervals where short neutron bursts are produced at a
cyclic rate. It should be appreciated that the corona regula-
tor tube 55 is included with the transistor to prevent the
ion source capacitance from being completely discharged; thus,
the peak current through the transistor is reduced and the ion
source voltage will reach the firing point more rapidly than
if the capacitance were completely discharged. This also
allows a lower voltage transistor to be used.
It should thus be appreciated that the neutron
sources built in accordance with the various embodiments of
the present invention produce vastly more flexible capability.
As an example of the measurements that can be made, the reac-
2~ )24N 27Al(n a)24Na and 23Na and Na(n,~)produce a 470 kev gamma ray with a 20 millisecond half life.
The reaction cross sections for these reactions are 48, 33 and
400 millibarns, respectively. The cross section-abundance
product for magnesium is sufficient to allow the measurement
to be valuable in lithology identification of formation rock.
Prior to this development, neutron sources have not been avail-
able that could be operated with an on-off cycle appropriate
for the selective detection of these elements.
Referring now to FIG. 6, a circuit is provided for
pulsiny the source in accordance with the various embodiments
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76
of the present invention. A pulse generator 70 is set to
produce a 10 microsecond wide negative pulse at the rate of
1000 such pulses per second and which are fed into a divide-by-
50 circuit 71 and also to an inverter 72 which drives the sync
wiclth single shot circuit 73. The outputs of the divide-by-50
circui~ 71 and the single shot circuit 73 are coupled into the
input of a NAND gate 74. The divide-by-50 circuit 71 is set to
trigger on the leading edge of the generator pulse from the
generator 70 and the single shot circuit 73 triggers on -the
-10 trailing edge of the inverted generator pulse. Thus, the
leading edge of the single shot output is delayed by about 10
microseconds with respect to the leading edge of the divide-by-
S0 output. These two outputs are fet to the NAND gate 74 which
produces 25 full-width sync pulses one millisecond apart foll-
owed by a 25 millisecond interval in which there are no sync
pulses. FIG. 7 provides a timing diagram of this circuit.
The sync p~lses so produced drive the high voltage pulser, for
example, the pulser 35 of FIG. 3, which causes the neutron
source to produce 25 neutron bursts one millisecond apart
followed by a 25 millisecond interval during which no neutrons
are produced. The sync pulse is also shaped by the shaper
circuit 75 and fed to a conventional ]ine amplifiex (not shown)
for transmission to the surface electronîcs along conductor 80
along with the output pulses from the radiation detector (not
shown) in the well logging instrument.
Re~erring now to FIG. 8, there is illustrated in
block diagram a surface electronic system which separates the
sync pulses from the amplified radioactivity detector pulses
and which delivers them to a single shot circuit and an inte-
grator circuit. The well logging conductor cable 80 is con-
nected through resistor 81 to an operational amplifier 82,
the output of which is connecte~ to a multichannel analyzer 83
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0176
and also into a sync separator circui~ 84. The output of the
sync separ~tor circuit 84 is connected to a delay single shot
circuit 85 and also into an integrator circuit 86. The output
o~ the delay single shot circuit 85 is connected to the input
of a width single shot circuit 87 having an output which is
connected into one of the two inputs of an OR gate 88. The
output of the integrator circuit 86 is connected into a delay
single shot circuit 89 having an output which is connected to
the input of a width single shot circui~ 90, which in turn has
an output which drives the other input to the OR gate 88. The
output of the OR gate 88 is also connected to the multichannel
analyzer 83.
The output of the. integrator 86 is also coupled into
the multichannel analyzer 83 as is the inverted output of the
integrator 86 through the inverter circuit 91.
The outputs o the multichannel analyzer 83 are
connected into a,pair of address decoders 92 and 93 which can
be constructed in accordance with applicant's U.S. Patent
4,013,874, issuea March 22, 1977. The outputs of the address
decoders 92 and 93 are connected, respectively, to the counting
; rate meters 94 and 9~. The outputs of the counting rate meters
9~ and 95 are connected to the inputs of a recorder 96. The
outputs of the counting rate meters 94 and 95 are also connected
into a ratio circuit 97 whose output is also recorded by the
recorder 96.
In the operation of the circuitry of FIG. 8, consider-
ed in conjunction with the timing diagram of FIG. 7, it should
be appreciated that the integrator circuit 86 integrates the
25 pulses of each cycle to produce an approximately symmetri-
cal 20 cycle per second square wave. The integrator output con-
trols one-half of the memory storage of the multichannel analyzer
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and the inverted integrator output controls the other half. The
integrator output drives a gate delay single shot circuit 89 which
in turn drives a gate width single shot circuit 90. The width
output of t~le circuit 90 is fed into one input of the OR gate 88
whicn drives the coincidence input of the multichannel analyzer
83. The sync separator circuit 84 drives a similar pair of single
shot circuits which provide a second input of the OR gate 88. The
single shot circuits associated with a sync separator are adjusted
to furnish a pulse that is 600 microseconds wi~e and which begins
350 microseconds after each sync pulse. The single shot circuits
associated with the integrator are adjusted to produce a pulse
that is 24.9 milliseconds wide and which begin one millisecond
after the last sync pulse of each cycle.
Thus, the multichannel analyzer 83 is made to sequen-
tially store in alternate halves of the memory those pulses pro-
du~ed by thermal neutron capture in the time intexval 350 to 950
microseconds after each sync pulse and those pulses produced by
neutron activation wnich occur in the time interval one milli-
second to 25.9 milliseconds after the last sync pulse in each cy-
cle. The two address decoders, fabri~ated in accordance with the
aforementioned U.S. patent no. 4,013j874, decode the address out-
put and drive the count rate meters 94 and 95 which in turn drive
a recorder. The ratio of the count rate meter output is also de-
rlved and recorded. If one decoder, for example, is set to pass
pulses corresponding to gamma rays produced b~ thermal neutron
capture by calcium, and the second is set to pass pulses corres-
ponding to gamma rays produced by the 20 millisecond magnesium
activation, a ratio responsive to the dolomitization of limes~one
may be recorded.
Thus it should be appreciated that there have been
illustrated and described herein the preferred embodiments of a
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new and improved pulsed neturon source finding special utility
in the logging o earth boreholes. F~lrthermore, circuitry has
been provided for utilizing the outputs of detected radiation
emanating from the earth formations as a result of irradiating
such formations with the sources in accordance with the pxesent
invention. The well logging instrument, including the radio-
activity detectors, have not been illustrated since any conven-
tional radioactivity logging instrument can be utilized, for
example, as is illustrated and described with respect to FIG. 1
of applicant's U.S. Paten~ No~ 3,787,686, issued January 22, 1974.
Although only the preferred embodiments of the present invention
are illustrated and described herein, obvious modiications to
these embodiments will occur to those skilled in the art. For
example, while the use of a resistor is contemplated as the means
o providi~g a shunt between the cathode and anode of the pulsed
neutron source, other or additional impedance means may be used.
It should also be appreciated by those in the axt that detection
circuitry such as is described in applicant's U.S. Patent Nos.
3,379,882 and 3,379,884, issued April 23, 1968, can also be used
in the borehole instrument to measure the rate of decline of the
thermal neutron population following the short burst o neutrons
described herein, in addition to the other measurements relating
to either the derivation of an indication o inelastic scatter
gamma rays or those measurements relating to activation logging.
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