CA1043020A - Pulsed neutron generator using shunt between anode and cathode - Google Patents

Abstract

Abstract of the Disclosure. A pulsed neutron generator for well logging is provided having a resistor connected between the anode and cathode. In an alternative embodiment, one secondary coil of a pulsing transformer is connected in series with a resistor between the anode and cathode. In an alternative embodiment of a corona regulator in series with the collector-emitter of a transistor is connected between the cathode and anode of the neutron source and the base drive to the transmitter is provided by a light-responsive solar cell activatable by an external lamp. Circuitry is provided for utilizing the various neutron sources.

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Description

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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 o~ 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. 3,309,5~2 to Arthur Ha Youmans et al and those illus*rated in my U.S. Patent No.
3,787,686. It is also known that such 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 i~ the pulsing frequency is much low~r than 1000 cycles per second. Although this characteristic is of little or no consequence while pulsing the generator at that rate or highex, it has been put to use in the "subtraction method'~
o detecting gamma rays produced by inelastic scattering of fast neutrons as descirbed in my aforementioned Patent No. 3,787,686.
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; This tendency to pre-ignite or to go into a steady state conduction presents a disadvantage whenever it is desired to make loggin~ runs which provide short half-li~e a~ti vation measurements, i.e., those which might occur several milli-seconds after the termination of ~he short burst of ast 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 atmas-phere substantially composed of a heavy isotope of hydrogen. Means - are provided to ionize the atmosphere and a target containing a heavy isotope of hydrogen is arranged to receive atmosphere ions.
Pulsing means are interconnected with the ionization means to periodically ionize the atmosphere.

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- 1~43~20 ., These and other objects, features and advantages of the present invention will be more readil~ appreciated from the following d~tailed specification and drawings, in which:
FIGs. 1 and 2 are schematic diagrams of pulsed . neutron sources in the prior art;
FIG. 3 is a schematic diagram of a pulsed 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 the pres-ent invention;
FIG. 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 . the invention; and - FIG. 8 is a block diagram of circuitry in accor-dance with the present invention which is utilized at the eart~'s surface as a part of a well logging operation.
Referring now to the drawings in more detail, especially to FIG. lj there is illustrated a pulsed neutron souroe whi~h is built in acc~rdance with th~ prior art. ~or ,~ .

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3q~ o example, such a neutron sou~ce is fully described in the U. S.
Patent No. 3,309,522 which issued on March 14, 1967 to Arthur H, Youmans et al, and which is ass~gned to the asslgnee of the present invention. In brief, the neutron source 10 of FIG. 1 includes an accelerator tube 11 ha~ing 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 ~an de ~raaf high yoltage generator. A belt-shaped target 15, generally formed of a thin strip of titanium,and impregnated with either deuterium or tritium, or a mixture oP both, is formed on the inslde 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 suppressor ~; rings 16 by way o~ 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. The pulse generator 17 is adapted to supply a s~equence o~ negatlve pulse$ through the capacitor 18 to the suppressor rings 16 at a ~i~ed ~reselected frequency, or at a ~ate det'ermlned by control app~ratus generally located at the su~face.' ~ince the'oper'atlon o~ the neutron source in accordance ~ith FIG. 1 is fully described in the aforementioned U. S. Patent No. 3,309,522 and also in my U. S. Patent No.
3,787,686 which issued on ~anuary 22, 1974, and which is also assigned to the assignee of the present invention, further description of this prior art neutron source need not be given here.
Referring now to ~I~. 2, there is illustrated another prior art neutron source whlch is also described ln the afore ;.

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mentioned U. S. Patent No. 3,309,522, and also in my U. S.
Patent No. 3,787,686. With this particular neutron source, a pulse generator 20 is provided whlch supplies negati~e pulses through a coupling capacitor 18 to the suppressor ring 16 but 5 which also supplies positi~e pulses to the electrode 21 to further aid in the pulsing of the neutron source.
As fully explained in the a~orementioned U. S. Patent No. 3,787,686, the neutron sources Or FIG.'s 1 and 2 have a common characteristic, i.e., i~ the pulse repetition rate is too low, the neutron source pre-ignites and goes into a con-tlnuous mode and is no longer operating as a pulsed neutron source. ~hile 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, ~or example, in actlvation logging.
Referring now to FI~. 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-ignitlon. The neutron source 30 is substantially identical to that o~ the circuit o~ G. 2 e~cept ~or ha~ing 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 ~ixed to the inside sur~ace of the source. Since the space between the tip ; o~ 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 flo~ between the cathode and ground (except ~or the beam current) will be concentrated between the corona point 34 and the nearest sur~ace of the cathode 33.
Since the corona point looks at the cathode and the 3~)20 anode ls connected to the high ~oltage termlnal o~ the Van de Graaf generator, the corona current will thus flow through the resistor 31. The value of the reslstor 31 is determined by the firing voltage of the ion source and the current available ~rom the Yan de Graa~ 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 o~ the ion source.
Then, With all control elements ad~usted for normal source operation but with the pulser 35 disabled, all current rrom the Yan de Graa~ 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 o~ appropriate amplitude from the pulser 35 is applied to the pulsing electrode 36, the voltage o~ the anode ~ 15 32 ~ith respect to the cathode 33 increases until the lon 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 ~an de ~raa~ and the next high ~-oltage pulse causes the cycle to repeat. It should be appreciated that the system can remain energized ~or an almost inde~ini~e period of tlme be~ore a high yoltage pulse ls ap~lied to cau~e a burst of neutrons to result.
This is in sharp contrast to the prior art embodiments of FIG.'s 1 and ~ 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.
~ ith the source acco~ding to FIG. 3, the wldth of the neutron burst is determined by~ the combined ef~ects of the high ~oltage pulse shape and amplitude, ~an de Graaf charging current and the ~alue o~ the shunt reslstor 31. The source 30 o~ FIG. 3 ; may be pulsed at a frequency as lo~ as desired to as high as ~L~P43~2C~
perhaps 10,000 cycles per second. It should also be appreciated that the pulsing frequency can be modulated at some lower frequency to provide a cycle appropriate for almost any measure-ment that one might wish to make.
Referring no~ to ~. 4, there is illustrated an alternatl~e embodiment of the present in~ention 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 ~ollowing differences. A positi~e trigger pulse is coupled into the anode 41 by means of trans-~ormer 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 fa~orable circumstances, adequate coupling m~y be e~ected without a magnetic core. The core 42, which may ~orm the support column for the upper and lower pulleys 43 ~ and 44 of the ~an de Graaf generator~ also supports a primary - winding 45 o~ the transformer ~hich ls connected to recei~e trigger pulses from any suitable pulse generator 46 and a 2Q secondar~ ~inding 47 ~hich is connected between the anode 41 and the cath~de 48. ~ current limitlng resistor 49 is connected ln ser~es with`the secondar~ coil 47.
In the operation of the source of FIG. Il, it should be appreciated that the shunt resistance of the secondary coil 47 and resistor 49 function much like the resistor 31 in ~IG. 3 which allows the source 40 to remain off until pulses are generate~ by the pulse generat~r 46.
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Referring now to FI~. 5, there is illustrated an ; alternati~e embodiment of the present lnYention wherein a neutron source 50 is ~abricated essentially llke the prior art sources of FIG.'s 1 and 2 except for the ~ollowing differences~
The anode 51 is connected to the hlgh voltage side of the ~an .

~L~)43~Z~;) de Graaf generator 52. 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 Victoreen GV 1 series ~Victoreen Instrument Co., Cleveland, Ohio) of appro-priate value in series with the collector~emitter of transistor 56 wherein one side of the corona regulator 55 is tied to the anode 51 of ths source 50 and the emitter of transistox 56 is tied to the cathode 54 of the source 50~ The base of transistor 56 is tied to one or more small area silicon solar cells 57, for exam~
ple, the Centralab 58C (Centralab Electronics Division, Globe Union Inc., Milwaukee, Wisconsin), which in turn are connected through a current limiting resistor 58 to the cathode 54. The ;~
corona regulator 55 is selected to have a regulating voltage about 50 to 100 volts below the firing voltage of the ion source.
A light source 59 with a focusing lens is situated within the tank opposite the solar cells 57L 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 desiredO If used, the pulses 61 is connected to the pulsing electrode 62 and also to the suppressor ~ r~ngs 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, if the light source S9 is extinguished, there is no base current for the transistor 56 generated by the solar cells 57 and the source wilI operate substantially as shown in my aforementioned Patent No. 3,787,686. If the light source 59 is actiYatedr transistor .~ , 56 is caused to conduct by the base current generated by the solar cells 57 and the Van de Graaf current flows through the corona 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 disa~led and no neutrons will be produced. Then, when the light source is again extinguished, the source rw/~' ~

9L~43~:0 will operate as though there ~ere no shunt. With a ~ast responding light source, the ion source is caused to generate short bursts of ions which produce correspondingly short bursts of neutrons. However, in con~unction with the pulser 61, the source may be operated in the nor~lal m~nner when short bursts of neutrons at a periodic rate are required and the shunt transistor 56 operated when longer on-o~f periods are desired.
Add~tionally, the two can be operated in conjunction, i.e., the pulser 61 used in con~unction with the lamp power source 60, to produce intervals where no neutrons are produced interspersed wlth intervals where short neutron bursts are produced at a cyclic rate. It should be appreciated that the corona regulator 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 polnt more rapidly than if the capacltance were completely discharged. This also allows a lower voltage transistor to be used.
~t should thus be appreciated that the neutron sources built in accordance with the varlous embodiments of the present lnYention produce vastly ~ore ~lexible capabillty. As an e~ample o~ the mea6urements that can be made, the ~eactions 4Mg (n,p)24Na, 27Al(n,a)24Na and 23Na(n,r~2~Na produce a 470-kev gamma ray ~ith a 20 millisecond half life. The reaction - 25 cross sections for these reactions are 48~ 33 and 400 millibarns,respectively. The cross section-abundance product ~or magnesium is sufficient to allow the measurement to be valuable in lithology identification of ~ormation rock. Prior to this development, neutron sources have not been available that could be operated ~ith an on-o~f cycle appropriate for the selective detection of these elements.
~eferring now to ~I~. 6, a circuit is pro-vided for : :

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pulsing the source in accordance with the ~arious embodiments of the present in~ention. A pulse generator 70 is set to pro-duce a 10 microsecond wide negati~e pulse at the rate of 1000 such pulses per second and which are Ped into a divide-by-50 circuit 71 and also to an inverter 72 which drives the sync width slngle shot circuit 73. The outputs of the divide-by-50 circuit 71 and the single shot eircuit 73 are coupled into the input o~ a NAND gate 74. The dlvide-by-50 circuit 71 is set to trigger on the leading edge of the generator pulse ~rom the generator 70 and the singlé shot clrcuit 73 triggers on the trailing edge o~ the in~erted generator pulse. Thus, the leading edge o~ the single shot output is delayed by about 10 microseconds with respect to the leading edge o~ the divide-by-50 output. These two outputs are ~ed to the NAND gate 74 which produces 25 rull-width sync pulses one millisecond apart followed by a 25 millisecond interval in which there are no syne pulses. ~IG. 7 pro~ides a timing dlagram o~ this circuit. The sync pulses So produced drlve the high Yoltage pulser, ~or example, the pulser 35 of ~I~. 3, ~hich causes the neutron source to produce 25 neutron bursts one millisecond apart Pollowed by a 25 miIlisecond inter~al during ~hlch no neutrons a~e produced. The~s~nc pulse is also shaped by the shaper elrcuit 75 and ~ed to a con~entlonal line ampli~ier (not shown) or tran~mi~ion to:the sur~ace electronics along conductor 80 along ~ith the output pulses ~rom the radiation detector (not shown~ in the well logging instrument.
~ eperring now to FI~. 8, there is illustrated in block diagram a sur~ace electronic system which separates the sync pulses from the ampli~ied rad~oactivity detector pulses and which deli~ers them to a slngle shot circuit and an integrator circult. The ~ell logging conductor cable 80 is connected through resistor 81 to an operational ampliPier 82, ~43~3Z(~
the output of which is connected to a multichannel analyzer 83 and also into a sync separator circuit 84. The output of the sync separator circuit 84 is connected to a delay single shot circuit 85 and a~so into an integrator circuit 86. The output of 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 circuit 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 mul-tichannel analyzer 83.
The output of the integrator 85 is also coupled into the multichannel analyzer 83 as is the inverted output of the integxator 86 through the inverter ci_cuit 91.
The outputs of 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, issued March 22, 1977. The outputs of the address decoders 92 and 93 are connected, respectively, to the counting rate meters 94 and 95. The outputs of the counting rate meters 94 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 recoxded by the recorder 96, In the operation of the circuitry of FIG. 8, con-sidered in conjunction with the timing diagram oE 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 the circuit 90 is fed into one input of the OR gate 88 which 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 wide and which begins ; 10 350 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 wnich begln one millisecond after the last sync pulse of each cycle.
Thus, the multichanneI analyzer 83 is made to sequen-- tially store in alternate halves of the memory those pulses pro-duced by thermal neutron capture in the time interval 350 to 950 - microseconas 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. Tne two address decoders, fabricated in accordance witi~ the - a~orementioned U.S. patent no. 4,013,874, 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-rived and recorded. If one decoder, for example, is set to pass pulses corresponding to gamma rays produced by thermal neutron capture by calcium, and the second is set to pass pulses corres-ponding to gamma rays produced b~ the 20 millisecond ma~nesium - activation, a ratio responsive to the dolomitiæation o~ limestone may be recorded.
~-30 Thus it should be appreciated that there have been illustrated and described herein the preferred embodiments o~ a ~1 ,. . ' , r~7/

3L'~43~Z~) new and improved pulsed neturon source finding special utility in the logging oE earth boreholes. Furthermore, 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 present invention. The well logging instrument, including the radio-activity detectors, have not b~en illustrated since any conven tional radioactivity logging instrument can be utilized~ for example, as iq illustrated and described with respect to FIG. 1 of applicant's U.S. Patent No. 3,787,686, issued January 22, 1974.
Although only the preferred embodiments- of the present invention are lllustxated and described herein-, obvious modifications to these embodiments will occur to those skilled in the art. For examplel while the use of a resistor is contemplated as the means of providing a shunt between the cathode and anode of the pulsed neutron ~ource, other or additional impedance means may be. used.
It should also be appreciated by those in the art that detection circuitry such as is described in applicant's U.S. Patent ~05 .
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 of neutrons described hsrein, in addition to the other measurements relating to either the derivation of an indication of inelastic scatter gamma rays or those measurements relating to activation logging. .

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A pulsed neutron source, comprising:
an ion beam accelerator having an anode and a cathode and corona current shunting circuitry between said anode and said cathode, said accelerator also having a static atmosphere substantially composed of a heavy isotope of hydrogen, means to ionize said atmosphere and a target containing a heavy isotope of hydrogen arranged to receive atmosphere ions;
and pulsing means interconnected with said ionization means to periodically ionize said atmosphere.

2. The pulsed neutron source according to claim 1 wherein said shunting circuitry comprises a resistor.

3. The pulsed neutron source according to claim 1 wherein said shunting circuitry comprises an inductor.

4. The pulsed neutron source according to claim 3 wherein said shunting circuitry also comprises a resistor.

5. The pulsed neutron source according to claim 1 wherein said shunting circuitry comprises a transistor.

6. The pulsed neutron source according to claim 5 wherein said shunting circuitry also comprises a corona regulator in series with said transistor.

7. The pulsed neutron source according to claim 6, including in addition thereto, means for providing base current to said transistor as a function of a light source.

8. A pulsed neutron source, comprising:
an ion beam accelerator having an anode and a cathode and corona current shunting circuitry between said anode and said cathode, said accelerator also having a static atmosphere substantially composed of a heavy isotope of hydrogen, means to ionize said atmosphere and a target containing a heavy isotope of hydrogen arranged to receive atmosphere ions; and pulsing means interconnected with said ionization means to periodically ionize said atmosphere, said pulsing means comprising:
a pulse source; and a transformer having a primary coil connected to said pulse source and a secondary coil forming at least a portion of said shunting circuitry.

9. The pulsed neutron source according to claim 8 wherein said shunting circuitry also comprises a resistor in series with said secondary coil.