CA1050176A - Pulsed neutron generator using shunt between anode and cathode - Google Patents

Abstract

PULSED NEUTRON GENERATOR USING
SHUNT BETWEEN ANODE AND CATHODE

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, the secondary coil of a pulsing transformer is connected in series with a resistor between the anode and cathode. In an alternative embodiment, 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.

Description

~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.

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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 rw/i~'',,.'~

~5~6 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-pb/ - 3 -~SI:)~7~
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 pb/ - 5 -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 P~/

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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 nh /

~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 P~/ .

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 ~ ' P~ / .

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 tO

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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 r =_ ~
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~L 135~L'76 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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Claims (5)

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 means to vary the effective impedance of said shunting circuitry.

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

3. The pulsed neutron source according to claim 2 wherein said shunting circuitry also comprises a corona regulator in series with said transistor.

4. The pulsed neutron source according to claim 3, including in addition thereto, means for providing base current to said transistor as a function of a light source.

5. The pulsed neutron source according to claim 1, including in addition thereto, pulsing means interconnected with said ionization means to periodically ionize said atmosphere.