US2973444A - Neutron source for well logging apparatus - Google Patents

Description

Feb. 2s, 1961 J. T. DEWAN NEUTRON SOURCE FOR WELL LOGGING APPARATUS Filed April 9, 1952 INVEN'ron.

JOHN' T. DEWAN W MFMGM v ls A oRNEYs.

States 2,973,444 Patented Feb. 2S, 1961 NEUTRON SOURCE FOR WELL LGGING APPARATUS lFiled Apr. 9, 1952, Ser. No. 281,378

S Claims. (Cl. 313-61) The present invention relates to well logging and, more particularly, to a neutron source suitable for well logging.

The primary object of the invention is to provide a source of substantially monoenergetic neutrons which is suiciently compact and rugged to be passed through a borehole for logging subsurface formations.

The invention contemplates providing an ion source, an ion accelerator and a target that includes a substance adapted to react with bombarding ions of sutlicient velocity to produce neutrons. The source, accelerator and target are all contained within an envelope filled with ionizable gas at a pressure such that the mean free path of the ions is of the order of the spacing between the ion source and the target. In the preferred embodiment the ionizable gas is deuterium and the substance in the target is tritium. The gas pressure is chosen sufficiently high to provide an adequate number of ions with which to bombard the target and at the same time low enough to prevent the gas molecules from appreciably retarding the ion beam.

In order that the invention may be more fully understood, it will now be disclosed in detail, reference being had to the accompanying drawing wherein the sole ligure is a View partly in longitudinal section of a neutron source constructed in accordance with the present invention and located in a .housing adapted to `be passed through a borehole traversing earth formations.

Referring to the figure, there is shown a cylindrical pressure-resistant housing 11 supported by an electrical cable 12 and containing a source 13 of neutrons, a radioactive radiation detector 14 and electronic equipment 15 including a high-voltage power supply. The neutron source 13 comprises an ion source 16, an ion beam accelerator 17 and a target 18 contained within an envelope 19 and lled with an ionizable gas at a very low pressure, preferably deuterium at a pressure in the order of 10-4 millimeters of mercury.

The ion source 16 ltakes the form of a Philips ionization gauge, the theory of which is fully described in Vacuum Equipment and Techniques by Guthrie and Wakerling (McGraw-Hill, 1949) on pages 12S-137. The ion source 16 comprises a pair of disc- shaped cathode electrodes 21 and 22 spaced apart along the axis of a magnetic field formed by a permanent magnet 23 mounted outside the envelope 19. A hollow cylindrical anode electrode 24 is disposed substantially coaxially with the magnetic axis between the two cathode electrodes 21 and 22 and preferably extends substantially the full distance therebetween. The anode electrode 24 is maintained at a moderate positive potential, say 500 Volts, with respect to the cathode electrodes 21 and 22 by means of a conductor 25 connected with the power supply in the electronic equipment 15. The cathode electrodes 21 and 22 are connected to the power supply by means of kconductors 26 and 27, respectively. The electrode 22 is preferablyv maintained at a somewhat greater negative potential, say l volts more, than the potential on the electrode 21 and has an aperture 28 therein through which positive ions may project toward the accelerator 17.

In the operation of the ion source 16 a free electron near the cathode electrode 21 tends to be accelerated toward the anode electrode 24 under the action of the electric field therebetween. However, the axial magnetic lield deilects the electron and causes it to move in a tight helix having an axis parallel to the direction of the magnetic i iield. Accordingly, the electron does not reach the anode electrode 24 but approaches the opposite cathode electrode 22 where its direction is reversed and it returns toward the rst cathode electrode 21 where it undergoes a further reversal. This oscillation causes the path of the electron to be many times longer than the interelectrode spacing and affords the electron an opportunity to ionize gas molecules even when the gas pressure is so low that the mean free path of the electron is very long. The aperture 28 in the cathode electrode 22 permits many of the positively charged ions, which in the case of deuterium gas are termed deuterons, to pass out of the ion source 16.

The ion beam accelerator 17 in the path of theion beam emerging from the ion source 16 may take the form of one or more cylindrical electrodes 29 and 31 maintained at progressively higher negative potentials in the direction of travel of the beam by conductors 32 and 33, respectively, connected to the power supply in the equipment 15. The electrodes 29 and 31 likewise serve to focus the ion beam upon the target 1S. The iinal electrode of the ion beam accelerator 17 is a cathode electrode 34, which is illustrated as of cup-like shape facing the aperture 2S in the cathode electrode 22. The potential applied to the electrode 34 by a conductor 35 from the power supply is negative, say in the range from 20 to 100 kilovolts, with respect to the cathode electrode 22.

The target 18 is mounted in the electrode 34, which may be from a foot to several feet from the cathode electrode 22 when the pressure is on the order of 10-4 millimeters of mercury. It is necessary that the spacing between the ion source 16 and the target 18 should not substantially exceed the mean free path of the ions in order to avoid excessive retardation of the ion beam, The mean free paths are approximately live feet and six inches at pressures of lO-4 and 10-3 millimeters of mercury, respectively. The target 18 contains a substance adapted to react with the impinging ions to produce neutrons and preferably contains tritium to react with bombarding deuterons. For example, the target 18 may comprise a tungsten disk coated with zirconium impregnated with tritium.

The reaction of the accelerated deuterons on the tritium yields substantially monoenergetic neutrons having energies of approximately 14 million electron volts. The reaction resulting from deuteron particles bombarding a tritium-bearing target is preferable, but other reactions producing neutrons may be employed. For example, deuteron particles bombarding a deuterium-bearing target will produce substantially monoenergetic neutrons at approximately 2.5 million electron volts. The neutron flux intensity, however, is lower than in the preferred reaction with the same ion current and accelerating voltage.

The neutron flux intensity is dependent upon the number of deuterons in the bombarding beam and may be increased also by increasing the negative potential on the cathode electrode 34.

Since the ion source 16, when energized, uses up the gas molecules in the envelope 19 and unwanted gas may leak in, the envelope 19 must occasionally be reevacuated and relled with the desired gas at the appropriate pressure. For `this purpose a plug 36 is provided in the housing 11 opposite a nipple valve 37 in the envelope 19. In order to prevent depletion of an appreciable proportion of the gas molecules during a single logging run in a borehole, the volume of the gas is preferably as vlarge as possible. The greater the gas volume the'more gradual will be the increase in the gas pressure and-the resulting retardation of the ion beamY caused by a leak in the envelope. Y Y- Y Y 'The neutron source 13 is quiescent at all times except when the housing 11 is within a borehole opposite earth formations which the operator desires to log. At such time the electronic equipment 15 is energized by an alternating'current power source (not shown) outside the borehole; The apparatus may then -be t employed in accordance with the logging methods disclosed by Clark Goodman in copendingapplication Serial No. 275,932, entitled Neutron Well Logging and tiled March l1, 1952; A shield 38 is located between the target 18 and the radioactive radiation detector 14., The soft beta emission associated with the decay of tritium is easily absorbed by the housing 11.

t Since the invention may assume many dilerent forms, the illustrated embodiment is merely exemplary and the Scope of the invention is limited only by the appended claims. Y f

Iclaim: Y Y

1. A neutron source suitable for well logging comprising an ion source including cathode means and anode means, means for lengthening the path of the electrons passing between said cathode means and said anode means, an ion accelerator, a target including a substance adapted to react with impinging ions to produce neutrons, and a sealed envelope containing said ion source, said accelerator and said target and lilled with ionizable gas at a pressure such that the mean free path of the ions is of the order of the spacing between said ion source and said ion target. v

2. A neutron Isource suitable for well logging comprising a deuteron source including cathode means and anode means, means for lengthening the path of the electrons passing between said cathode means and said Vanode means, a deuteron accelerator, a target including tritium adapted to react with impinging Vrdeuterons to produce neutrons, and a sealed envelope containing said deuteron source, said deuteron accelerator and saidV target and filled with detiterium gas at a pressure such that the mean free path of the deuterons is of the order of the spacing between `said deuteron source and said target.

3. A neutron source suitable for well logging comprising an ion source for generating a beam of ions including means for establishing an axial magnetic field, an anode electrode substantially coaxial with the magnetic axis, a pair of cathode electrodes spaced apart along the v magnetic axis, one of said cathode electrodes being maintained at a potential more negative than the potential on the other cathode electrode vand having an aperture therein through which a beam of ions may project; an ion beam accelerator for accelerating the beam of ions; a target containing a substance adapted to react with impinging ions to produce neutrons; and an envelope Vcontaining the electrodes of said ion source, said ion accelerator and said ion target and filled with ionizable gas at a pressure suciently high to provide asupplypof ions and yet suiciently low to offer little retardation of the ion beam impinging on said target. Y

4. A neutron source suitable for well logging comprising an ion source for generating deuterons including means for establishing an axial magnetic iieId, an anode electrode substantially coaxial with the magnetic axis, a pair of cathode electrodes spaced apart along the magnetic axis, one of said cathode electrodes being maintained at a potential more negative than the potential on the other cathode velectrode and having an aperture therein through which deuterons may project; an ion accelerator :for accelerating thedeuterons that'hproject" through the apertured cathode electrode; an ion target containing tritium gas adapted to react with impinging deuterons to produce neutrons; and an envelope containing the electrodes of said ion source, said ion accelerator and said ion target and iilled with deuterium gas at a pressure such that the mean free path of the deuterons is of the order of the spacing between said ion source and said target. i

5. A neutron source suitable for well logging comprising an ion source for generating deuterons includingV means for establishingpan axial magnetic eld, an anode electrode substantially coaxial with the magnetic axis, a pair of cathode electrodes spaced apart along the magnetic axis, one of said cathode electrodes being maintained at a potential more negativeV than the potential on the other cathode electrode and having an aperture therein through which deuterons may project; an ion accelerator for accelerating the deuterons that project through the apertured cathode electrode; an ion target containing tritium gas adapted to react with impinging deuterons to produce neutrons; and an envelope containing the electrodes of said ion source, said ion accelerator and said ion target and iilled with deuterium gas at a pressure of the order of 104 millimeters of mercury the distance between said apertured cathode electrode and said ion target being approximately equal to the mean free path of said deuterons in said deuterium gas at said pressure. Y

6. A neutron source comprising an annular permanent magnet, a sealed envelope extending axially through said magnet and containing a deuteron source, a target, a deuteron accelerator and a deuterium gas, said deuteron source including a pair'of circular cathode electrodes spaced apart coaxially of the magnetic iield set up by said magnet, and a hollow cylindrical anode electrode spaced inwardly of said magnet and coaxially with said magnetic eld and extending between said cathode electrodes,V said anode electrode being at a positive potential with respect to the potential on said cathode electrodes in order to ionize said deuterium gas in said envelope and produce deuterons, one of said cathode electrodes having an aperture formed therein through which deuterons may be projected, said target extending inwardly from one end of said envelope in confronting relation to said apertured cathode electrode and spaced from said apertured cathode electrode a distance on the orderY of but not exceeding six inches, said target containing tritium fior reaction with said deuterons to produce high energy neutrons, said deuteron accelerator including at least one annular accelerating. electrode supported in axial alignment intermediate said apertured cathode and said target, said accelerating electrode being maintained v at a relatively'high negative ptoential with respect to the potential on said cathode electrodes to acceleratezsaid deuterons for bombardmentV of Vsaid target, said deuterium gas contained in said envelope being at a pressure on the order of l0*3 millimeters of mercury. Y

7.'A neutron source suitable for Well logging comprising a pressure resistant housing adapted to be disposed in awell bore, an Vannular permanent magnet mounted inside of said housing, a sealed envelope extending axially through said magnet and containing a deuteron source, a target, a deuteron accelerator, and a deuterium gas, said deuteron source including a pair of circular cathode electrodes spaced apart coaxially of the magnetic field setv up by said permanent magnet, one ofV said cathode electrodes having an aperture formed therein through which deuterons mayA be projected, and a hollow cylindrical anode electrode spaced inwardly of said permanent magneti and coaxially withfsaid magnetic eld'and extending Vessentially thefull distance between` said cathode electrodes,

said anode electrode being at a positive Vpotential Withire-t` said apertured cathode electrode and spaced from said apertured cathode electrode a distance on the order of but not exceeding six inches, said target containing tritium for reaction with said deuterons to produce high energy neutrons, said deuteron accelerator including a cathode electrode within which said target is mounted and having a relatively high negative potential applied thereto with respect to the potential on said cathode electrodes of said deuteron source, said deuterium gas contained within said enevelope being at a pressure sutliciently high to provide a supply of deuterons and yet suliciently low to offer little retardation of the deuterons impinging on said target.

8. A neutron source suitable for Well logging comprising a pressure resistant housing adapted to be disposed in a Well bore, an annular permanent magnet mounted insidesaid housing to provide an axial magnetic field, a sealed envelope extending axially through said magnet and containing a deuteron source, a deuteron accelerator, a target and a deuterium gas, said deuteron source including a pair of at circular cathode electrodes spaced apart coaxially of said magnetic field, one of said cathode electrodes having an aperture formed therein through which deuterons may be projected, and a hollow cylindrical anode electrode spaced inwardly of said magnet and coaxially with said magnetic eldand extending substantially the full distance between said cathode electrodes, said anode electrode being at a positive potential with respect to the potential on said cathode electrodes in order to ionize said deuterium gas and produce deuterons, said target extending inwardly from one end of said envelope in confronting relationship to said apertured cathode electrode and spaced from said apertured cathode electrode a distance on the order of but not exceeding six inches, said target containing tritium for reaction with said deuterons to produce high energy neutrons, said target being at a high negative potential with respect to the potential on said cathode electrodes, said deuteron accelerator including at least one annular accelerating electrode supported in axial alignment intermediate said apertured cathode electrode and said target, said annular accelerating electrode being at a high negative potential relative to the potential on said cathode electrodes in order to accelerate said deuterons for bombardment of said target, and said deuterium gas contained in said envelope being at a presure on the order of 10*3 millimeters of mercury.

References Cited in the le of this patent UNITED STATES PATENTS 2,211,668 Penning Aug. 13, 1940 2,287,619 Kallmann et al June 23, 1942 2,287,620 Kallmann et al .Tune 23, 1942 2,297,416 Kallmann et a1 Sept. 29, 1942 2,489,436 Salisbury NOV. 29, 1949 2,507,652 Smith May 16, 1950 2,600,151 Backus June 10, 1952 2,689,918 Youmans Sept. 21, 1954 2,712,081 Fearon et al. June 28, 1955 OTHER REFERENCES Sourcebook on Atomic Energy, by Samuel Glasstone, 'published by Van Nostrand Co., copyright 1950, pages A250 to 257. i

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