US3364355A - Neutron generator with occluded gas ion source - Google Patents

Description

2 Sheets-Sheet l I... RUBY ETAL ATTORNEY S r S N E m E W T Y H NBHCH E U m E T V R T R E E N m E B N 0 c U E N V H K E P R Y E E WNSL AOOA LTJD H/ 2 W 4 2 2 V 3 8 I A] M A Q B 3 L A O 5 A f 6 4 I m 3 L hvilii- 1 NEUTRON GENERATOR WITH OCCLUDED GAS ION SOURCE Jan. 16, 1968 Filed July 13, 1965 PULSE POWER //4 SUPPLY PULSE POWER SUPPLY Jan. 16, 1968 L. RUBY ET AL NEUTRON GENERATOR WITH OCCLUDED GAS ION SOURCE 2 Sheets-Sheet 2 Filed July 13, 1965 PULSE FORMING NETWORK POWER SUPPLY R E G G R T GENERATOR PULSE FORMING NETWORK SUPPLY INVENTORS LAWRENCE RUBY TONY VULETICH JOSEPH B. RECHEN DALE KENNETH WELLS B /flwmqw ATTORNEY United States Patent 3,364,355 NEUTRON GENERATOR WITH OCCLUDED GAS ION SOURCE Lawrence Ruby, Berkeley, Tony Vuletich, Piedmont, Jo-

seph B. Rechen, Berkeley, and Dale Kenneth Wells, Concord, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed July 13, 1965, Ser. No. 473,568 11 Claims. (Cl. 25084.5)

ABSTRACT OF THE DISCLOSURE Ion source including an evacuated cylindrical insulator having paralleled metallic film strips deposited on the interior walls thereof, said strips interrupted by gaps. On application of an electrical current pulse, to form an arc across said gaps, gas occluded on said strips is released and ionized to provide a cloud of ions.

The present invention relates generally to ion sources and more particularly to a source of the occluded gas type which is particularly adapted for releasing a rapid pulse of ions.

Occluded gas sources are a particularly convenient means for obtaining ions without necessitating the use of heavy, cumbersome apparatus ordinarily associated with other types of ion sources. For example, the usual sealed-tube pulsed neutron generator, as utilized in geophysical surveys such as oil-well logging, requires a bulky storage system to provide gas for conversion into ions. In contrast, an occluded gas ion source can be relatively small and lightweight, and is well adapted for providing a sudden pulse of ions of controllable and consistent intensity.

To prepare an occluded gas source, a metal such as titanium or erbium is heated and then cooled in an atmosphere of the desired gas. During cooling, the metal absorbs large quantities of the gas. Such gas is retained in the metal, even in a vacuum, but the gas can be readily released by reheating the metal.

The occluded gas ion source of the present invention may readily be used as a pulsed neutron generator, the ionized gas from the source being accelerated to a high velocity and directed toward a target which also contains occluded gas. Either the target or the source contains occluded deuterim while the other contains occluded tritium. When the ionized gas from the source is accelerated into the target, the tritium combines with the deuterium to form helium and a neutron (H +H He +n). The neutrons readily pass through the walls of the generator and are available for use externally.

A form of occluded gas ion source is disclosed in US. Patent No. 2,786,143, issued Mar. 19, 1957, to Ruby et al. and in the periodical, Review of Scientific Instruments, vol. 31, No. 3, March 1960, pages 235-240 an occluded gas pulsed neutron source is discussed. The sources described in such publications and the present invention all utilize metallic electrodes of occluding a gas and each including a spark gap. A high voltage power supply is connected across the electrodes and an arc is caused to form across the gap, generating heat at the gap and releasing ionized gas. Generally, the high voltage is applied in a very short pulse and the quantity of ionized gas released is predictable for several hundred pulses.

The present invention is advantageous over the previous devices in that both the quantity of released gas and the position at which the gas is released are very consistent in the present invention. Such advantages in the present invention are obtained by utilizing a novel configuration in which the gas occluding elements, spark lab Lil

forming elements and accelerating field means are all formed by deposits plated on the inside surface of a compact insulative cylinder.

It is an object of the present invention to provide a pulsed ion source of the occluded gas type having a physical configuration providing highly improved performance relative to previous sources of such type.

It is another object of the present invention to provide a physically rugged and shock resistant pulsed ion source.

It is another object of the present invention to provide a pulsed ion source in which the quantity of ionized gas released is accurately predeterminable.

It is another object of the present invention to provide a pulsed ion source which is convenient and inexpensive to construct.

It is another object of the present invention to provide a pulsed ion source in which the location at which gas will be released is accurately predeterminable.

It is another object of the present invention to provide a pulsed ion source in which potential vacuum leaks are avoided by eliminating electrical wiring feedthroughs.

The invention will best be understood by reference to the accompanying drawing of which:

FIGURE 1 is a longitudinal section view of one embodiment of the pulsed ion source,

FIGURE 2 is a cross-sectional view taken along line 22 in FIGURE 1,

FIGURE 3 is a longitudinal section View of an alternate embodiment of the pulsed ion source,

FIGURE 4 is a cross-sectional view taken along line 44- in FIGURE 3, and

FIGURE 5 is a broken-out section of a pulsed neutron tube embodying the pulsed ion source of FIGURE 1.

Referring now to FIGURES l and 2, there is shown a cylinder 11 of insulative ceramic material. A thin band of conductive material 12 is plated in :a ring on one end of the cylinder 11. The thickness of the plating shown in the drawings is greatly exaggerated so that the details thereof are more readily apparent. A similar band 13 is plated on the opposite end of the cylinder 11 and the bands are connected to opposite sides of a pulse power supply 14. In typical operation, a 1000-volt pulse is applied across the bands 12 and 13, and the length of the cylinder 11 must be sufficiently long to avoid arcing between the bands.

A plurality of metallic strips 16 are plated on the inside surface of cylinder 11 and extend in a longitudinal direction thereon, the metal being of a type which Will occlude substantial quantities of gas. Titanium is suitable for this purpose. One end 17 of each strip overlaps and electrically contacts band 12. A narrow slit 18 approximately one mil wide (0.001 inch) is provided near the midpoint of each strip 16. A resistor 19 is plated on the cylinder 11 between the band 13 and the free end 21 of each of the metallic strips 16, the ends of each resistor overlapping the strips and the band to obtain electrical contact therewith. The resistance value of each resistor 19 is not critical, but for the particular embodiment of the invention described, the optimum resistance is about forty ohms.

In the preparation and operation of the invention a gas such as deterium or tritium is loaded into the metallic strips 16 by occlusion. The assembly is then enclosed in a vacuum chamber 22 as indicated by a dashed line in FIGURE 1. A pulse of current applied across the parallel connected strip 16-resistor 19 combinations causes an arc across each gap 18. The current is caused to divide essentially equally across each gap 18 owing to the presence of the resistors 19. If, for instance, the imped ance at one gap 18 should be lower than the impedance at the other gaps 18, the voltage drop across the resistor 19 in series with the low impedance gap 18 will be higher than across the other resistors 19, thus reducing the voltage across such low impedance gap. Conversely, a higher potential will be applied across any high impedance gap 18. Therefore, the currents across all the gaps 18 are caused to be approximately equal. The heat of the are formed across each gap l8 causes the occluded deuterium to be released. Since the heating at the various gaps 18 is approximately equal, the quantity of deuterium released at each of the gaps is approximately the same and the total quantity of gas released for each pulse is accurately controlled. Also, the gaps 18 can be provided at any desired location along each strip 16, so the region within the cylinder 11 at which the gas is released is accurately predeterminable.

Another embodiment of the invention is shown in FIGURES 3 and 4 wherein an insulative cylinder 31 is provided with a first and a second conductive band 32 and 33 at opposite ends thereof. A band 34 of gas occluding metal is plated completely around the inside of the cylinder 31 coaxially therewith and approximately midway between the bands 32 and 33. A first plated contact strip 36 is formed between the first band 32 and the gas occluding band 34. A second plated contact strip 37 is plated on the inside surface of the cylinder 31, 180 degrees from the first strip 36, and electrically connecting between the second conductive band 33 and the gas occluding band 34. A pulse power supply 38 is electrically connected between the first and second conductive bands 32 and 33. The cylinder 31 is disposed within an envelope forming a vacuum chamber 39. Two electrically parallel current paths are provided between contact strip 36 and contact strip 37 by the gas occluding metal band 34. One or more are gaps 40 can be provided in each half of the metal band 34. Operation of the ion source is essentially similar to the ion source of FIGURES 1 and 2.

The ion source of FIGURES l and 2 is shown in FIG- URE 5, modified to constitute a neutron generator 49. The thickness of the plating on the cylinder 11 as shown in FIGURE is not exaggerated as in FIGURES l to 4. In FIGURE 5, an electrically conductive end cover 51 is utilized to provide a vacuum seal across one end of the cylinder 11 and is soldered or welded to the band 13. An electrically conductive ring 52 is similarly soldered or welded to band 12, the ring being folded back over a portion of the outer surface of cylinder 11 and ending in an outwardly projecting flange 50. All of the elements of the neutron generator 49 are disposed coaxially with respect to cylinder 11. A grid support ring 53 is soldered or welded coaxially to the ring 52 and has a central aperture 55 in which a grid 54- or open mesh is provided across the open end of the cylinder 11. A target support ring 56 is attached to the outwardly projecting flange 50 of the ring 52, the support ring being bonded to an insulative glass cylinder 57 which in turn is bonded to a target cap 58. The target cap 53 is spaced from the grid 54 and provides a vacuum tight closure over the open end of the cylinder 11. A target 59 having occluded deuterium or tritium therein as previously discussed is afiixed to the center of the inside surface of the target cap The pulse power supply 14, which provides current for the gas occluding strips 16, is connected from the ring 52 to the end cover 51. Internally within the pulse power supply 14- there is a DC. power supply 61 connected through a current limiting resistor 62 to a pulse forming network 63. One side of the pulse network output is connected direct to end cover 51 while another side of the output is connected to the anode of a switch tube 64, such as a thyratron. The cathode of the tube 6 is connected to the ring 52 so that when conduction of the tube 64 is triggered on, a current pulse is created across the gaps 18. A pulse output signal from a trigger generator 66 is applied to the control electrode of the tube 6 through an input transformer 67. Operation of the trigger generator 66 thus controls the instant at which ions of the occluded gas are released from the strips 16.

Simultaneously with the release of the gas, the trigger generator causes a negative voltage pulse to be applied to the target 59, thereby accelerating toward the target ions released from the strips 16. To create such target pulse, a target pulse power supply 65 including a DC, power supply 68, pulse forming network 63, resistor 71, and thyratron tube 72 are provided as in pulse power supply 14, the tube '72 being triggered on through an input transformer 73 by the same control pulse from the trigger generator as is used to trigger the pulse power suppl 14. Output pulses from the target pulse power supply 65 are applied through a step-up autotransformer 74 across the target 59 and the ring 52 the target being provided with a relatively negative potential.- Thus a high potential of approximately 150,000 volts is established between the grid 54 and the target 59, causing ionized gas particles released from the strips 16 to be accelerated toward the target 59 and causing the nuclear interactions previously discussed. This in turn causes a pulse of neutrons to be emitted from the target 59.

Each of the two embodiments of the ion source, i.e. that of FIGURE 1 and that of FIGURE 3 provide advantageous operation over the other in certain respects The embodiment of FIGURES 3 and 4 is the more corn pact and is particularly designed to minimize the chance of a spurious are which would cause current to lay-pass the gas occluding metal band 34, In particular, the em bodiment of the invention shown in FIGURES 3 and 4 has been designed so that the contact strips 36 and 37, across which the full power supply voltage is impressed, are spaced apart as far as possible. As previously discussed, the embodiment of the invention shown in FIG- URES l and 2 is advantageously in providing a relatively constant quantity of ions since individual variations in production of ions at the various gaps are averaged out because of the large number of gaps provided and be= cause of the regulating effect of the series resistors. Also the total quantity of ions per pulse can be higher than in the embodiment of FIGURES 3 and 4 because of the higher number of gaps available in the embodiments of FIGURES l and 2.

Both embodiments of the invention are physically rugged and inexpensive to construct. A further important advantage is obtained in that no electrical lead wires must pass through the wall of the vacuum chamber 22 and 39, thereby minimizing problems of vacuum leaks.

Therefore, while the invention has been disclosed with respect to particular embodiments, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a pulsed ion source, the combination comprising an annular insulator, a first and a second spaced apart conductive band plated on said insulator, at least one strip of metal plated on a surface of said insulator, said metal being of a type capable of occluding a gas, each of said metal strips having at least one narrow spark gap thereacross, means electrically coupling said metal strip between said first and second conductive bands, 21 pulse power supply connected across said conductive bands, and means providing a vacuum at said cylinder.

2. A pulsed ion source as described in claim 1 wherein a plurality of said metal strips are plated on said insulator and wherein said gaps in said metal strips are provided at substantially the same longitudinal position along said insulator.

3. A pulsed ion source as described in claim 1 wherein said power supply is of a type providing a peak pulse potential of substantially 1000 volts and wherein said gaps have a width of substantially 0.001 inch.

A pulsed ion source as described in claim 1, wherein said metal strip is plated on an inside surface of said annular insulator.

5. In a pulsed ion source, the combination comprising a cylindrical insulator, a first and a second spaced apart conductive band plated on said cylinder, a power supply electrically connected across said first and second conductive bands, a plurality of gas occluding metals strips plated on said cylinder between said conductive bands, each of said strips having a narrow gap therein, a plurality of resistive strips plated on the surface of said cylinder each one being connected in series with a separate one of said metal strips, each said metal strip and tre associated resistive strip being electrically coupled between said first and second conductive bands, and means providing a vacuum adjacent said metal strips.

6. A pulsed ion source as described in claim 5, wherein said metal strips and said resistive strips are longitudinally aligned on said cylinder, said gaps in each of said metal strips provided at a common longituidnal position along said cylinder.

7. A pulsed ion source as described in claim 5, wherein said resistive strips have an impedance of about forty ohms, and wherein said power supply has a peak output pulse potential of about 1000 volts.

8. In a pulsed ion source, the combination comprising an insulative cylinder, means providing a vacuum within said cylinder, a first and a second conductive band plated at opposite ends of said cylinder, a power supply electrically coupled across said conductive bands, a plurality of equally spaced occluding metal strips plated longitudinally on the inside surface of said insulator with a first end of each strip contacting said first conductive band, each of said metal strips having a narrow gap provided thereacross at a common longitudinal position, a plurality of resistive strips each having a first end contacting said second conductive band and having a second end contacting a second end of a separate one of said metal strips.

9. In a pulsed ion source, the combination comprising a cylindrical insulator, a first and a second plated conductive band deposited on opposite ends of said insulator, a power supply electrically connected across said first and said second conductive bands, a ring of gas occluding metal deposited on the inside surface of said cylinder and disposed approximately midway between said first and said second conductive bands and having at least two narrow gaps dividing said ring into at least two segments, a first contact strip deposited from said first conductive band to a first of said segments of said metal ring, a second contact strip deposited from said second conductive band to another of said segments of said metal ring, and means providing a vacuum in said cylinder.

10. In a pulsed neutron generator, the combination comprising a cylindrical insulator, at least one strip of a gas occluding metal deposited on the inside surface of said insulator, each of said strips having at least one narrow gap thereacross, an anode of gas occluding metal spaced from one end of said insulator, means providing a pulse of current through said strip of gas occluding metal, and means simultaneously applying a high potential negative voltage pulse to said anode relative to said strips.

11. A pulsed neutron generator as described in claim 10, and comprising the further combination of a grid disposed adjacent said strips between said strips and said anode, said grid being electrically connected to the adjacent ends of said strips.

No references cited.

JAMES W. LAWRENCE, Primary Examiner. RODNEY D. BENNETT, Examiner.

C. E. WANDS, S. A. SCHNEEBERGER,

Assistant Examiners.

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