US3183356A - Neutron source - Google Patents

Description

United States Patent 3,183,356 NEUTRON SOURCE Julian H. Cherubini, Quincy, Mass, assignor to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts N0 Drawing. Filed July 30, 1962, Ser. No. 213,130 1 Claim. (Cl. 250-845) This invention relates to neutron sources and more particularly to a novel lithium tritide target and to an improved method of preparation thereof.

Neutron sources of the type in which a neutron emissive target is bombarded by a high energy deuteron beam or the like often require targets having an abundant yield of high energy neutrons. It is desirable that such targets have a reasonably long life. The current state of the art reveals no target which effectively meets all of the above criteria. Long life targets such as those made of beryllium or deuterated wax yield relatively low energy neutrons. Polonium-beryllium and lithium-beryllium targets are limited in their neutron output. In order to obtain both high energy neutrons and high neutron outputs, the tritium target has often been resorted to. However, conventional tritium targets do not last very long if the accelorator producing the high energy deuteron beam is operated at full power. Typically, these targets consist of titanium or zirconium into which tritium gas has been absorbed. The neutron yield decreases all too readily when the target is bombarded by a deuteron beam and the resulting rapid target decomposition necessitates frequent replacement thereof. A further problem associated with prior art neutron source targets is that various target materials react with air and moisture to produce a metal hydroxide and a hydrogen gas. Thus, when not in the proper environment such as a vacuum system, the target may spontaneously decompose.

Accordingly, it is a principal object of this invention to provide, in a neutron source, a new and improved high energy, high yield target member.

It is another object of this invention to provide a new and improved method of producing a neutron source target member.

it is another object of this invention to provide a neutron source target member comprising discrete amounts of lithium tritide dispersed within a metal matrix.

It is another object of this invention to provide a long life neutron source target adapted to produce a copious supply of high energy neutrons.

It is another object of this invention to provide a lithium tritide neutron source target that is impervious to air and moisture.

:It is another object of this invention to provide a practical neutron source target member having a lower cross sectional stopping area and consequent higher neutron output than the conventional titanium-tritium target.

It is another object of this invention to provide a neutron source target member comprising lithium tritide disposed in an aluminum matrix wherein said lithium-tritide is greater than twenty percent by volume of the mixture thereof.

These, together with other objects and features of this invention will become apparent from the following detailed description of the novel target compositions and preferred fabrication method. While specific materials and apparatus are referred to herein, it is intended that they be taken as illustrative of the principles of the invention and not in a limiting sense, the scope of the invention being defined by the appended claim.

The present invention comprehends a neutron source 3,183,356 Patented May 11, 1965 target member comprising discrete amounts of lithium tritide dispersed in a metal matrix. It has been determined that there is considerable advantage in the high energy neutron yield of a lithium-tritide target over that of the conventional titanium-tritium target. In addition to having a lower cross sectional stopping area than titaniumtritium, there is a Li (d, n)Be reaction associated with the lithium-tritide target that produces neutrons at substantially the same energy as does the tritium. These unique characteristics have been recognized and utilized in combination with other features hereinafter disclosed to provide a long life target having a neutron output approximately three times that of prior art targets of this type.

It is a particular feature of this invention to disperse the lithium-tritide in a metal matrix such as aluminum. The lithium-tritide, being a salt, exhibits very low thermal conductivity. Therefore, in order to obtain adequate neutron output it is necessary to heat it to a high temperature (operate at a high power level). This, however, effects rapid decomposition of the target by thermal dissociation. Accordingly, the lithium-tritide is dispersed in an aluminum, or other highly conductive metal, matrix producing a target having a thermal conductivity that is substantially an average of that of the salt and the metal. The resulting high thermal conductivity target permits the maintaining of as low a temperature as possible, or, alternatively, allows operation at a higher power level at a safe lithium tritide temperature.

The lithium-tritide target as described above is per se atmospherically unstable. That is, it reacts spontaneously with moisture in the air to produce lithium hydroxide and hydrogen gas. In order to permit convenient handling and storing of these targets and to prevent their degradation when outside of the vacuum system, it is another feature of this invention to vapor coat them as a final step of the fabrication process. Such a vapor coat of copper or aluminum or the like isolates the lithium from atmospheric contact without significantly affecting the etficiency of the target.

The method of fabrication of the subject target comprises the following steps. An evacuated chamber such as a bell jar is filled with tritium gas at a pressure of .approximately 7 p.s.i.g. Melted lithium is then introduced therein whereby a salt forming reaction takes places between the lithium and the tritium gas. The salt thus formed is then ground to form a powder of less than 170 mesh size (U.S. standard sieve). Aluminum powder of substantially the same or greater mesh size is then mixed therewith. The proportion by volume of lithium tritide salt to aluminum may range from 20% lithium tritide/ aluminum to 80% lithium tritide/20% aluminum by volume depending upon the proposed use of the target. Targets having less than 20% lithium tritide are limited by their neutron output while those having more than 80% lithium tritide are limited by fabrication parameters and by decreased thermal conductivity. The mixture is then compacted into a foil through a cold pressing process or through a modified-evacuated pack rolling technique to a thickness of the order of one or two mils and to a density exceeding 98% of theoretical. Coupons of the desired target size are then ptulched out of the foil. The coupons are then vapor coated with a metal such as copper or aluminum. This is accomplished by placing the coupons together with a hot metal source in an evacuated container wherein the metal evaporates and condenses on contact with the coupon. Finally the coupon thus fabricated is soldered onto a backing disc of high conductivity metal such as copper.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

A neutron emissive target adapted to bombardment by a high power charged particle beam for extended periods of time comprising a coupon consisting of a compacted mixture of powdered lithium tritide and powdered aluminum, a copper backing plate afiiXed thereto, and a surface coating of vaporized aluminum powder, said lithium tritide and said aluminum being powdered to less than 170 mesh size and compacted to a density exceeding ninety-eight percent of theoretical, said mixture containing not less than twenty percent nor more than eighty percent by volume of lithium tritide powder and being adapted to References Cited by the Examiner UNITED STATES PATENTS Fahrenwald 75-138 Czochralski et al 75-138 Kallmann 31361 X Anderson 176- 10 X Carroll 17610 X Fearon et a1. SIB--61 Goodman 31361 Manning et al. 176l6 Gordon 29--182.5 Thomas et al 313- 61 X Tittle et a1. "a 250-845 15 CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEI N, Examiner.