The present invention relates generally to neutron generation and, more particularly, to increasing the half-life of tritium-titanium targets when bombarded by deuterium ion beams.
In many applications, particularly in the medical field, there is an increasing need for high-intensity neutron generators. A relatively simple and inexpensive neutron generator utilizes a tritium-loaded, titanium-coated, rotating copper target which is bombarded by deuterium ions accelerated by a few hundred keV. However, a serious roadblock to the practical utilization of such generators is caused by the relatively short useful life-time of the target. As described by R. Booth and H. H. Barschall in "Tritium Target For Intense Neutron Source," Nuclear Instruments and Methods No. 99, February, 1972, pages 1-4, the depletion rate of the tritium-titanium target is very strongly influenced by the presence of molecular ions in the deuteron beam. Specifically, deuterium beams, such as are obtained from duoplasmatron-type ion sources, contain deuterons (i.e., monatomic deuterium ions) as well as diatomic and triatomic molecular deuterium ions. For a given beam energy, the deuterons penetrate the target material to a greater depth or range than the diatomic ions which, in turn, penetrate to a greater depth than the triatomic ions. This difference in penetration or range is due to the differences in sharing of acceleration energy by the ions of different mass. At or near the end of their penetrations, the ions displace tritium atoms from the titanium matrix in a relatively thin layer. The neutron-producing reactions, on the other hand, are most prevalent for each ion type a short distance back (i.e., closer to the target surface from the location of maximum penetration; that is, neutron generation is most prevalent where the ions still have approximately 100 keV of residual energy. The diatomic ions, with lesser penetration depths than the atomic ions, tend to displace tritium atoms at a depth which corresponds to the maximum neutron-producing depth of the atomic ions. The depletion of tritium at this depth seriously reduces the useful life of the target. Similarly, the triatomic ions tend to displace tritium atoms at the most efficient neutron-producing depth of the diatomic ions. The triatomic ions themselves are not very effective in neutron production because of their relatively low particle energies and because their maximum neutron-production region is at or near the target surface which is often contaminated by oxygen, carbon and organic material from vacuum seals. The end result is a useful target life which is far less than optimum.
A number of approaches to extending the useful target life have been suggested. One approach accelerates the mixed ion beam to full energy and then separates the ions with a powerful magnet before bombardment of the target. Only one of the thusly separated mass constituents of the beam is permitted to strike the target. This method eliminates the harmful interference by one ion type with another, but does so by introducing a number of disadvantageous features. For one thing, since two ion types are eliminated from bombardment, the particle accelerator is required to deliver between two and three times the beam current that is actually used. Moreover, the beam-separating magnet must be quite large to achieve sufficient separation of the high energy particles. Further, the stops for the rejected beam components must be capable of dissipating a substantial amount of energy. The resulting structure is thus large, costly and wasteful of energy.
Another prior art approach to increasing target life involves magnetic separation of the ion constituents before the beam is accelerated. This is performed in the high voltage terminal of the apparatus where the ions are at low energy levels and can therefore be deflected sufficiently with a relatively small magnet. This approach requires only the selected ion mass to be accelerated to full energy; however, such desirable result is achieved only with the introduction of additional complexity. The high energy beam must be extracted and focussed, by means of an electrostatic lens, onto a cooled apertured plate arranged to reject the two discarded ion types. This plate is located beyond the mass-separating magnet but still inside the high voltage terminal. A second lens is required to re-focus the single-constituent beam through the accelerator and onto the target. Still another control adjustment is necessary in order to obtain the correct deflection angle through the magnet. The apertured mass-rejection plate requires that there be a cooling facility and a vacuum-pumping capability within the high voltage terminal.
In both of the aforementioned prior art arrangements, "in-line" accelerator configurations are not possible because all three beam constituents must be deflected off-line during separation. In the second-mentioned apparatus, the beam position is also unstable because the long distance between the separation magnet and the target renders automatic control difficult.
It is therefore an object of the present invention to provide a simple and inexpensive approach to improving the useful life of the target in a neutron generator of the type described.
It is an object of the present invention to achieve increased target life in a neutron generator of the type described with minimal waste of energy.
It is another object of the present invention to take maximum advantage of the neutron-producing characteristics of the different ion mass constituents of a deuterium ion beam to achieve increased useful life of a tritium-titanium target.
It is still another object of the present invention to achieve improved target life in a neutron generator of the type described wherein an "in-line" accelerator may be employed.