Energetic ion acceleration from relativistically intense laser interactions is a very active area of research with a wide spectrum of possible applications in scientific, medical andengineering communities. One such utilization of these high energy ion beams is a compact, high-brightness, short duration and relatively low cost neutron source, and with these qualities would be advantageous for fast neutron radiography, transmutation of nuclear waste and fusion research. A high power laser system can be used to accelerate deuterons, which fuse deuterons in either the target itself or a separate deuterated 'catcher'. The d(d,n)-³He fusion reaction produces 2.45 MeV center-of-mass energy neutrons and measurements of these have been used as a diagnostic for the high-energy ion beam accelerated during the laser interactions (1–7). Fig. 1 shows an experimental configuration for experiments in laser driven neutron production carried out on the T³ laser system.

Determining the ion acceleration and dynamics from these interactions can be difficult because in attempting to leave the target, global electromagnetic fields may influence the ions. Therefore, the externally measured ion, or electron, spectra is possibly not representative of conditions within the target. Neutron measurements have inferred the deuteron acceleration direction at the front surface of deuterated targets for a high contrast ratio (2) or a low contrast ratio where shock ion acceleration was diagnosed (7).

Since the neutron directionality is influenced by the deuteron beam characteristics, it can be envisaged that laser generated neutron sources could be used for neutron radiography. It is particularly interesting if the 2.45 MeV d(d,n)-³He fusion neutrons can be up-shifted to higher energies due to the fact that the deuteron beam has momentum in a particular direction (4). The fusion reactions can occur within the deuterated targets itself, a solid target (1; 2), clusters (3), gas(5) or a droplet (6), as the deuterons move through it. Or alternatively a secondary deuterated target known as a catcher, which is placed in the deuteron beam path, can be used (5; 6). This can be thought of as the pitcher-catcher method and has the advantage that it will naturally select a collimated ion beam, required for the collimated neutron beam. Another neutron generation method is to use a proton beam into a LiF catcher target to generate neutrons through the 7 Li(p,n)-7 Be reaction (8).

References

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