CONCEPTUAL DESIGN OF A POSITRON-ANNIHILATION SYSTEM FOR GENERATION OF QUASI-MONOCHROMATIC GAMMA RAYS*

A conceptual design is presented for a system consisting of the following: an electron accelerator and production target to produce positrons, a dipole magnet and wedge to compress the positron momenta to be nearly monochromatic, a magnetic transport system to focus and direct the positrons to a converter, and a converter in which the positrons annihilate in flight to produce quasimonochromatic gamma rays. The system represented is designed to produce ~10 MeV gammas, but it can also be tuned for other energies. OVERVIEW Beams of nearly monoenergetic gamma rays tunable in the energy range around 10 MeV are useful for research and for a number of industrial applications, such as for scanning of cargo for fissionable materials by inducing photofission reactions in those materials, which then are identified by detection of the delayed fission neutrons or gammas as a telltale signature of their presence [1]. The primary application of the system described is a field deployable cargo scanning system motivated by nonproliferation and homeland security. To satisfy the demands of being field deployable the system has been designed to fit within 2 standard semi-trailers. An ideal system would provide an intense, nearly monoenergetic gamma ray beam that can be tuned to the energies of interest. In practice, an energy spread of about 1 MeV is small enough because photofission of actinide nuclei proceeds through a giant dipole resonance that is fairly broad [2].