2. Neutron Sources

Abstract

This chapter discusses the neutron sources. All neutron sources are based on nuclear reactions that free neutrons bound in nuclei. These are categorized according to the mechanisms by which this is done: fission chain reactions, fusion reactions, e - -bremsstrahlung-induced photoneutron and photofission reactions, charged-particle nuclear reactions, and spallation. The last three together termed as accelerator-based reactions. Muon-catalyzed fusion and the neutron focus represent other categories of neutron-producing reactions. Electron-bremsstrahlung sources produce evaporation neutrons with an average energy of a few mega-electron-volts, whether due to photofission or photonuclear reaction. Since the neutron production mechanisms involve first producing bremsstrahlung photons, which then interact to produce neutrons, these sources produce very large gamma-ray fluxes as well, which sometimes represent a troublesome background. Neutron shielding is like that for a reactor. Spallation sources also produce predominantly evaporation neutrons, which boil off in the process of “cooling off” of highly excited nuclei. The types of accelerators that provide the highest intensities are resonant accelerators, which all have inherent pulse structure. In cyclotrons, synchrotrons, and storage rings, bunches of particles circulate at frequencies of a few megahertz.