Actinide-loaded glass scintillators for fast neutron detection

Fast neutron detection has applications in a number of topic areas, including national security and nuclear fuel cycle safeguards. Ideally, these sensors would have high sensitivity to fission (fast) neutrons but be insensitive to thermal neutrons and gamma rays. This paper describes a study of actinide-loaded glass for fast neutron detection, wherein a threshold fission reaction in the actinide produces fission products depositing approximately 200 MeV of energy. The prominent advantage of this approach to fast neutron detection, when compared to fission chambers, is the potential for several orders of magnitude increase in the mass of fissionable atoms per unit volume-thereby improving the absolute detection efficiency and offsetting the small (approximately 1 barn) threshold cross-sections. The detectors tested to date are glass-matrix based, loaded with thorium and a rare-earth activator. The threshold cross-section for fission of 232Th (100% natural abundance) provides insensitivity to thermal neutrons, but the primary research question is whether the 200 MeV of energy released per fission is sufficiently distinct, in terms of total scintillation light and/or scintillation decay time, to be clearly distinguished from gamma rays and the inherent alpha emissions in the actinide. The high density of ionization and resulting propensity for recombination is expected to reduce the scintillation production well below that corresponding to 200 MeV electron equivalent. Technical challenges to be overcome in the fabricating actinide-loaded glasses and the testing of these early samples are significant. These include making a glass containing thorium and the proper activator that will scintillate, discriminating fast neutron events from the natural radioactivity of 232Th and its daughters and minimizing the severity of the non-linearity of energy to light conversion for the fission fragments. The first obstacle has been overcome; scintillating glass samples containing thorium have been produced and spectra on these samples have been acquired. These spectra as well as initial tests of fast neutron detection are presented here, along with a discussion of the research paths to be explored in the future.