Tuning the index of refraction of a polyvinyl toluene and polystyrene copolymer toward a heterogenous, index‐matched neutron detector

Abstract

One class of neutron detectors for illicit nuclear materials are capture‐gated detectors, which use organic scintillators to slow neutrons while emitting fluorescent light and elements that have high neutron capture cross‐sections to provide a second signal. Homogeneous detectors composed of neutron capturing metallo‐organics within plastic darken due to their chemical instability, while heterogeneous detectors frequently result in non‐transparent material due to a mismatch of the refractive index. These detectors are often polymerized through bulk polymerization, but there is little data available on this process applied to mixtures of polystyrene (PS) and polyvinyl toluene (PVT), two commonly used polymers in plastic scintillators. This work presents bulk polymerization processing toward an index‐matched, heterogeneous capture‐gated neutron detector based on PS and PVT copolymers with a range of refractive indices. Specifically 1:3, 1:1, and 3:1 PS:PVT ratios were manufactured and their refractive indices, measured by refractometry, were compared to a theoretical model based on a mixture of the refractive indices of pure PS and PVT. Finally, a composite of PS/PVT and an Ohara S‐BAL42 glass was developed to confirm the index‐matching capability of the process as a step toward developing a heterogenous, capture‐gated neutron detector with high light transmission efficiencies allowed by index‐matched materials.