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.

Uranium and thorium oxides have critical roles as fuels in existing nuclear power plants, as well as in proposed reactor concepts. The thermal conductivity of these materials determines their ability to transfer heat from the reactor core to the surrounding coolant. Additionally, these actinide compounds are of interest in condensed matter physics because of the 5f orbitals and unique electron interaction, coupling, and scattering events that can occur. Because of the radioactivity of thorium and uranium, thin film measurements of actinide materials are used to limit the amount of operator exposure, but standard thermal characterization methods are not well suited for thin films. This paper presents the process of depositing thin film UOx and ThOx samples of nm-μm thicknesses and the results of thermal property measurements. Thin films were deposited on silicon and glass substrates via dc-magnetron sputtering using an argon/oxygen mixture as the working gas. The thermal properties of the films were measured by the Thermal Conductivity Microscope (TCM). This uses one laser to generate thermal waves and a second laser to measure the magnitude and phases of the thermal waves to obtain the conductivity of materials. The results of the research show that the UOx film properties are lower than bulk values and that the role of the substrate has a considerable effect on determining the measured properties. Future work aims at improving the deposition process. Epitaxial film growth is planned. Additional understanding of thermal property measurements is targeted.

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