Neutron and Photon Imaging Capabilities of Bismuth-loaded Plastic

Plastic scintillators utilizing iridium complex fluorophores offer substantial improvements in light yield, and their light yield is not significantly quenched in compositions with bismuth metalorganic loading at 21% weight. These advances may resolve significant capability gaps for low-cost, portable, and durable dual-particle imaging (DPI) systems for nuclear safety, security, and safeguard purposes. However, all candidate materials should first undergo investigation utilizing industry standards to quantify and evaluate their capabilities. As such, a 21% bismuth-loaded polyvinyl toluene (BiPVT) scintillator fabricated by Lawrence Livermore National Laboratory (LLNL) is computationally and experimentally evaluated as a small, pixelated radiographic array, with individual pixel dimensions of 2×2×19 mm. To facilitate direct comparisons, the same evaluations are conducted for two same-sized arrays made from EJ-200 and EJ-256 scintillator, respectively. ASTM standard test methods and practices are utilized to calculate the modulation transfer function and basic spatial resolution for each array, both from measured and simulated data. Measurements are recorded by pressure coupling all three arrays to a commercial a-Si digital radiographic panel, and the computational model replicates the experimental design. Computational and experimental results are compared for all three arrays in the x-ray and fast neutron environments. The x-ray results demonstrate equivalent performance between the evaluated BiPVT array and the more ideally manufactured EJ-200 array, while the BiPVT array outperforms a similar array made from EJ-256. The agreement between simulated and experimental x-ray results validates the applied computational methodology and suggests more ideally manufactured BiPVT arrays may significantly outperform similar arrays made from EJ-200. Experimental results in a fast neutron environment demonstrate superior performance of the BiPVT array compared to the EJ-256 array, while the EJ-200 array is found to outperform both. Additionally, the performance of a second array made from a separate 21% bismuth-loaded plastic (Ir-Bi-Plastic) is evaluated experimentally in both x-ray and neutron environments using the same radiographic panel and methodology. The Ir-Bi-Plastic array consists of 64 pixels with individual dimensions of 5×5×20 mm, and the results suggest it will outperform similar arrays made from EJ-200 in both x-ray and neutron environments. These findings suggest plastic scintillators with iridium complex fluorophores and 21% weight bismuth-loading hold promise over more traditional material alternatives for DPI applications supporting nuclear safety, security, and safeguard missions.

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