Recent research in manufacturing plastic scintillators via photopolymerization has reported various non-aromatic acrylic-based resins for easy 3D printing. However, the absence of traditionally used aromatic matrices, such as polystyrene or poly(vinyl toluene) (PVT), resulted in a limited scintillation performance. In this research, the feasibility of accommodating a high ratio of PVT with pentaerythritol tetraacrylate was demonstrated by the synthesis of plastic scintillators with efficient pulse shape discrimination. Moreover, the research described the understanding of the current inferior performance of photocured plastics compared to thermally cured analogs and showed 3D printability of studied resins in different shapes.
KEYWORDS: Digital electronics, Very large scale integration, Single photon avalanche diodes, Analog electronics, Silicon photomultipliers, Semiconducting wafers, Photodetectors, Radiography, Scintillators, Field programmable gate arrays
For decades, fast neutron radiography performed using pulse-counting detectors employed PMTs coupled to plastic scintillator pixel arrays. SiPM-based systems are now sought to replace fragile PMTs, but conventional, “analog” SiPMs suffer from intrinsic limitations which limit their achievable performance. Among these limitations, a complete analog readout and digitizer chain is required, a counterintuitive approach when considering that the single-photon avalanche diode (SPAD), the basic unit cell of SiPMs, is a Boolean detector providing digital detection at the sensor level. This paper outlines a new concept for neutron radiography instrumentation by using photon-to-digital converters (PDCs, aka digital SiPMs), a fully digital solution to sense the scintillation light.
Recent material advancements in plastic scintillators enable marked increases in material light yield, detection efficiency, pulse-shape discrimination, and array production rates. These advances may resolve significant capability gaps for lowcost, portable, and durable dual-particle imaging (DPI) systems for nuclear safety, security and safeguard purposes. Two such materials, both 21% bismuth-loaded plastics utilizing iridium complex fluorophores (Ir-Bi-Plastic) were experimentally evaluated for DPI purposes as a small, pixelated radiographic array and compared to similar arrays made from EJ-200 and EJ-256 (5 wt% Pb). Experimentation involved separate exposures to 370 kVp x-rays and 14.1 MeV neutrons when paired with a digital radiographic panel, and array performance was evaluated using ASTM methods for dSNRn determination. Additionally, the development of fast-curing plastic scintillator (FCPS) formulations is highly attractive because it facilitates the 3D-printing of complete pixelated plastic scintillator arrays for radiation detection and localization. Future advancements in this area will significantly reduce the time and costs associated with current array manufacturing techniques. Some early investigations of FCPS samples sensitized with 5 wt% Bi is discussed herein, with their gamma detection efficiencies and associated light yields compared to an equivalent sample of EJ-256. These early unoptimized samples provided similar but not superior performance to EJ-256, and this is an ongoing area of research at the Air Force Institute of Technology.
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 a loading level of 21 wt% bismuth metal. This new bismuth plastic (Ir-Bi-Plastic) offers improved detection efficiency over commercial plastic scintillators. One application for Ir-Bi-Plastic is in low-cost, portable, and durable dual-particle imaging (DPI) systems supporting nuclear safety, security, and safeguards. However, new materials must undergo investigation using industry standards to quantify their capabilities. In this work, an Ir-Bi-Plastic was experimentally evaluated as a small, pixelated radiographic array in a fast neutron environment, with individual pixel dimensions of 2×2×19 mm. For comparison, identical evaluations were conducted for two similarly sized arrays made from EJ-200 and EJ-256. A separate Ir-Bi-Plastic array with 5×5×20 mm pixels was also evaluated. ASTM methods were leveraged to determine the modulation transfer function and spatial resolution for each array. Edge response measurements of a 2-in thick tungsten block were recorded by pressure coupling all four arrays to a commercial a-Si digital radiographic panel. Experimental results were then compared for all four arrays, and the results demonstrated that the Ir-Bi-Plastic outperforms similar arrays made from EJ-200 and EJ-256 (5 wt% Pb). These findings suggest that DPI systems utilizing Ir-Bi-Plastic hold promise for continued development over older, more traditional, alternatives.
The multi-institution Single-Volume Scatter Camera (SVSC) collaboration led by Sandia National Laboratories (SNL) is developing a compact, high-efficiency double-scatter neutron imaging system. Kinematic emission imaging of fission-energy neutrons can be used to detect, locate, and spatially characterize special nuclear material. Neutron-scatter cameras, analogous to Compton imagers for gamma ray detection, have a wide field of view, good event-by-event angular resolution, and spectral sensitivity. Existing systems, however, suffer from large size and/or poor efficiency. We are developing high-efficiency scatter cameras with small form factors by detecting both neutron scatters in a compact active volume. This effort requires development and characterization of individual system components, namely fast organic scintillators, photodetectors, electronics, and reconstruction algorithms. In this presentation, we will focus on characterization measurements of several SVSC candidate scintillators. The SVSC collaboration is investigating two system concepts: the monolithic design in which isotropically emitted photons are detected on the sides of the volume, and the optically segmented design in which scintillation light is channeled along scintillator bars to segmented photodetector readout. For each of these approaches, we will describe the construction and performance of prototype systems. We will conclude by summarizing lessons learned, comparing and contrasting the two system designs, and outlining plans for the next iteration of prototype design and construction.
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