The Single Volume Scatter Camera (SVSC) Collaboration aims to develop portable neutron imaging systems for a variety of applications in nuclear non-proliferation. Conventional double-scatter neutron imagers are composed of several separate detector volumes organized in at least two planes. A neutron must scatter in two of these detector volumes for its initial trajectory to be reconstructed. As such, these systems typically have a large footprint and poor geometric efficiency. We report on the design and characterization of a prototype monolithic neutron scatter camera that is intended to significantly improve upon the geometrical shortcomings of conventional neutron cameras. The detector consists of a 50 mm×56 mm× 60 mm monolithic block of EJ-204 plastic scintillator instrumented on two faces with arrays of 64 Hamamatsu S13360-6075PE silicon photomultipliers (SiPMs). The electronic crosstalk is limited to < 5% between adjacent channels and < 0.1% between all other channel pairs. SiPMs introduce a significantly elevated dark count rate over PMTs, as well as correlated noise from after-pulsing and optical crosstalk. In this article, we characterize the dark count rate and optical crosstalk and present a modified event reconstruction likelihood function that accounts for them. We find that the average dark count rate per SiPM is 4.3 MHz with a standard deviation of 1.5 MHz among devices. The analysis method we employ to measure internal optical crosstalk also naturally yields the mean and width of the single-electron pulse height. We calculate separate contributions to the width of the single-electron pulse-height from electronic noise and avalanche fluctuations. We demonstrate a timing resolution for a single-photon pulse to be (128 ± 4) ps. Finally, coincidence analysis is employed to measure external (pixel-to-pixel) optical crosstalk. We present a map of the average external crosstalk probability between 2×4 groups of SiPMs, as well as the in-situ timing characteristics extracted from the coincidence analysis. Further work is needed to characterize the performance of the camera at reconstructing single- and double-site interactions, as well as image reconstruction.

We report on the design and characterization of a prototype monolithic neutron scatter camera that is intended to significantly improve upon the geometrical shortcomings of conventional neutron cameras. The detector consists of a 50 mm × 56 mm × 60 mm monolithic block of EJ-204 plastic scintillator instrumented on two faces with arrays of 64 Hamamatsu S13360-6075PE silicon photomultipliers (SiPMs). The electronic crosstalk is limited to < 5 pairs. SiPMs introduce a significantly elevated dark count rate over PMTs, as well as correlated noise from after-pulsing and optical crosstalk. In this article, we characterize the dark count rate and optical crosstalk and present a modified event reconstruction likelihood function that accounts for them. We find that the average dark count rate per SiPM is 4.3 MHz with a standard deviation of 1.5 MHz among devices. The analysis method we employ to measure internal optical crosstalk also naturally yields the mean and width of the single-electron pulse height. We calculate separate contributions to the width of the single-electron pulse-height from electronic noise and avalanche fluctuations. We demonstrate a timing resolution for a single-photon pulse to be 128 (±4) ps. Finally, coincidence analysis is employed to measure external (pixel-to-pixel) optical crosstalk. We present a map of the average external crosstalk probability between 2-by-4 groups of SiPMs, as well as the in-situ timing characteristics extracted from the coincidence analysis. Further work is needed to characterize the performance of the camera at reconstructing single- and double-site interactions, as well as image reconstruction.

Contextual sensors can be equipped to radiation detection systems to allow free-moving 3D gamma-ray imaging through scene data fusion. Scene data fusion captures relevant scene information to construct a 3D map of an environment, which can be used to constrain image reconstruction techniques. While developed to enable free-moving imaging measurements, scene data fusion also has applications in cases that require multiple static measurements to generate 3D images, where the scene information can improve results. One way we seek to further leverage contextual scene information is to use the 3D map to automatically identify the radiological object of interest in a scene, and then determine optimal measurement poses around that object for quantitative Compton imaging. We also seek to improve the use of contextual information by identifying materials present in an environment to inform attenuation estimates for intervening materials. We present the approach for further leveraging the contextual information, and the preliminary results of using an SDF-enabled gamma-ray imager to find optimal measurement positions for a distributed source scenario to produce quantitative Compton images with minimization of associated uncertainties.

Detecting, localizing, and quantifying concealed nuclear material is a problem relevant to locating sources outside of regulatory control, containing contamination, and preventing illegal transport of radioactive materials. Recent advances include the ability to locate and quantify point sources in 3D more accurately by taking into account the attenuating materials between the source and a gamma-ray detector and by obtaining scene information through LiDAR measurements. These efforts have greatly improved the localization and quantification of obstructed sources, but they neglect internal structure composed of gamma-ray attenuating materials that LiDAR cannot detect. In this work, we report upon model-based efforts to advance the state-of-the-art by exploring the increased performance in quantitative gamma-ray imaging through the use of Through-Wall Radar Imaging (TWRI) to gain more information about the scene.

A handheld system developed to digitize a contextual understanding of the scene at a chemical, biological, radiological, nuclear and/or explosives (CBRNE) events is described. The system uses LiDAR and cameras to create a colorized 3D model of the environment, which helps domain experts that are supporting responders in the field. To generate the digitized model, a responder scans any suspicious objects and the surroundings by carrying the system through the scene. The scanning system provides a real-time user interface to inform the user about scanning progress and to indicate any areas that may have been missed either by the LiDAR sensors or the cameras. Currently, the collected data are post-processed on a different device, building a colorized triangular mesh of the encountered scene, with the intention of moving this pipeline to the scanner at a later point. The mesh is sufficiently compressed to be sent over a reduced bandwidth connection to a remote analyst. Furthermore, the system tracks fiducial markers attached to diagnostic equipment that is placed around the suspicious object. The resulting tracking information can be transmitted to remote analysts to further facilitate their supporting efforts. The paper will discuss the system's design, software components, the user interface used for scanning a scene, the necessary procedures for calibration of the sensors, and the processing steps of the resulting data. The discussion will close by evaluating the system's performance on 11 scenes.

Reflective materials such as Teflon and Enhanced Specular Reflector are commonly used to maximize light collection in scintillator-based radiation detection systems. While effective, in most cases, the spatial information carried by the light is diminished or lost entirely. Retroreflectors, in contrast, better preserve the spatial information and have been shown to improve the localization performance of such systems. In this work, the relative retroreflectivity of a selection of commercial retroreflective tapes and their performance impact when coupled to a plastic scintillator are reported. We demonstrate improved localization performance with some tapes compared to Teflon and black paint.

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.

KCaI3:Eu is a scintillator composition that is promising for national security applications, with a high light yield and good energy resolution (similar to 70,000 ph/MeV and similar to 3% E.R. at 662 keV, respectively). In addition to its high performance, we have been successful at growing KCaI3:Eu at larger sizes (38 mm diameter) without cracking. In this work we utilized a multi ampoule growth station to grow four 38 mm diameter crystals simultaneously. Three of these crystals achieved an energy resolution of similar to 4% at 662 keV. The three crystals were hermetically packaged and a collimation study was carried out This study revealed that light yield at a given irradiation location is dominated by photon path lengths and self-absorption, while the energy resolution dependence is affected heavily by local crystal quality. A 50 mm diameter KCaI3:Eu crystal was grown that achieved an energy resolution of 4.6% at 662 keV. GEANT4 was used to model a gap-style package that results in reduced photon path lengths, and therefore reduced self-absorption probability.

The feasibility of a full-scale lithium fluoride zinc sulfide (LiF/ZnS) based neutron multiplicity counter has been demonstrated. The counter was constructed of modular neutron detecting stacks that each contain five sheets of LiF/ZnS interleaved between six sheets of wavelength shifting plastic with a photomultiplier tube on each end. Twelve such detector stacks were placed around a sample chamber in a square arrangement with lithiated high-density polyethylene blocks in the corners to reflect high-energy neutrons and capture low-energy neutrons. The final system design was optimized via modeling and small-scale test. Measuring neutrons from a Cf-252 source, the counter achieved a 36% neutron detection efficiency (epsilon) and an 11.7 mu s neutron die-away time (tau) for a doubles figure-of-merit (epsilon(2)/tau) of 109. This is the highest doubles figure-of-merit measured to-date for a He-3-free neutron multiplicity counter. (C) 2018 Elsevier B.V. All rights reserved.