Matthew A Blackston

A Comparison of GEANT4 and DETECT2000 for the simulation of light transport in scintillation detectors

Monte Carlo simulation techniques are powerful tools for investigating the performance of imaging detectors. DETECT2000, a Monte-Carlo package for light transport simulation, has been commonly used to model the optical properties of scintillation detectors and is able to realistically estimate the response of photodetectors. However, DETECT2000 generally runs slowly when complex detector geometries are specified and the lack of visualization tools makes it difficult to accurately define complex geometries. GEANT4 is a simulation toolkit that can also realistically model optical photon transport for scintillation detectors. This paper describes a case study in which GEANT4 was found to be significantly faster both in computing time and, aided by visualization tools, in the user time required to develop the geometry of a scintillation detector. In addition, because the detector geometry can be easily parameterized using GEANT4, it was possible to perform automated searches of large amounts of the solution space for an optimal design. In this work, we compared the results from simulations of a custom-designed scintillation detector obtained using both packages. Both yielded similar flood images and were able to resolve the same number of pixel elements from a segmented light guide with slight differences in the peak-to-valley ratios. A simulation-speed comparison on a common computer using a simplified geometry showed that GEANT4 is about 40% faster than DETECT2000. In this paper we compare simulation results for the initial design with experimental data and describe subsequent simulations that were used to arrive at the optimal design.

Enhancing pixelated fast-neutron block detector performance using a slotted light guide

The first fast-neutron scintillation block detector has been developed for use in neutron imaging applications. The low-cost, large field of view (FOV) block detector consists of a 10 × 10 array of plastic scintillator crystals, each with a size of approximately 1 × 1 × 5 cm3. To achieve both low cost and fast response time, the detector uses a 2 × 2 array of photomultiplier tubes (2-in. Photonis XP20D0) optically coupled to the array of scintillation crystals through a custom-made light guide. The light guide is segmented in such a way that all scintillator crystals can be resolved, including the edge crystals, to fully utilize the entire FOV of the detector. The thickness and depth of the light-guide segments were also optimized to maintain nearly uniform linearity between the true pixel locations and the reconstructed positions. Flood images from simulation and experiment show that each of the 100 pixels is resolved with an average peak-to-valley ratio for 14 MeV neutrons of 9:1. A mean deviation of 3.3 mm is obtained between the resolved image peaks and the respective true positions demonstrating good linearity. A coincidence measurement performed using a deuterium-tritium neutron generator, in which the neutrons were measured with the block detector and the corresponding alpha particles were measured with another scintillation detector revealed a total time resolution of 1.3 ns full width at half maximum.

3D millimeter event localization in bulk scintillator crystals

One of the primary goals of scintillator-based gamma-ray detector development is to obtain spatial resolutions in bulk crystals at the millimeter level in all three spatial dimensions. An even more challenging goal is to disentangle multiple simultaneous energy depositions with comparable spatial resolutions. We are exploring a new technique to achieve this level of performance through the use of close-coupled, coded-aperture shadow masks placed between the crystal and a position-sensitive phototransducer. We report on simulations of such a device using Monte Carlo light transport simulations performed with GEANT 4. Initial indications are promising; however, the technique will require a very high level of performance from the phototransducer.

Passive and Active Fast-Neutron Imaging in Support of Advanced Fuel Cycle Initiative Safeguards Campaign

Results from safeguards-related passive and active coded-aperture fast-neutron imaging measurements of plutonium and highly enriched uranium (HEU) material configurations performed at Idaho National Laboratory s Zero Power Physics Reactor facility are presented. The imaging measurements indicate that it is feasible to use fast neutron imaging in a variety of safeguards-related tasks, such as monitoring storage, evaluating holdup deposits in situ, or identifying individual leached hulls still containing fuel. The present work also presents the first demonstration of imaging of differential die away fast neutrons.

Position-Sensitive Fast-Neutron Detector Development in Support of Fuel-Cycle R&D MPACT Campaign

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Fast-neutron elastic-scatter imaging for material characterization

Fast-neutron associated-particle imaging (API) using neutrons created via the deuterium-tritium (DT) fusion reaction is a powerful technique for imaging the internal geometry of target items, even if they contain large amounts of shielding. Although traditional neutron transmission radiography and tomography performed using API techniques provide detailed information about geometry, they provide little or no information about the materials in the items under inspection. Fortunately, material-specific information is available using neutrons that elastically scatter in the target and are subsequently detected. Because the API technique provides knowledge of the time and initial direction of 14 MeV neutrons produced in the DT reaction, the angular deviation and time of flight of detected neutrons are used to identify these elastically scattered neutrons. The most distinctive elastic-scatter signal exists for materials with low mass numbers (4), so initial development has focused on producing low-4 images. This paper describes the elastic-scatter imaging approach, describes the imaging system, describes analysis and image reconstruction approaches with a focus on elastic scatters from A = 1 nuclei, and presents the first elastic-scatter images from laboratory measurements.