Nathan R. Hilton

Active coded aperture neutron imaging

Because of their penetrating power, energetic neutrons and gamma rays (>~1 MeV) offer the best possibility of detecting highly shielded or distant special nuclear material (SNM). Of these, fast neutrons offer the greatest advantage due to their very low and well understood natural background. We are investigating a wholly new approach to fast-neutron imaging-an active coded-aperture system that uses a coded mask composed of neutron detectors. The only previously demonstrated method for long-range fast neutron imaging is double-scatter imaging. Active coded-aperture neutron imaging should offer a highly efficient alternative for improved detection speed, range, and sensitivity. We will describe our detector including design considerations and present initial results from a lab prototype.

The performance effects of crystal boundaries in cadmium zinc telluride radiation spectrometers

The specific performance effects of extended crystal defects in cadmium zinc telluride (CZT) radiation spectrometers have been investigated. While it is generally accepted that defects such as grain boundaries and decorated twins can have a significant deleterious impact on the performance of semiconductor radiation spectrometers, the detailed mechanisms of degradation have been previously unexplored. Because of the difficulties of the high pressure Bridgman (HPB) growth process, CZT materials generally contain a large number of such defects, and the yield of defect-free crystals is comparatively low. High-resolution gamma-ray response mapping has been combined with analytical spectral simulations to relate the anomalies in the measured detector response to the likely originating defect. Examples of several different types of boundaries have been examined in a range of commercial CZT detectors, with the relevant parameters of the boundary being its trapping or channeling nature and the properties of the material on each side of the boundary.

Alpha-emitting radioisotopes for switchable neutron generators

Abstract Traditionally, radioisotopic neutron generators mix an alpha-emitting radioisotope with beryllium. The disadvantage of such an alpha–Be source is that they emit neutrons at a steady rate even when stored. These conventional generators are extremely awkward to use in many applications because of the neutron shielding required to prevent exposure to personnel and sensitive electronics. Recently, at our laboratory and others, the possibility of using switchable radioactive neutron sources has been investigated. These sources rely on a mechanical operation to separate the alpha-emitting radioisotope from the Be target, thus allowing the source to be switched on and off. The utility of these new switchable sources is critically dependent on the selection of the alpha-emitting radioisotope. In this paper we discuss issues that determine the desirability of an alpha-emitting source for a switchable neutron generator, and select alpha emitters that are best suited for use in this application.

SPATIAL MAPPING OF CADMIUM ZINC TELLURIDE MATERIALS PROPERTIES AND ELECTRICAL RESPONSE TO IMPROVE DEVICE YIELD AND PERFORMANCE

Abstract Cadmium zinc telluride has experienced tremendous growth in its application to various radiation sensing problems over the last five years. However, there are still issues with yield, particularly of the large volume devices needed for imaging and sensitivity-critical applications. Inhomogeneities of various types and on various length scales currently prevent the fabrication of large devices of high spectral performance. This paper discusses the development of a set of characterization tools for quantifying these inhomogeneities, in order to develop improvement strategies to achieve the desired cadmium zinc telluride crystals for detector fabrication.

Development of an ideal observer that incorporates nuisance parameters and processes list-mode data.

Observer models were developed to process data in list-mode format in order to perform binary discrimination tasks for use in an arms-control-treaty context. Data used in this study was generated using GEANT4 Monte Carlo simulations for photons using custom models of plutonium inspection objects and a radiation imaging system. Observer model performance was evaluated and presented using the area under the receiver operating characteristic curve. The ideal observer was studied under both signal-known-exactly conditions and in the presence of unknowns such as object orientation and absolute count-rate variability; when these additional sources of randomness were present, their incorporation into the observer yielded superior performance.

Null-hypothesis testing using distance metrics for verification of arms-control treaties

We investigate the feasibility of constructing a data-driven distance metric for use in null-hypothesis testing in the context of arms-control treaty verification. The distance metric is used in testing the hypothesis that the available data are representative of a certain object or otherwise, as opposed to binary-classification tasks studied previously. The metric, being of strictly quadratic form, is essentially computed using projections of the data onto a set of optimal vectors. These projections can be accumulated in list mode. The relatively low number of projections hampers the possible reconstruction of the object and subsequently the access to sensitive information. The projection vectors that channelize the data are optimal in capturing the Mahalanobis squared distance of the data associated with a given object under varying nuisance parameters. The vectors are also chosen such that the resulting metric is insensitive to the difference between the trusted object and another object that is deemed to contain sensitive information. Data used in this study were generated using the GEANT4 toolkit to model gamma transport using a Monte Carlo method. For numerical illustration, the methodology is applied to synthetic data obtained using custom models for plutonium inspection objects. The resulting metric based on a relatively low number of channels shows moderate agreement with the Mahalanobis distance metric for the trusted object but enabling a capability to obscure sensitive information.

Development of a list-mode ideal observer to perform classification tasks when imaging nuclear inspection objects under signal-known-exactly conditions

We developed a signal-known-exactly version of the ideal observer that processes data in list-mode format to perform binary classification, a useful task for arms-control treaty applications. This observer offers the best possible performance and future observer models developed in our work will be compared to this model. The two examined sources were plutonium inspection objects developed by Idaho National Lab. We modeled a fast-neutron coded-aperture imager, developed by Oak Ridge National Lab and Sandia National Labs to acquire simulation data. Monte Carlo simulations using the GEANT4 toolkit tracked photons and neutrons from these objects to the imager. The observer model was evaluated using the area under the ROC curve for multiple background strengths.