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The size of the image pixels in a coded-aperture image is a function of the mask hole size, the focal length (mask-to-detector spacing), and the distance to the scene being imaged. Gamma-ray imaging instruments designed for field use can have a focal length that is comparable to the thickness of the detector, resulting in radially blurred images because a range of image pixel sizes contributes to the image. If so equipped, one can use a detector’s depth-of-interaction resolution to eliminate this blurring by binning images formed from different depths within the detector to a common image scale. However, binning to a common image scale requires handling a number of issues including spatially dependent energy weighting, spatially varying offset corrections, and spatially correlated uncertainties. This paper explores these issues and provides effective solutions that have been implemented for online use in a field-portable instrument.

The ability of gamma-rays to penetrate matter makes them an excellent signature for the detection of nuclear materials—except that this very ability makes their detection difficult. This is particularly true if one wishes to make images, since general-purpose focusing optics do not exist. Various indirect imaging techniques have been successfully applied to obtain gamma-ray images, including Compton and coded-aperture imaging. This paper reviews the different approaches, and discusses their advantages and disadvantages as illustrated with results obtained from different instruments designed for use in nuclear security applications.

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.

Direct measurement of plutonium and uranium X-rays and gamma-rays is a highly desirable non-destructive analysis method for the use in reprocessing fuel environments. The high background and intense radiation from spent fuel make direct measurements difficult to implement since the relatively low activity of uranium and plutonium is masked by the high activity from fission products. To overcome this problem, we make use of a grazing incidence optic to selectively reflect K α and K β fluorescence of Special Nuclear Materials (SNM) into a high-purity position-sensitive germanium detector and obtain their relative ratios.

The use of a grazing incidence optic to selectively reflect K-shell fluorescence emission and isotope-specific lines from special nuclear materials is a highly desirable nondestructive analysis method for use in reprocessing fuel environments. Preliminary measurements have been performed, and a simulation suite has been developed to give insight into the design of the x ray optics system as a function of the source emission, multilayer coating characteristics, and general experimental configurations. The experimental results are compared to the predictions from our simulation toolkit to illustrate the ray-tracing capability and explore the effect of modified optics in future measurement campaigns.

The amounts of fissile Pu and U in spent nuclear fuel are of primary concern to the safeguards community. In particular, there are issues when safeguards transitions from an item accountancy basis (such as fuel bundles) to a fissile material mass basis as occurs when spent fuel enters a reprocessing plant. Discrepancies occur because item accountancy requires estimating the content of fissile material using indirect techniques such as the fuel burn-up and item-level measurements of radiation emissions from fission by-products. Direct measurement of the fissile content by monitoring line emissions from fissile species themselves is impossible because the lines are much weaker than those emitted by shorter-lived isotopes in the fuel. The goal of this project is to develop a technique to directly measure these weaker lines despite the presence of overwhelming radiation from other isotopes. This is achieved by using gamma-ray mirrors as a narrow band-pass filter. The mirrors reflect only energies of interest toward a HPGe detector that is shielded from direct view of the spent fuel and its fierce emissions. This can significantly improve the reliability with which the mass of fissile material is tracked.

大気球シンポジウム 平成24年度（2012年10月16-17日. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS)）, 相模原市, 神奈川県), Balloon Symposium 2012 (October 16-17, 2012. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan, 著者人数: 31名, 資料番号: SA6000066018, レポート番号: isas12-sbs-018