Simulation Study of a Shield-Free Directional Gamma-Ray Detector Using Small-Angle Compton Scattering

Gamma-ray imaging is a technique for visualizing the spatial distribution of radioactive materials. Gamma-ray imaging has recently been applied to research on environmental restoration and decommissioning of the Fukushima Daiichi Nuclear Power Station (FDNPS). In this paper, we present an elemental technology study of the gamma-ray imager using small-angle scattering (GISAS), which is intended for application at the FDNPS decommissioning site. GISAS consists of a set of directional gamma-ray detectors that do not require a shield. In this study, we investigated the feasibility of a shield-free directional gamma-ray detector by simulation. The simulation result suggests that by measuring scattered-electron energies of several keV using a scatterer detector, gamma rays with ultrasmall-angle scattering could be selected. Using Compton scattering kinematics, a shield-free detector with a directivity of about 10 deg may be feasible. By arranging the directional gamma-ray detectors in an array, we expect to be able to realize the GISAS, which is small, light, and capable of quantitative measurement.