High Resolution Time-Encoded Imaging of Plutonium and MOX using 3-D Position-Sensitive CdZnTe Detectors

Measuring the detailed spatial distribution of special nuclear material (SNM) in unknown objects is important to nuclear emergency response. The spatial distribution of radioactive material can be measured using coded aperture where source emissions are selectively modulated to encode an image. High resolution images can be made by encoding signal in the time domain, avoiding degradation mechanisms, such as detector spatial non-uniformity, that commonly limit image quality in traditional CdZnTe based coded aperture. This paper builds upon the work of Brown et al. and demonstrates high resolution, time-encoded imaging of high-burnup plutonium and MOX fuel at the Zero Power Physics Reactor using the MIRA system and digital CdZnTe detectors. The low-burnup plutonium Radiation Signature Training Device (RSTD) was similarly imaged. ²⁴¹Am was found to be a bright, imagable surrogate for high-burnup plutonium while x-rays and plutonium gamma rays were used to image low-burnup plutonium. The resolution of reconstructed time-encoded images was much better than 1 cm². Estimation of high-quality, directional spectra was experimentally demonstrated using ¹³³Ba, ⁵⁷Co and ²⁴¹Am check sources.