Time-Encoded Thermal Neutron Imaging using Large-Volume Pixelated CdZnTe Detectors

Abstract

CdZnTe detectors are commonly used for room-temperature gamma-ray spectroscopy and imaging in a variety of applications including nuclear security, nuclear medicine, and space science. The material's long-established sensitivity to thermal neutrons, however, is less utilized. Generally speaking, the performance of neutron detectors based on the Cd capture reaction is limited by the physical nature of the reaction itself. Multiple gamma rays are emitted promptly following each capture event, which consists of one realization of many possible combinations of gamma-ray lines. Although the gamma-ray cascade can reduce photopeak efficiency in conventional devices, this work demonstrates that pixelated CdZnTe can recover losses by reading out each gamma-ray interaction separately. Including coincident events, the measured 558-keV photopeak efficiency for a 3 by 3 array of 2 cm by 2 cm by 1.5 cm pixelated CdZnTe detectors was about 10%, i.e., ten 558 keV photopeak events per 100 incident thermal neutrons. This was in good agreement with its calculated value. Initial measurements also show that neutron-gamma discrimination beyond simple energy windowing is possible when incorporating the 3-D interaction locations of gamma rays provided by the pixelated readout.

In this work, we developed and successfully demonstrated a proof-of-principle time-encoding system for thermal neutron imaging using pixelated CdZnTe. Time encoding was chosen because it is not limited by the detector's position resolution or spatial extent. These issues are exacerbated by Cd capture due to the dispersal of cascade gamma rays throughout the device. The system was first tested using a MURA-based, W-metal mask with both Co-57 and U-metal gamma-ray sources. About 0.3-degree angular resolution within a 22-degree field of view was achieved for gamma rays, and good image uniformity was observed for objects of moderate spatial extent. A MURA-based thermal neutron mask was then constructed using 1-mm-thick BN tiles, which attained roughly 4-degree angular resolution within a 50-degree field of view when measuring HDPE-moderated Cf-252. Two different thermal neutron imaging measurements were taken, with one and two moderators within the field of view. Reconstructed images corresponded well with the 3-D locations and sizes of moderators, and had predictable signal-to-noise ratio. We believe the experimental imaging results provided here warrant further studies on the use of CdZnTe for other thermal neutron imaging scenarios.