Thermal-neutron and gamma-ray coded-aperture imaging using a CdZnTe detector with a Gd converter: A Monte Carlo study

Compact neutron/gamma-ray imaging is of considerable interest for nuclear safety and nonproliferation, yet realizing dual-particle coded-aperture imaging with a semiconductor detector platform remains challenging. In this work, a pixelated CdZnTe (CZT) detector, which can directly record gamma-ray shadowgrams, is extended to indirect thermal-neutron coded-aperture imaging by coupling it with a gadolinium film, while preserving its gamma-ray imaging capability. A composite mask assembled from boron nitride tiles and tungsten is introduced to modulate thermal neutrons and gamma rays within the same compact architecture. The signal formation processes are described separately for the two particle types: gamma rays are detected through direct interactions in CZT, whereas thermal neutrons are inferred from secondary photons generated after neutron capture in the Gd converter. Based on these processes, source-specific analytical system matrices are developed for direct gamma-ray imaging and converter-assisted thermal-neutron imaging, and image reconstruction is performed using the maximum-likelihood expectation-maximization algorithm. Monte Carlo simulations with a ⁵⁷Co gamma-ray point source and a moderated ²⁵²Cf neutron point source demonstrate separate far-field angular localization of both source types. The results provide a modeling and reconstruction framework for CZT-based dual-particle coded-aperture imaging, while also showing that indirect thermal-neutron imaging remains limited by low imaging efficiency and blurred neutron-induced projections.