Unconventional Advancements in Single Crystal, Time-Encoded Imaging

Abstract

Coded-aperture masks have been used in the field of radiation imaging for a few decades. In recent years, these masks have been used in time-encoded imaging for nuclear nonproliferation applications and the imaging of special nuclear material. This work includes the design and construction of two rotating coded-aperture masks around single crystal, non-position-sensitive radiation detectors to perform cylindrical time-encoded imaging. Also, there is an exploratory spherical coded-aperture mask simulated for a new time-encoded imaging modality. These systems were designed for the advancement of coded-aperture systems for nuclear nonproliferation applications.

The first system has a compact, 1D coded-aperture design for imaging both gamma rays and neutrons. The goal of this system is to produce images despite lacking a pointlike detector inside of the mask. This motivates the use of cylindrical coded-aperture masks for handheld or man-portable imagers. In contrast, the second system focuses on gamma rays and optimizing the setup for typical high-purity germanium detectors. The second system has a larger 2D coded aperture utilizing a low-density, 3D-printed, tungsten-loaded plastic mask. Both coded apertures allow for the reconstruction of images with angular resolutions better than the geometric angular resolution inherent to the open voxels in the masks. Comparisons are made between an inorganic scintillator and a semiconductor detector to study the potential imaging improvement enabled by incorporating higher energy resolution. In particular, the use of high-purity germanium detectors was postulated to improve the reconstructed image quality by better differentiating radiation background and scatter events from photopeak interactions. Indeed, a 137Cs measurement with the 1D system reconstructing counts only in photopeak regions-of-interest led to an angular resolution improvement of 41.5% with HPGe and 13.4% with CLLBC.

Depleted uranium was imaged near a 239Pu proxy with the 2D system at three distances. An experimentally-based system response matrix was created with 133Ba, allowing for improved image reconstruction for low-energy sources (i.e., 241Am, 133Ba, 235U) and showed small improvements for high-energy sources (i.e., 137Cs, 238U). This response matrix reduced image noise leading to an angular resolution improvement of 5.3% in FWHM at 60 keV.

An unconventional coded aperture was simulated in a spherical geometry to explore a new modality of time-encoded imaging. Ray-traced and radiation transport models were simulated for the preliminary validation of a coded-aperture system utilizing spherical time-encoded imaging. Introducing multiple axes of rotation showed differences in count rate modulation that can be utilized for improved image reconstruction. This would be beneficial for setups where there is a source along one such axis, and source localization is only possible by rotating around other axes. These simulations provide a proof-of-concept for a free-moving, spherical time-encoded imaging system.