The work presented in this report describes the current status of the High-Resolution Gamma-ray Tomography System (HRGTS) under development at the University of Michigan (UM) for high-resolution measurements of void fractions in complex geometries such as fuel bundles and high-pressure test sections, including various test measurements. The system consists of a high-resolution fan-beam gamma tomography system based on an Ir-192 source and a custom modular detector array. The module arrangement is composed of eight detectors, each consisting of a LYSO (Lu1.9Y0.1SiO5) scintillator read out by two Silicon Photomultipliers (SiPMs) arranged in parallel for improved light collection. Custom pulse-processing electronic boards for each module amplify the analog signals and count events at two independently-defined pulse height thresholds per detector. The individual detector modules have WiFi capabilities so that the detector arc can be easily expanded, requiring only a single PC to operate the entire array remotely. Reconstructed images of test phantoms have confirmed a spatial resolution of about 1.5 mm. Further tests were performed using a static mock-up of a 5x5 fuel assembly. The complete detector arc is mounted on a rotating stage with a large inner hole of 470 mm in order to accommodate flow channels, such that the source and detector are rotated around the stationary channel in order to collect the range of projection angles needed to perform tomographic reconstruction.
