Evaluation of MSNDs for Fast-Neutron Detection and the TREAT Hodoscope

The need for next-generation, accident-tolerant fuels has generated significant interest in the restart of domestic fuels testing at the Transient Reactor Test (TREAT) facility. A key feature of the TREAT reactor core is an open, horizontal channel that allows line-of-sight viewing of test specimens using various instrumentation during steady-state and transient operation. This instrumentation includes the hodoscope, which consists of over 300 steel-collimated channels coupled to Hornyak buttons (and other detectors) for detection of fast neutrons emitted from fission of the test specimen. Posterior analysis can be used to reconstruct fuel motion and other properties from the collimated channel signals. A large-scale effort is underway to develop next generation instrumentation for transient testing at TREAT (and, potentially, other facilities). Work at KSU specifically aims to provide improved detection systems for hodoscope applications. It was initially proposed to adapt KSU-developed Microstructured Semiconductor Neutron Detectors (MSNDs) for use as fast-neutron detectors. The current generation of MSNDs is designed for thermal-neutron detection based on ⁶Li and can achieve intrinsic efficiencies nearing 30% owing to the large (n, t) cross section of ⁶Li and clever optimization of the geometry. In order to evaluate MSNDs for fast-neutron detection, two approaches were considered: (1) use of a hydrogenous material to thermalize neutrons to exploit the intrinsic efficiencies of current MSNDs, and (2) substitution of a fast-sensitive reactant for ⁶Li. The former approach is attractive because it makes use of proven, off-the-shelf technology and leads to reasonable time resolution and efficiency. Unfortunately, use of thermalization leads to prohibitive degradation of spatial resolution. The second approach will require further experimental efforts, but scoping analyses suggest ²³⁵U- or ²³⁷Th-loaded MSNDs may achieve sufficient efficiencies. The rest of this summary examines these issues in more detail. (authors)