Numerical evaluation of fast-sensitive microstructured semiconductor neutron detectors for TREAT hodoscope

Abstract

Presented is the numerical study of fast-sensitive, actinide and hydrogenous microstructured semiconductor neutron detectors (MSNDs) for the hodoscope at the Transient REActor Test facility (TREAT) using Geant4 and MCNP6. Neutron converters considered were 237Np, 235U, natural uranium, and 232Th for actinide MSNDs and paraffin wax for hydrogenous MSNDs (H-MSNDs). Paraffin wax was found to have a larger fission-spectrum-weighted macroscopic cross section (0.32 cm −1) than the actinide materials (the best being 0.067 cm −1 for 237Np). However, actinide reactants were found to allow higher lower-level discriminator (LLD) settings due to the large energy of fission fragments. Actinide MSNDs filled with 235U, natural uranium, and 232Th were evaluated in Geant4 using the fission fragment generator. With the LLD set to 5 MeV, the intrinsic neutron-detection efficiency of the 235U-filled MSNDs was 1.2% for a 2-cm device length and saturated at 2.6% for lengths beyond 14 cm, where 20-μm trench and 10-μm wall widths were assumed. The deposited energy in 235U predicted by Geant4 differed from the MCNP6-predicted value by about −0.7%. With the LLD set to 300 keV, Geant4 and MCNP6 predicted an efficiency of about 10% for a 2-cm long H-MSND with 20-μm trench and 10-μm wall widths and an efficiency of about 26% for a detector length of 20 cm. For a LLD set to achieve a signal-to-noise (S/N) ratio of 100 when including gamma-ray noise, the best-case, Geant4-predicted efficiencies were 2.5% and 9.6% for 2-cm and 20-cm long devices with 60-μm trench and 40-μm wall widths.