Present Status of the Microstructured Semiconductor Neutron Detector-Based Direct Helium-3 Replacement

A small form-factor, third-generation microstructured semiconductor neutron detector (MSND)-based ³He replacement (HeRep Mk III) detector has been developed to serve as a direct replacement to expensive small-diameter, high-pressure ³He gas-filled proportional neutron counters. Previously, the larger, 2-in diameter by 6-in-long HeRep Mk II utilized thirty 4-cm² active area single-sided MSNDs to replace a similarly sized 4-atm ³He counter. The net count rate of the HeRep Mk II was 95.15% ± 9.04% of the net count rate of a similarly sized 4-atm ³He detector for bare ²⁵²Cf at a distance of 1 m. The HeRep Mk II net count rate measured 102.71% ±2.65% of the ³He detector net count rate when each were placed in 6-in diameter by 9-in-long cylindrical high-density polyethylene (HDPE) moderator casks. The HeRep Mk III was developed as a test bed for new-generation dual-sided MSNDs (DSMSNDs), as well as for reduced-power electronics. The 0.75-in diameter by 4.50-in-long HeRep Mk III was populated with twelve 1-cm² active area DSMSNDs. The pvp-type diodes have thus far realized 53.54% ± 0.61% intrinsic thermal-neutron detection efficiency. The HeRep Mk III was tested against a 0.75-in diameter by 3.0-in-long 10-atm ³He detector with a 30-ng ²⁵²Cf source at a distance of 0.25 m. The HeRep Mk III reported 45.4% ± 0.3% and 56.7% ± 0.2% of the count rate of the 10-atm ³He detector for detectors with no exterior moderator and in a 3-in by 5.75-in cylindrical HDPE moderator cask, respectively. The HeRep Mk III gamma-ray sensitivity was measured at a dose rate of 50 mR/hr from ¹³⁷Cs resulting in a net count rate of 0.01 cps, which corresponds to a gamma-ray rejection ratio of approximately 5:10⁷.