Fourth-generation microstructured semiconductor neutron detector (MSND)-based 3He replacement (HeRep) for high pressure 3He detectors

Two fourth-generation
³
He replacement (HeRep Mk IV) detectors based on microstructured semiconductor neutron detector (MSND) technology have been fabricated and characterized against a 0.75-in diameter by 3.0-in long, 10-atm
³
He neutron proportional counter. The HeRep Mk IV detectors have a 0.75-in by 0.75-in square form factor with one HeRep Mk IV measuring 3.1-in long and the other measuring 6.1-in long. Each HeRep Mk IV was populated with 15 DS-MSNDs that have an intrinsic thermal-neutron detection efficiency of approximately 50%. The void spaces between adjacent detectors in the 6.1-in long HeRep Mk IV were filled with HDPE moderator. The 3.1-in long HeRep Mk IV recorded 92.8 ± 1.8% of the count rate of the 10-atm
³
He detector for bare 26.5-ng
²⁵²
Cf, bare detector and 82.2 ± 0.9% for moderated
²⁵²
Cf source, moderated detector configurations. The 6.1-in long HeRep Mk IV count rate was 108.5 ± 2.0% and 113.8 ± 1.2% of the count rate of the 10-atm
³
He detector for bare
²⁵²
Cf, bare detector and moderated
²⁵²
Cf, moderated detector, respectively. The gamma-ray rejection ratio was determined for
¹³⁷
Cs at a dose rate of 50 mR/hr. The gamma-ray rejection ratio was 1:1.5×10
⁸
and 1:4.6×10
⁸
for the 3.1-in long and 6.1-in long HeRep Mk IV, respectively. The HeRep Mk IVs exhibit a 116% (3.1-in long) and 33% (6.1-in long) improvement in intrinsic detection efficiency over the 0.75-in diameter by 4.5-in long HeRep Mk III for moderated
²⁵²
Cf at 25 cm.