Characterization of a mid-sized Li foil multi-wire proportional counter neutron detector

A 550 cm2 thermal neutron detector was constructed with five parallel sheets of 75 µm thick 6Li foil (95% enrichment) spaced 1.63 cm apart. Anode wire banks containing a plurality of anode wires were strung on both sides of each foil, six banks in total. The chamber was backfilled with P-10 proportional gas and over-pressured to 1.1, 1.5, 2.0, and 2.8 atm (111, 151, 202, and 284 kPa). The design was tailored to allow the products from the 6Li(n,t)4He reaction to escape both sides of the Li foil simultaneously, thereby, allowing for concurrent measurement in the proportional gas. The measured intrinsic thermal neutron detection efficiency of the detector with normal incident thermal neutrons to the foil sheets was 53.8±0.20%. When the detector was angled (55° from normal) such that a 0.5 cm diameter thermal neutron beam intersected all of the foil layers, the intrinsic thermal neutron detection efficiency increased to 58.6±0.21%. A 252Cf neutron source positioned at a distance of 2.0 m yielded an absolute neutron detection efficiency of 0.73 cps ng−1. The gamma-ray rejection ratio (GRR) was 7.67×10–9 as measured from a 60Co source for an exposure rate of 40 mR hr−1. Theoretical pulse-height spectra obtained with MCNP6 agreed well with experimental data and allowed pulse-height spectra and discriminator settings to be energy-calibrated. These results demonstrate the potential for the Li foil multi-wire proportional counter (MWPC) as a viable 3He neutron detector replacement.