A compact moderator for thermal neutron reference radiation field with an 241Am-Be source

Abstract

To calibrate thermal neutron detectors, State Key Laboratory of NBC Protection for Civilian plans to establish thermal neutron reference radiation field (TNRRF) based on the existing 7.4 × 1011 Bq 241Am-Be source. An 82 cm (L) × 50 cm (W) × 50 cm (H) moderator was designed through Monte Carlo simulation to thermalize fast neutrons. Each component was determined to optimize thermal neutron fluence rate φ, proportion of thermal neutrons ρ, and uniformity factor μ of the radiation field. A 3He proportional counter was used to evaluate the fabricated moderator. Results show that surrounding reflector is the main origin of thermal neutrons. D2O tank can promote ρ and its length can be utilized to adjust φ. Homogenizing lens will improve μ and reduce φ. Experimental φ, cadmium ratio η, and μ at the reference position are 1808.82 ± 85.96 cm−2 s−1, 315.80, and 5.29 %, respectively, which are consistent with simulated results. Next, TNRRF will be established after long-term stability testing.