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

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 x 10(11) Bq Am-241-Be source. An 82 cm (L) x 50 cm (W) x 50 cm (H) moderator was designed through Monte Carlo simulation to thermalize fast neutrons. Each component was determined to optimize thermal neutron fluence rate phi, proportion of thermal neutrons rho, and uniformity factor mu of the radiation field. A He-3 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 rho and its length can be utilized to adjust phi. Homogenizing lens will improve mu and reduce phi. Experimental phi, cadmium ratio eta, and mu 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 longterm stability testing.