Thermal neutron reference radiation facility with high thermalization and large uniformity area

Abstract

A new thermal neutron facility based on 12 241Am-Be neutron sources has been established at the National Institute of Metrology, China. It has two independent irradiation fields, the Inner field and Outer field, constructed with high-purity graphite and heavy water as moderators, respectively. Three innovative designs, including the reflection cavity, reflection layer, and homogenizing lens, were introduced to improve the performance of the facility. The reflection layer can increase the thermal neutron fluence rate by about 3~4 times, the reflection cavity also enhances thermal fluence by 3~4 times by taking advantage of multiple scattering of neutrons, and the homogenizing lens mounted on the neutron exit surface of the moderator improve the distribution of thermal neutron. The characteristics of the two fields were investigated by Monte Carlo simulations and experimental measurements. The results show that the new facility has high thermalization and large uniformity area with these innovative designs. For the Inner field, the thermal neutron fluence rate at the reference position is (21433.3 ± 407.2) cm−2∙s−1, the cadmium ratio is 20.6, the nonuniformity is 0.1% over the 40 cm × 40 cm region. For the Outer field, the thermal fluence rate at the reference position is (2046.0 ± 49.1) cm−2∙s−1, the cadmium ratio is 1433, and the nonuniformity is 0.7% in the 70 cm × 70 cm region. The facility is expected to be suitable for the large-size detector, and its excellent performance is attractive for various thermal neutron experiments.