Analysis of the Neutron Thermalization in an Accelerator Room

The estimation of the thermalized neutrons in the accelerator room is one of the important shielding problems in the facility design and the evaluation of the worker's exposure. In an accelerator room, high energy neutrons produced by an accelerator lose their energy by the multiple collisions with the wall, floor, or ceiling and are reflected from concrete wall's surfaces as thermal neutrons. The simple empirical formula, phi(th) =CQ/S,iswidely used as an estimation for the neutron thermalization. Where phi(th) is a thermal neutron flux in the room, Q is the intensity of the source neutron, S is the inner surface area for an accelerator room and C is the coefficient as a fitting parameter. In this study, various factors including S were examined to evaluate the value of C based on the MCNP simulation. The dependence on room size, room shape, energy of the source neutron and distance from the source were analyzed by using MCNPX code. As a result, the modified estimation, phi(th) = Sigma(g) C(g)Q(g)/S, and C-g dependant on energy group of the source neutron were proposed. The proposed estimation was also compared with the results from other estimation method and MCNPX calculations.