Measurement and calculation of thermal neutrons induced by the 24 GeV/c proton bombardment of a thick copper target

The CERN High-Energy AcceleRator Mixed field (CHARM) facility provides a secondary particle field, produced by irradiating a thick target with 24 GeV/c protons supplied by the proton synchrotron. In order to investigate the thermalization process of secondary neutrons in the CHARM facility, we measured the thermal neutrons using the gold foil activation method. Bare and Cd-covered gold foils were placed at 35 positions to deduce the thermal neutron distribution in the CHARM facility. The 197Au(n, γ)198Au reaction rates and thermal neutron fluxes measured in this study were compared with the Monte Carlo simulation codes, PHITS, FLUKA, and MARS. The comparison between the measured and simulated values gives an agreement better than a factor of two. Besides, we investigated the simple empirical formula to estimate a thermal neutron flux in the accelerator room, ϕth = CQ/S, where Q is the neutron source intensity and S is the total surface area of a room. The coefficient C estimated in this study did not significantly depend on the incident proton beam energy.