Thermal neutron profile inside J-PARC main ring tunnel

Abstract To estimate the beam losses and activation in the J-PARC main ring (MR) tunnel, thermal neutron distribution was measured using gold foil activation detectors with the Cd difference method. Hundred pairs of bare and Cd-covered gold foils were prepared to cover the entire area of the MR tunnel. The relative and absolute activities of the gold foils were measured using an imaging plate and an HP-Ge detector for some reference foils, respectively. A total of 97% of the thermal neutrons were detected in the injection section, where a beam collimation system was installed to localize the beam loss in the MR tunnel. The thermal neutron fluxes were compared with the calculation results from a Monte Carlo simulation code, PHITS. The calculation results were generally in good agreement with the experimental thermal neutron flux distribution. Furthermore, we investigated a simple empirical formula, ϕ th = C Q ∕ S , to estimate the thermal neutron flux in the accelerator room, where Q is the neutron source intensity and S is the total surface area of the room. A coefficient C of 1.61 ± 0.15 was deduced in the MR tunnel, which is within the error range of the previous value measured in the CERN CHARM facility using a 24 GeV/c proton beam despite the different projectile energies and room shapes. The γ dose rates at the same positions as the gold foils were measured as well. A strong correlation was observed between the thermal neutron fluxes and γ dose rates with respect to the surrounding concrete types. This result suggests that the dose rate on the concrete wall can be used to estimate the distribution of the thermal neutron fluxes and the amount of beam loss in the MR.