Measurement of ambient neutrons in an underground laboratory at Kamioka Observatory

TL;DR

This study measured the ambient neutron flux and spectrum in the Kamioka underground laboratory, providing new detailed spectral data crucial for background estimation in rare event experiments.

Contribution

It presents the first detailed neutron energy spectrum measurement at Kamioka, combining experimental data with Monte Carlo simulations to characterize ambient neutron backgrounds.

Findings

Total neutron flux: (23.52 ± 0.68 stat. +1.87/-2.13 sys.) × 10^{-6} cm^{-2} s^{-1}

Neutron spectral shape differs from previous studies, indicating new background characteristics.

The ratio of thermal to total neutron flux depends on rock thermalizing efficiency.

Abstract

Ambient neutrons are one of the most serious backgrounds for underground experiments searching for rare events. The ambient neutron flux in an underground laboratory of Kamioka Observatory was measured using a $\mathrm{^3He}$ proportional counter with various moderator setups. Since the detector response largely depends on the spectral shape, the energy spectra of the neutrons transported from the rock to the laboratory are estimated by Monte-Carlo simulations. The ratio of the thermal neutron flux to the total neutron flux was found to depend on the thermalizing efficiency of the rock. Therefore, the ratio of the count rate without a moderator to that with a moderator was used to determine this parameter. Consequently, the most-likely neutron spectrum predicted by the simulations for the parameters determined by the experimental results was obtained. The result suggests an interesting spectral shape, which has not been indicated in previous studies. The total ambient neutron flux is $(23.52 \pm 0.68 \ \mathrm{_{stat.}} ^{+1.87}_{-2.13} \ \mathrm{_{sys.}}) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$. This result, especially the energy spectrum information, could be a new and important input for estimating the background in current and future experiments in the underground laboratory at Kamioka Observatory.