Ambient neutron measurement with a He-3 proportional counter in an underground laboratory at the Kamioka Observatory

Neutron is one of the most serious backgrounds for underground experiments searching for rare events.

Ambient neutron flux in an underground lab. of Kamioka Observatory was measured using a He-3 counter with various moderator setup. Since the detector response largely depends on the spectrum shape, energy spectra of neutrons transported from the rock to the lab. are estimated by Monte-Carlo simulations. The ratio of thermal neutron flux to the total one was found to depend on the thermalizing efficiency in the rock. Therefore, the ratio of the count rate without a moderator to that with a moderator was used to determine this parameter. As a result, the most-likely neutron spectrum predicted by the simulations with the parameters determined by the experimental results was obtained. The neutron flux is (23.52+/-0.68(stat.)+1.87-2.13(sys.)) μ/cm²s.

The result suggests an interesting spectrum shape in high energy region (-MeV), which has not been indicated in previous studies. To clarify that, a scintillator is prepared. The new result will be reported.