Abstract
Ambient neutrons are one of the most serious backgrounds for underground experiments in search of rare events. The ambient neutron flux in an underground laboratory of Kamioka Observatory was measured using a 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 were 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. Thus, 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 [1]. In this paper, we explain our method of the result and discuss our future plan.
Highlights
We evaluated an ambient neutron spectrum with 3He proportional counter at Kamioka
We considered the natural sources of ambient neutrons in a wall rock using data-driven Monte-Carlo (MC) simulation to estimate the shape of the neutron energy spectrum
The ambient neutrons were measured at Kamioka Observatory
Summary
Ambient neutrons are one of the most serious backgrounds for future underground experiments, such as neutrinoless double beta decay searches, neutrino measurements, and direct dark matter. We evaluated an ambient neutron spectrum with 3He proportional counter at Kamioka. We considered the natural sources of ambient neutrons in a wall rock using data-driven Monte-Carlo (MC) simulation to estimate the shape of the neutron energy spectrum. We will review the current method and propose a future plan to measure the comprehensive energy spectrum of ambient neutron in an underground laboratory. To measure high-energy neutrons (in the MeV range), moderators and a shielding material were used. A polyethylene moderator (outer radius of 9.9 cm, length of 51 cm, and thickness of 6.5 cm) was used to thermalize the high-energy neutrons so that they can be detected by the 3He proportional counter. It was observed that the B sheet shielded the ambient thermal neutrons, thereby making setup B to be sensitive mainly to high-energy neutrons. The errors of a simple waveform cut, evaluation of total counts, and 5% detector gain fluctuations including the electronics gain difference between setups were taken into account as systematic errors
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