Environmental sub-MeV neutron measurement at the Gran Sasso surface laboratory with a super-fine-grained nuclear emulsion detector

Abstract

The measurement of environmental neutrons is particularly important in the search for new physics, such as dark matter particles, because neutrons constitute an often irreducible background source. The measurement of the neutron energy spectra in the sub-MeV scale is technically difficult because it requires a very good energy resolution and a very high $\ensuremath{\gamma}$-ray rejection power. In this study, we used a super-fine-grained nuclear emulsion, called nano imaging tracker, as a neutron detector. The main target of neutrons is the hydrogen (proton) content of emulsion films. Through a topological analysis, proton recoils induced by neutron scattering can be detected as tracks with submicrometric accuracy. This method shows an extremely high $\ensuremath{\gamma}$-ray rejection power, at the level of $5\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.16em}{0ex}}\ensuremath{\gamma}/{\mathrm{cm}}^{2}$, which is equivalent to five years accumulation of environmental $\ensuremath{\gamma}$ rays, and a very good energy and direction resolution even in the sub-MeV energy region. In order to carry out this measurement with sufficient statistics, we upgraded the automated scanning system to achieve a speed of 250 g/(year machine). We calibrated the detector performance of this system with 880 keV monochromatic neutrons: a very good agreement with the expectation was found for all the relevant kinematic variables. The application of the developed method to a sample exposed at the INFN Gran Sasso surface laboratory provided the first measurement of sub-MeV environmental neutrons with a flux of $(7.6\ifmmode\pm\else\textpm\fi{}1.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}n/({\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}\mathrm{s})$ in the proton energy range between 0.25 and 1 MeV (corresponds to neutron energy range between 0.25 and 10 MeV), consistent with the prediction. The neutron energy and direction distributions also show a good agreement.