Measurement of the ambient neutron flux and spectral distribution at the Canfranc Underground Laboratory

Abstract

We report on the measurement of the ambient neutron flux and its energy distribution over a broad range of neutron energies, conducted in Hall A of the Canfranc Underground Laboratory using the High Efficiency Neutron Spectrometry Array (HENSA), with a total livetime of 248 days. In particular, we obtained a thermal neutron flux of $3.4(2)\times 10^{-6}$ cm${}^{-2}$ s${}^{-1}$ and a total energy-integrated flux of $14.8(2)\times 10^{-6}$ cm${}^{-2}$ s${}^{-1}$. This work represents the first long-term measurement of the ambient neutron flux that includes a full spectral characterization across a wide energy interval (from $10^{-4}$ eV to 20 MeV), performed in an underground laboratory to date. The results are relevant for rare-event search experiments located underground, ranging from nuclear astrophysics to astroparticle physics.

5 pages, 3 figures, 1 table