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

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) × 10⁻⁶ 3.4 ( 2 ) × 10 - 6 cm ⁻² - 2 s ⁻¹ - 1 and a total energy-integrated flux of 14.8(2) × 10⁻⁶ 14.8 ( 2 ) × 10 - 6 cm ⁻² - 2 s ⁻¹ - 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 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.