Study of the neutron background at the Canfranc Underground Laboratory

A quantitative study of the neutron environment in the Canfranc Underground Laboratory is being carried out, specially focused on the IGEX Dark Matter Experiment. A set of simulations based on GEANT4 and FLUKA codes together with data obtained in different experimental conditions in the IGEX set-up allow us to quantify the effect of neutrons in the low-energy spectrum of the IGEX-DM detector. As an intermediate result, we have estimated the flux of neutrons from radioactivity of the rock (4 × 10 − 6 n cm − 2 s − 1 ), the flux of muon-induced neutrons in the rock (2 × 10 − 9 n cm − 2 s − 1 ), and the rate of neutron production by muons in the lead shielding (4×10 − 9 n cm − 3 s − 1 ). As a first conclusion, 40 cm of neutron moderator should be enough to push far below the present level of sensitivity of the IGEX-DM detector, the contribution to the low energy region (from 4 to 10 keV) of neutrons coming from fission and ( α ,n) reactions in the rock. The contribution of muon-induced neutrons in the rock is also estimated to be far below the present level of sensitivity, so the residual background of IGEX-DM in the low energy region cannot be attributed to neutrons.