Flux measurement of fast neutrons in the Gran Sasso underground laboratory

Neutron energy spectrum for energies extending up to 15 MeV has been measured in the Hall A of INFN Laboratori Nazionali del Gran Sasso in Italy, by using a \(\mathrm {1.5~m^3}\) stainless-steel tank filled with 1.2 ton Gd-doped liquid scintillator. A two-pulse signature, due to proton recoils and delayed gammas from neutron capture on Gd, has been used for event selection. The neutron energy spectrum has been obtained from the detected proton-recoil spectrum, after unfolding the detector response, which had been simulated with GEANT4. Although this spectrum has been measured previously, the present work represents an improvement in the energy resolution with respect to previously published results. The results obtained with this technique are compared with other measurements existing in literature.