Direct measurement of the atmospheric neutron flux in the energy range 10–500 MeV

The results of a direct measurement of the atmospheric neutron flux in the energy range 10–500 MeV performed at 42°25′11″ N, 13°31′2″ E, rigidity cutoff 6.3 GV, altitude 970 m a.s.l. (LNGS external site) on November 2008, during minimum solar activity, are reported.The detector consists of a 1.5 × 1 × 1 m3 stainless steel tank filled with 1.2 tons of 0.1% Gd doped liquid scintillator, monitored by three photomultipliers and surrounded by a 4π active muon veto. The measurement is performed by observing events formed by two un-vetoed pulses inside a temporal window 95 μs long: the first one due to a recoiling proton scattered by a neutron, the second one due to the neutron capture (n, Gd).The resulting atmospheric neutron fluxes are:Φ(E>10MeV)=47±5neutronss-1m-2;Φ(E>20MeV)=42±4neutronss-1m-2.