Thermal neutrons at Gran Sasso

We have used a set of proportional counters filled with 3He to measure the thermal neutron background flux in the deep underground Gran Sasso National Laboratory (LNGS). The flux was equal to (5.4±1.3)×10−7 neutrons/(cm2s). During one week's exposure we registered about 650 neutron signals above about 160 background signals, so using this method we would be able to measure 10 times smaller neutron flux.