Verification of He-3 proportional counters' fast neutron sensitivity through a comparison with He-4 detectors: He-3 and He-4 proportional counters' fast neutron sensitivity and evaluation of the cosmic neutron fluxes at ESS

In the field of neutron scattering science, a large variety of instruments require detectors for thermal and cold neutrons. Helium-3 has been one of the main actors in thermal and cold neutron detection for many years. Nowadays, neutron facilities around the world are pushing their technologies to increase the available flux delivered at the instruments; this enables a completely new science landscape. Complementary with the increasing available flux, a better signal-to-background (S/B) ratio enables to perform new types of measurements. For instance, in neutron reflectometry, the time resolution for kinetic studies is limited by the available S/B. An improved S/B opens the possibility of sub-second kinetic studies. To this aim, this manuscript re-examines the background sensitivity of today's gold standard neutron detection. Fast neutrons and gamma rays are the main background species in neutron scattering experiments. The efficiency (sensitivity) of detecting fast neutrons, cosmic rays and gamma rays, for a Helium-3-based detector is studied here through the comparison with Helium-4 counters. The comparison with Helium-4 allows to separate the thermal (and cold) neutron from the fast neutron contributions in Helium-3-based counters which are otherwise entangled, verifying previous results from an indirect method. A relatively high sensitivity is found. Moreover, an estimate for the cosmic neutron fluence, also a source of background, at ground level at ESS is presented in this manuscript.