Abstract
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.
Highlights
A sharp drop in the spectrum is expected, and it extends down to low energies. (A further peak due to the gamma rays occurs at very low energies.) The presence of thermal neutrons in the incoming cosmic flux can be verified by the measurement in configuration (S)
New science is enabled by more powerful instruments at neutron sources that are increasing the available neutron flux delivered at the instruments
Gamma rays and fast neutrons are the main species of background in a neutron scattering facility, and the flux on a detector of such radiations can exceed the thermal neutron flux which carries the scientific information
Summary
A source of background in a neutron scattering facility is mainly due to gamma rays, epi-thermal, fast neutrons and cosmic rays [14] When any of these radiations is detected, it results in spurious events recorded as “good” events. An indirect method based on a series of measurements with different absorbers and simulations has indicated that Helium-3 has a fast neutron sensitivity of the order of 10−3 [57]. In this manuscript, a complementary method is presented. The measurement with cosmic rays, a possible source of background, is shown here to verify the actual thermal and fast cosmic neutron fluxes present at the ground level at ESS
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