  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 ratio (S/B) enables to perform new types of measurements.
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 to Helium-4 allows to separate the thermal (and cold)
neutron to 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.
