  The 3He-based neutron detectors are no longer the default solution for
neutron scattering applications. Both the inability of fulfilling the
requirements in performance, needed for the new instruments, and the shortage
of 3He, drove a series of research programs aiming to find new technologies for
neutron detection. The characteristics of the new detector technologies have
been extensively tested to prove their effectiveness with respect to the
state-of-the-art technology. Among these, the background rejection capability
is crucial to determine. The signal-to-background ratio is strongly related to
the performance figure-of-merit for most instruments. These are designed to
exploit the high flux expected from the new high intensity neutron sources.
Therefore, an inadequate background rejection could significantly affect the
measurements, leading to detector saturation and misleading events. This is of
particular importance for the kind of techniques in which the signals are
rather weak. For the first time, the sensitivity of 3He detectors to fast
neutrons, up to En = 10 MeV, has been estimated. Two independent measurements
are presented: a direct calculation based on a subtraction method used to
disentangle the thermal and the fast neutron contribution, while a further
evidence is calculated indirectly through a comparison with the recently
published data from a 10B-based detector. Both investigations give a
characterization on the order of magnitude for the sensitivity. A set of
simulations is presented as well in order to support and to validate the
results of the measurements. A sensitivity of 4x10-3 is observed from the data.
This is two orders of magnitude higher than that previously observed in
10B-based detectors.
