Abstract: An activation detector for fast-neutrons has a yttrium target exposed to a neutron source. Fast-neutrons which have energy in excess of 1 MeV (above a threshold energy level) generate gamma rays from a nuclear reaction with the yttrium, the gamma rays having an energy level of 908.96 keV, and the resultant gamma rays are coupled to a scintillator which generates an optical response, the optical response of the scintillator is coupled to a photomultiplier tube which generates an electrical response. The number of counts from the photomultiplier tube provides an accurate indication of the fast-neutron flux, and the detector is exclusively sensitive to fast-neutrons with an energy level over 1 MeV, thereby providing a fast-neutron detector which does not require calibration or the setting of a threshold.