Using plastic scintillator for fission neutron detection

The difference in speed of fast neutrons (E-n-2 MeV) and gamma rays has been investigated as a mechanism for distinguishing neutron and gamma interactions in plastic scintillators. Successive scattering of neutrons in a scintillator results in pulses that are broader than those from gamma rays because of the longer time interval (similar to 3ns) between scatterings. However, the experimentally observed pulse widths for these neutrons appear to be insufficiently different from those of gamma rays to permit discrimination on a pulse-by-pulse basis. An alternate technique using scintillators separated by an air gap has also been investigated using time-of-flight measurements. For this configuration the efficiency for neutron detection has been measured along with the interference from gamma rays when sources emitting both neutrons and gamma rays are present.