The Response of Heavily Shielded Plastic Scintillator-Photomultiplier Combinations to Nanosecond Neutron Pulses

The effect of neutron transport in massive shields surrounding scintillation detectors has been experimentally determined. Beam target prompt neutron production to measure ion beam current and the measurement of thermonuclear neutron production in particle beam fusion experiments require nanosecond measurements fram scintillator-photomultiplier combinations shielded with massive amounts of lead. The lead shield is necessary to attenuate the intense x-ray bursts characteristic of these experiments. The response of shielded detectors to several-nanosecond-wide neutron pulses has been established for both 2.45 MeV and 14.1 MeV neutrons. The characteristic neutron pulse shape measured for lead shields up to 20.3 cm in thickness show a sharp leading edge with a few nanoseconds risetime, a FWHM of 6-32 ns, and a tail that can extend for tens of nanoseconds. This pulse shape is predicted from an analysis of elastic, inelastic, and (n, 2 n) reactions that occur in the lead shield.