Fast Neutron and Gamma Ray Risetime Differences for 8 to 10 MeV Pulses in a Large NaI(Tl) Crystal

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Abstract
A dissimilarity in the risetime distribution of 8 to 10 MeV pulses from a 22.9 by 30.5 cm NaI(Tl) crystal due to fast neutrons and those due to gamma rays of equivalent energy has been observed. The time spectra were obtained using low and high level discriminators triggering at 100 keV and 2.5 MeV, respectively. The NaI (Tl) crystal was viewed by six matched RCA 4524 bialkali photomultiplier tubes. The time resolution of this system (using a small plastic scintillator to obtain one time reference) for 4.43 MeV gamma rays using leading edge timing is 2.5 nsec. The fast neutrons undergo inelastic collisions within the crystal prior to capture. The mean time between collisions for 3 MeV neutrons in NaI is 2.8 nsec; thus, the time interval between the two discriminators is longer for a typical fast neutron than for a gamma ray of equivalent energy. This results in a degradation of the time spectrum due to the fast neutrons. The 8 to 10 MeV gamma ray risetime distribution is symmetric, having a time resolution (FWHM) of 2.75 nsec. The neutron risetime distribution has a tail on the side corresponding to longer time intervals between the two discriminator outputs. It is shown that it is possible to reduce the number of pulses due to neutrons in the 8 to 10 MeV region by ⅓ by setting a time window. Estimates of an increased capability to discriminate through the use of a pulsed accelerator and a beam pickoff (thereby allowing the time interval between neutron emission and first inelastic scattering to be observed) are presented.

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