Time Dependence of Scintillations and the Effect on Pulse-Shape Discrimination

The time dependence of the light following excitation by gamma rays and neutrons was measured for stilbene and for the liquids NE-213, NE-213M, and NE-218.+ In addition, the number of photoelectrons produced at the cathode of an RCA-8575 photomultiplier per keV energy loss in the scintillator has been determined for each of these scintillators. A straightforward technique for pulse-shape discrimination is described; it consists in measuring the time difference between the start and the instant at which the integrated photomultiplier pulse reaches a specified fraction of its final amplitude. From the measured scintillation decay, the probability distribution of these rise times has been calculated for different fractions and for different photon and neutron energies on the assumption of Poisson statistics. These calculated distributions are in agreement with the experimental results obtained with the above-mentioned technique. The pulse-shape discriminating system is very stable and will accept 80% of the neutrons at 350 keV and only approximately 1% of the gamma rays producing scintillations with equal integrated light.