STUDY OF N-GAMMA DISCRIMINATION WITH NE213 AND BC501A LIQUID SCINTILLATORS OF DIFFERENT SIZE

A comparative study of n-gamma discrimination by the digital charge comparison method was carried out for NE213 and BC501A scintillators of different size coupled to a 130 mm diameter XP4512B photomultiplier. Particularly, the scintillators of diameter 50 mm x 50 mm size are compared with those of 41 volume (160 mm in diameter and 200 mm in depth). The figure of merit, M, of 3.81 measured with the diameter 50 mm x 50 mm BC501A scintillator at 1 MeV energy lost by recoil electrons, is much better than M of 2.05 as observed with the 41 volume cell. This is the effect of the reduced photoelectron yield by about 50% determined for the 41 volume scintillator. Moreover, the slowing down of the light pulse and multiscattering of neutrons have a further influence on the lowering of the M factor. The study of the M factor versus the initial delay and width of the gate set at the slow component showed that the early light of the slow component represented often by the intermediate component with the decay time constant of about 30 ns is of great importance for the n-gamma discrimination. Particularly, for the 41 volume cells it is sufficient to limit the width of the gate at the slow component to about 300 ns. The comparison of the NE213 and BC501A scintillators showed that the BC501A scintillator exhibits a larger light yield evidently observed for the 41 volume cell and thus giving a better n-gamma discrimination.