The γ-ray performance of a large volume plastic scintillator

Abstract

The γ-ray spectral and the background count of a large plastic scintillator (20″ × 10″ × 6 1 2 ″) viewed by two 7″ diameter photomultipliers (20th Century VMP/11/170 or E.M.I. 9623) were investigated. The spectral spread on the high energy side was compounded of three independent factors: (1) Statistical, (2) Geometrical, (3) Intrinsic, and contribution of each factor to the total spread was calculated from experimental γ-ray spectra. The first two factors were then independently estimated (1) by determining the average number of photoelectrons released by a known energy deposition in the scintillator, and (2) by directing collimated beams of γ-rays (from 137Cs source) into the scintillator. It was found that the optimum performance could be obtained by applying proper distribution of high voltage on the photomultiplier and by using short light guides. The background spectra (0–3 MeV γ-ray energy) was obtained inside a steel cubicle (5″ thick wall) and additional 1″ lead shielding reduced the background by 25%. The radioactivity of the photomultiplier and its contribution to the total background were measured. The perspex light guide (1″ or 1 1 2 ″ thick) reduced the background by about 10% which was due to the photomultiplier radioactivity. The photomultiplier noise which determines the lowest measurable energy was found to be an increasing function with the voltage between photocathode and first dynode.