Study of Counting Efficiency with Triple to Double Coincidence Ratio in Liquid Scintillation Counter by using Geant4

Liquid scintillation counter (LSC) is one of radioactivity measurement techniques, which is suitable for low level and low energy beta decay radionuclide. LSC counts the number of flushes of scintillation lights by charged particles of decay products. However, the counting rate may be partly lost due to quenching that reduces the light output of the scintillator. Therefore, the compensation of the count rate loss is essential to determine the radioactivity. We previously reported on the study of counting efficiency in double coincidence events of two photomultipliers (PMTs) by using a Geant4 Monte Carlo simulation. In this research, we have newly developed a simulation for LSC-LB7 manufactured by Hitachi, Ltd. that is equipped with three PMTs. The triple and double coincidence ratio (TDCR) method is an interest for quantitative measurement in the field of radionuclide metrology, because the method does not require a series of standard quenched samples for estimating the detection efficiency. For the purpose of studying the counting efficiency and TDCR, the simulation of LSC-LB7 system had been carried out. The liquid scintillation cocktail was assumed to Ultima Gold of PerkinElmer Co. Ltd. The photon yield of scintillation cocktail was adopted from the previous result in LSC-5100, that had been determined in the series of standard quenched sample of C-14. The physics-list took account for all physics processes such as radioactive decay, electromagnetic and optical photon processes. The quantum efficiency and photosensitivity of PMT were also taken into account. H-3, C-14, S-35 and P-32 were generated in scintillation cocktail in a glass vial, respectively. The correlation between TDCR and efficiency was studied by counting events that satisfy the double and triple coincidence in three PMTs. The results show that the correlation is described by a bijective function in the pure beta-decay nuclides.