Estimation of photon yield in liquid scintillation counter by using Geant4 Monte Carlo simulation

Liquid scintillation counter (LSC) is an efficient technique for quantitative measurement of radioactivity in low energy beta decay nuclide. However, the detection efficiency varies with quench effect that caused by missing energy in the conversion from the energy of beta-ray to the intensity of scintillating photons. For the purpose of investigating the effect of photon yield on efficiency and quench level, the Geant4 simulation has been performed. The simulation modeled LSC-5100 manufactured by Hitachi Ltd. It consists of two photomultipliers (PMTs) placed around a sample vial in an optical chamber, and the activity is determined by using the count of double coincidence event in two PMTs. The simulation accounts for radioactive decay, electromagnetic interactions, scintillation and optical photon processes. The LSC spectrum was calculated from optical photons reached at the PMT windows. The photon yields in the simulation were estimated to reproduce the measured spectra in C-14 and H-3 standard quenched samples of Ultima Gold provided by PerkinElmer Co. Ltd. The calculated efficiencies in the photon yields well reproduced the measured efficiencies. In addition, the estimated photon yields were verified in the simulation of the external standard channel ratio (ESCR) with Cs-137 source. The ESCRs in the simulation reproduced the measured ESCRs as well.