Validation of beta ray scintillation spectra in liquid scintillation counter using Geant4 simulation

Abstract

Liquid scintillation counter (LSC) has been used as a very efficient technique for a quantitative measurement of radioactivity. The LSC has been the most powerful tool in the fields of low level environmental radioactivity monitoring and the detection of low energy beta decay events of radioactivity. In LSC the sample is dissolved in a liquid scintillation cocktail in a sample vial. In order to quantify the nuclear decay in terms of activity, the LSC counts the number of flashes of scintillation lights. However, the sample's count rate varies with the detection efficiency that depends on the energy conversion from the released beta ray energy in nuclear decay to light flashes. Since the sample solution absorbs part of nuclear decay energy and photons of scintillation lights, which is known as quench, the observed count rate becomes less than the expected rate from actual energy. Therefore the compensation of count rate loss is essential to determine the sample's activity. There are many techniques to determine the efficiency, however, it is known that those techniques lead different results especially in low energy beta decay radionuclides such as 3H. In order to investigate the influence of quench phenomena in LSC, we developed a Geant4 based Monte Carlo simulation and validated the beta-ray scintillation spectra in LSC. The simulation takes into account radioactive decay, flashes of scintillation lights, and transportation of optical photons including reflection and refraction characteristics at material boundaries. In this paper, we report on the detail of our simulation and the comparison of beta ray spectra detected in LSC between the simulation and measurements.