Fundamental approach to development of plastic scintillator system for in situ groundwater beta monitoring

The performance of a plastic scintillator for use in an in situ measurement system was analyzed using simulation and experimental methods. The experimental results of four major pure beta-emitting radionuclides, namely ³H, ¹⁴C, ³²P, and ⁹⁰Sr/⁹⁰Y, were compared with those obtained using a Monte Carlo N-particle (MCNP) code simulation. The MCNP simulation and experimental results demonstrated good agreement for ³²P and ⁹⁰Sr/⁹⁰Y, with a relative difference of 1.95% and 0.43% between experimental and simulation efficiencies for ³²P and ⁹⁰Sr/⁹⁰Y, respectively. However, owing to the short range of beta particles in water, the efficiency for ¹⁴C was extremely low, and ³H could not be detected. To directly measure the low-energy beta radionuclides considering their short range, a system where the source could flow directly to the scintillator was developed. The optimal thickness of the plastic scintillator was determined based on the suggested diameter. Results showed that the detection efficiency decreases with an increase in the depth of the water. The detection efficiency decreased drastically to approximately 10 cm, and the tendency was gradually constant.