The effect of particle size of CaF2(Eu) powder and chamber thickness on solid scintillation counting for low-level tritiated water measurement

Abstract

With the development of the International Thermonuclear Experimental Reactor (ITER) and the China Fusion Engineering Test Reactor (CFETR), the measurement of tritium in low-level tritiated water is indispensable from the viewpoint of assessment and safety of tritium. In our work, CaF2(Eu) was chosen as the scintillation material in the research to design a tritium monitor for tritiated water. The effect of particle size of CaF2(Eu) powder and chamber thickness on detection efficiency and count rate were assessed by Geant4 simulation. Our results show that reducing the CaF2(Eu) particle size can effectively improve the probability of energy deposition in the scintillator. Under the same conditions, the use of CaF2(Eu) particles with a radius of 0.5 μm increases the energy deposition probability by a factor of more than 20 compared to particles with a radius of 50 μm. However, the coincidence detection efficiency rapidly decreases as the chamber thickness of the solid scintillation counting (SSC) system increases when the particle size is small. With a fine CaF2(Eu) powder (particle radius of 1 μm), the coincidence detection efficiency is greater than 50% when the chamber thickness is 1 mm. Only approximately 35% of tritium decay is coincidentally detected when the chamber thickness is 5 mm. In addition, filling the chamber with CaF2(Eu) particles of different sizes leads to different maximum count rates. With the finest CaF2(Eu) powder, the SSC system has the highest count rate when the chamber thickness is 5 mm. With such results, the replacement of the scintillation cocktail with CaF2(Eu) powder in low-level tritiated water measurements is very promising. The simulation result will help in the design of a SSC system applied to low-level tritiated water measurement, and such a system will be tested in our future work.