Optimization of Measurement Condition for Determination of Tritium in Air by Liquid Scintillation Counting

During the operation of the reactor, a large amount of tritium is released into the surrounding air through the waste gas treatment system and the plant ventilation system. Tritium is very easy to replace hydrogen in water molecules, and can enter the human body through breathing, resulting in internal radiation hazards. Therefore, it is necessary to study the measurement of tritium in ambient air. At present, there is no national or industrial standard for the measurement of tritium in air. In this paper, the liquid scintillation counter was used to measure the activity concentration of tritium in air. It is found that the detection limit of the activity concentration of tritium in the air mainly depends on ambient temperature, relative humidity, sample amount, detection efficiency of the instrument, background count rate of the instrument, measurement time, etc. Among them, ambient temperature and relative humidity are inherent characteristics of air samples. Therefore, in order to reduce the detection limit and shorten the measurement time as much as possible, three aspects must be considered which is reducing the background count rate, improving the detection efficiency and increasing the sample volume. Possible approaches include selecting background water and scintillation fluid with the lowest possible counts, selecting the appropriate counting area, and optimizing the mixing ratio of sample and scintillation fluid. In order to reduce the effect of water and scintillation fluid on the background, 11 different sources of background water and 3 types of scintillation fluid were selected for testing. The samples were prepared according to the ratio of 8 mL of water to 12 mL of scintillation fluid, and the long-term counting measurement was carried out after being stored in the dark for 24 h. The measurement results show that the count rate range of the background water sample varies from 2.373 min-1 to 3.441 min-1, and the difference between the maximum value and the minimum value is 1.068 min-1, which also shows that the selection of the background water sample is very critical. For three scintillation fluids including Gold Star Quanta, OtpiPhase HiSafe 3 and UItima Gold LLT, there is no significant difference in background and detection efficiency.In order to further reduce the detection limit, a figure of merit (FOM) was introduced, which is the ratio of the square of the detection efficiency of the radionuclide of interest to the background count r