Detection Limit of a NaI(Tl) Survey Meter to Measure ¹³¹I Accumulation in Thyroid Glands of Children after a Nuclear Power Plant Accident

Background: This study examined the detection limit of thyroid screening monitoring conducted at the time of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011 using a Monte Carlo simulation.Materials and Methods: We calculated the detection limit of a NaI(Tl) survey meter to measure 131I accumulation in the thyroid gland of children. Mathematical phantoms of 1- and 5-year-old children were developed in the simulation of the Particle and Heavy Ion Transport code System code. Contamination of the body surface with eight radionuclides found after the FDNPP accident was assumed to have been deposited on the neck and shoulder area.Results and Discussion: The detection limit was calculated as a function of ambient dose rate. In the case of 40 Bq/cm2 contamination on the body surface of the neck, the present simulations showed that residual thyroid radioactivity corresponding to thyroid dose of 100 mSv can be detected within 21 days after intake at the ambient dose rate of 0.2 μSv/hr and within 11 days in the case of 2.0 μSv/hr. When a time constant of 10 seconds was used at the dose rate of 0.2 μSv/hr, the estimated survey meter output error was 5%. Evaluation of the effect of individual differences in the location of the thyroid gland confirmed that the measured value would decrease by approximately 6% for a height difference of ±1 cm and increase by approximately 65% for a depth of 1 cm.Conclusion: In the event of a nuclear disaster, simple measurements carried out using a NaI(Tl) scintillation survey meter remain effective for assessing 131I intake. However, it should be noted that the presence of short-half-life radioactive materials on the body surface affects the detection limit.