Influence of short-lived radioiodines other than ¹³¹I on screening direct thyroid measurements with TCS-172 NAI(TL) survey meters

In a nuclear emergency, one of the actions taken for the sake of public is to monitor thyroid exposure to radioiodines. Japan's Nuclear Regulation Authority recently published a report on such monitoring and proposed direct thyroid measurements with conventional NaI(Tl) survey meters (e.g. Hitachi model TCS-172) as a primary screening method. A previous study proposed screening levels (SLs) used in these simplified measurements as the net reading values of the TCS-172 device. Age-specific SLs were derived from a thyroid equivalent dose of 100 mSv due to the inhalation intake of I-131. This study addressed the possible influence of short-lived iodine isotopes other than I-131 on the simplified measurements. In preparation for such measurements, the responses of the device for I-132 as an ingrowth component from Te-132, I-133, I-134 and I-135 in the thyroid were evaluated by numerical simulations using age-specific stylized phantoms in addition to those obtained for I-131 in the previous study. The radioactivity ratios of the relevant isotopes were taken from the inventory data of the Fukushima Daiichi Nuclear Power Plant. The results were used to predict the net readings of the device when Te-132-I-132 and I-133 as well as I-131 were inhaled at 24 or 72 h after the shutdown of a nuclear power plant. In these cases, the signals from Te-132-I-132 and I-133 become undetectable a couple of days after intake, which could lead to underestimations of the thyroid dose. To estimate the thyroid dose accurately from the simplified measurements, it is necessary to identify the exact time of intake after the shutdown and the actual physiochemical property of Te-132 that affects the thyroid uptake of I-132.