Screening levels of TCS-172 NaI(Tl) survey meters used for direct thyroid measurements in nuclear disasters.

It is a challenging task to establish a feasible and robust method for the population monitoring of individuals' thyroid exposure following an accidental intake of radioiodines in a nuclear emergency, because of the time restriction. The authors previously proposed a method for such monitoring to obtain as many reliable human data as possible and one of the components is simplified measurements by conventional NaI(Tl) survey meters that are intended to be used for the initial triage to identify significantly exposed individuals and get an overall picture of the exposure levels in a target population in a timely manner. This study determined screening levels (SLs) for a conventional NaI(Tl) survey meter (model TCS-172, Hitachi, Japan) using the conversion factor (131I kBq in the thyroid per μSv h-1) that were obtained from experiments and simulations with age-specific phantoms. The results demonstrated that the derived SLs for 100 mSv thyroid equivalent dose were as follows: 0.2 μSv h-1 (SL1) for the age group ≤ 5-y-olds, 0.5 μSv h-1 (SL2) for the 10- and 15-y-old age groups and 1.0 μSv h-1 (SL3) for adults. These SLs would be reasonably available within 1 week after the intake of 131I on the safe side.