Monte-Carlo simulations with mathematical phantoms to investigate the effectiveness of a whole-body counter for thyroid measurement

FASTSCAN (Canberra, Meriden, CT, USA), a standing-type whole-body counter with upper and lower NaI(Tl) detectors, has been mostly used to determine local residents' level of internal contamination with (CS)-C-134/137 following the Fukushima Daiichi Nuclear Power Plant (FDNPP) disaster in 2011. In the present study, we performed Monte-Carlo simulations with age-specific mathematical human phantoms to investigate the counting efficiencies of the FASTSCAN counter for I-131 thyroid measurements. As a result, although the lower detector had low sensitivity for I-131 in the thyroids of the adult and 15 y phantoms as well as the 10 y phantom standing on a 30-cm-high stool, the response of this detector may be used as an indicator of surface contamination overlooked on a subject. The total counting efficiency investigated in this study was 0.017-0.018 (cps/Bq) among all age groups. Based on this sensitivity, the determination level for thyroid activity could be calculated to be similar to 300 Bq under the slightly elevated background radiation level observed after the FDNPP disaster.