Design, development, and initial operation of BabyScan: An in-vivo counter for children around Fukushima

Abstract The Fukushima Dai-ichi accident has released large quantities of radionuclides, including 131 I, 134 Cs, and 137 Cs. These radionuclides, when inhaled or ingested, cause internal dose to the individual. Whole Body Counting [WBC], also known as in-vivo counting is a common method to assess internal radioactivity as a tool to evaluate internal dose. The FastScan WBC system was widely used following the Chernobyl and the Fukushima accidents for in-vivo measurements of the population. Although the FastScan was designed for adults, it was successfully used for children by having them stand on a small stool. However small children and infants cannot stand, and have a much lower quantity of radioactive cesium. That required the development of a much more sensitive WBC system, called the BabyScan. A very important element of the project was to make the unit look esthetically pleasing, while not compromising performance. The steel shield was enclosed in a molded fiberglass exterior skin, whereas a carbon-fiber liner was used on the interior, to keep the background low. The system was calibrated using MCNP; on-site testing with phantoms confirmed the adequacy of the mathematical efficiency calibrations. The system has a Minimum Detectable Activity with a 4-min measurement of approximately 20 Bq for infants approximately 10 kg in weight, and ~40 Bq for children approximately 30 kg in weight. The 40 K that is naturally present is also reliably detected at the appropriate quantity for infants as small as 6 kg. Data from the first 365 subjects counted showed that 40 K was detected in all of them, and that there was no 134 Cs or 137 Cs above the MDA levels.