Radioactivity in the Body and Dose Assessment
Whole-body counters (WBCs) can measure the radioactivity content in a body on the day of measurement. Similar to other radiation measuring devices, WBCs have a detection limit depending on their performance and counting time.
Given that radioactive cesium has a biological half-life of 70-100 days for adults (p.11 of Vol. 1, “Half-lives and Radioactive Decay”), around one year after the accident would be the time limit for estimation of the initial body burden (in the case of a single intake event at the beginning). As shown in the upper figure, the radioactivity of cesium incorporated into the body decreases in around a year to nearly zero, namely the level before the intake. Subsequent whole-body counting is performed for the purpose of estimating chronic exposure, mainly from foods (p.61 of Vol. 1, “Data on Internal Exposure Measured by Direct Counting”).
In contrast, children generally have a faster metabolism and shorter biological half-lives compared to adults, due to which the initial intake can be estimated only for a period of about half a year. Furthermore, the residual radioactivity accumulated in the body through the daily intake of trace amounts is also minimal in children, and their estimated chronic internal exposure is often below the detection limit. In such cases, it would be more reasonable to examine adults and estimate their internal doses in terms of understanding the internal exposure situation in detail, taking into account the fact that the committed effective dose coefficients are similar for both children and adults, despite the notable difference in their metabolism rates.
In order to estimate the committed effective dose (p.56 of Vol. 1, “Committed Effective Doses”) from the measurement result for the radioactivity in the body, it is necessary to use an appropriate intake scenario and an appropriate model aligned with the exposure circumstances, such as acute or chronic intake, inhalation or ingestion as a dominant route of intake, the time when the intake started, and so on.
Regarding radionuclides with short effective half-lives, such as I-131, the radioactivity in the body diminishes rapidly, making it difficult to detect such radionuclides as time progresses. Additionally, pure beta-emitters lacking γ-ray emission, such as Sr-90, also cannot be detected by a whole-body counter (WBC).
- Included in this reference material on March 31, 2013
- Updated on March 31, 2025