Explanation
Radiation doses and their correlations for radiological protection
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The International Commissions on Radiological Protection (ICRP) defines "effective dose" that can quantify radiation effect from low dose exposure.
The effective dose can be derived with the 2 steps from absorbed doses to organs and tissues
;1) multiplying absorbed doses to organs and tissues by tissue weighting factors that are allocated
to each organ and tissue by taking account for radiation sensitivity,
2) summing the multiplied dose over a whole body [1,2]. The effective dose, however, is unmeasurable and then quantities (doses) that are used in radiation measurements are also defined in radiological protection. "Ambient dose equivalent" is defined for area monitoring using radiation instruments, such as a survey meter, and "Personal dose equivalent" is defined for individual monitoring using a dosimeter. While effective dose is called as a "protection quantity", the two equivalent doses are called as "operational quantities". The unit of "Sievert (Sv)" is commonly utilized for the protection and the operational quantities. Survey meters and dosimeters are used for radiation control for workers in nuclear and radiation facilities. Since ambient dose equivalent and personal dose equivalent reasonably estimate effective dose without underestimation, radiation dose for a worker can be checked and safely controlled by the ambient dose equivalents and personal dose equivalents measured by survey meters and dosimeters. Radioactive materials are released and deposited in the environment in the TEPCO Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Thus, we shall appropriately estimate the effective dose of the public. Here, the difference of exposure condition should be taken into account between workers in the facilities and public in the environment. Fig. 1 depicts images of the irradiation conditions. |
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| Fig.1: The schematics of external radiation exposures in (a) work space of radiation facilities and (b) contaminated environment. |
The position of radioactive material can be specified or recognized in the facility and a worker may be usually faced with the material and wears a dosimeter at the frontal side on the human body. A dosimeter is also put on the frontal side on the human body in the monitoring for the public following the FDNPP accident. A person, however, can be exposed to radiations from the backside and the radiations can be shielded by the human body.
Then, we have analyzed data that can clarify correlations among ambient dose equivalent, personal dose equivalent and effective dose for the environment where radioactive material are deposited [3,4]. We also prepared conversion coefficients from radioactivity concentration of radioactive cesium to effective dose, ambient dose equivalent or personal dose equivalent for dose estimations following the FDNPP accident (Please see, “Analyses of personal dose equivalent measured by dosimeters for radioactive cesium deposited in environment”)
[1] ICRP Publication 60 (1991).
[2] ICRP Publication 103 (2007).
[3] D. Satoh et al., J. Nucl. Sci. Technol., 53, 69-81, (2016).
[4] D. Satoh et al., J. Nucl. Sci. Technol., 54, 1018-1027, (2017).