Effective Dose Factor in Isotropic Dispersion

Nuclear reactor incident might be involved with a wide dispersion of radionuclides. The external exposure of the human body at large contaminated area does not arise from a single direction, but from multiple isotropic isotopes surrounding the body. Single source exposure causes a greater dose to the nearest organs than the deeper organs which located at the other side of the body. According to the facts that there is a difference in the organs radiation sensitivity, the non-symmetrical distribution of organs in the body and the specific absorbed dose of each organ, the biological damage by exposure of the body from one side or from the other side is not identical. Thus, for the same exposure, there is a different effective dose (whole body biological damage) for different angels of body exposure (different side of the body). Exactly the same, there is effective dose difference for one directional exposure or surrounding isotropic radiation from all the sides together, as being in large contaminated area. Taking all of the above into consideration, ICRP (1) has calculated and published dose coefficients for exposure from different parts of the human body. Those coefficients depend on the direction of exposure and energy.

The work used the Chernobyl accident data (2) and focused on Iodine and Cesium dispersion. By taking into consideration each Iodine and Cesium radionuclides release activity, their emission probabilities and energies, an effective dose coefficient has been calculated for each photon. Finally, an effective dose coefficient, weighted for Iodine and Cesium isotopes, was calculated for large contaminated area with large dispersion and isotropic exposure. The 23 calculated photons energy range was 0.07-1.4 MeV. The final calculated conversion factor of external exposure to effective dose in reactor Iodine and Cesium large dispersion area founds as 0.7 Sv/Gy (at average photon energy of 0.48 MeV).

Reference

1. ICRP 116, Conversion coefficients for radiological protection quantities for external radiation exposure, 2010.
2. Environmental consequences of the Chernobyl accident and their remediation: Twenty years of experience, report of Chernobyl forum expert group "Environment", IAEA, 2006.