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Dose coefficients for external exposures to environmental sources.
This publication presents radionuclide-specific organ and effective dose rate coefficients for members of the public resulting from environmental external exposures to radionuclide emissions of both photons and electrons, calculated using computational phantoms representing the ICRP reference newborn, 1-year-old, 5-year-old, 10-year-old, 15- year-old, and adult males and females. Environmental radiation fields of monoenergetic photon and electron sources were firstly computed using the Monte Carlo radiation transport code PHITS (Particle and Heavy Ion Transport code System) for source geometries representing environmental radionuclide exposures including planar sources on and within the ground at different depths (representing radionuclide ground contamination from fall-out or naturally occurring terrestrial sources), volumetric sources in air (representing a radioactive cloud), and uniformly distributed sources in simulated contaminated water. For the above geometries, the exposed reference individual is considered to be completely within the radiation field. Organ equivalent dose rate coefficients for monoenergetic photons and electrons were next computed employing the PHITS code thus simulating photon and electron interactions within the tissues and organs of the exposed reference individual. For quality assurance purposes, further cross-check calculations were performed using GEANT4, EGSnrc, MCNPX, MCNP6, and the Visible Monte Carlo radiation transport codes. From the monoenergetic values, nuclide-specific effective and organ equivalent dose rate coefficients for several radionuclides for the above environmental exposures were computed using the nuclear decay data from Publication 107. The coefficients are given as dose rates normalised to radionuclide concentrations in environmental media, such as radioactivity concentration, in units of nSv h-1 Bq-1 m-2 or nSv h-1 Bq-1 m-3 and can be re-normalised to ambient dose equivalent (Sv Sv-1) or air kerma (Sv Gy-1). The findings showed that, in general, the smaller the body mass of the phantom, the higher the organ and effective dose due to (1) closer proximity to the source (in the case of ground contamination) and (2) the smaller amount of body shielding of internal organs in the younger and smaller reference phantoms. The difference in effective dose between an adult and an infant is 60-140% at a photon energy of 50 keV, while it is less than 70% above a photon energy of 100 keV, where the smaller differences are observed for air submersion and the largest differences are observed for soil contamination on the surface of the ground. For realistic exposure situations of radionuclide environmental contamination, the difference was found to be more moderate. For example, for radioactive caesium (134Cs, 136Cs, 137Cs/137mBa) deposited on and in the ground, the difference in effective dose between an adult and an infant was in the range of 20-60%, depending on the radioactivity deposition depth within the soil.
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