Influence of the irradiation geometry on the severity of acute radiation damage

The aim of this study was to demonstrate how the severity of radiation damage in the case of bone marrow syndrome due to acute irradiation depends on the non-uniform irradiation of the body. We used the calculation method that involves the evaluation of organism mortality as a function of bone marrow cells colony survival vs dose for different radiation profiles. It was presumed that the probability of the death for the organism is the same for the same value of survival level of the bone marrow cells regardless of the dose distribution by mass of the organ. The dose of uniform irradiation that is equivalent to the dose for the L-th case of non-uniform irradiation is calculate based on the survival level of the total marrow cells. After that the probability of the death of the organism is estimated according to the dose response curve. Dose distribution in bone marrow of computational MIRD-5-type stylized model adult man for different geometries of exposure by point source of ¹³⁷Cs was evaluated. Larger non-uniformity of the dose distribution in the bone marrow at the same dose in free air in the site of human location causes a greater probability of survival due to a greater proportion of bone marrow cells that have preserved the possibility of reproduction. The values of mean 50% lethal dose for cases of approximately uniform irradiation of the body surface (point source at a distance 10m) and sharply non-uniform irradiation (point source at a distance of 0.5 m) differ approximately by the factor of 1.7 – 2.5 depending on direction of irradiation. Additionally the values of conversion coefficients from the reading of an individual dosimeter to the value of an effective dose for various geometries of irradiation of emergency workers from the ¹³⁷Cs point source were calculated. The average value of the conversion coefficient from personal dose to effective dose for the considered exposure situations is 0.7 Sv Cy^-1 and the 90% confidence interval is 0.49 – 0.99 Sv Gy^-1.

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