The kinetics of injury accumulation during a single exposure to60 Co γ-rays is described in the adult rooster and 3-wk chicken, and the results compared with those previously reported for the 3-day chick. Single exposure periods vary from 30 to 1440 min, with exposures in the lethal range for early (0-2 day) and for later (3-30 day) acute mortality. For early mortality, an empirical model based on a constant repair or reversal rate during irradiation describes the dose-time relations. Individual repair rates are assumed to be normally distributed and independent of exposure rate. In addition, there appears to be little influence of age between 3 days and 6 mo. For the later (3-30 day) mortality, repair or reversal processes operating during irradiation are time limited as well as rate limited, with an effective period of about 360 min. The effectiveness of dose protraction decreases with age.

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