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Irradiation at conventional dose-rates of an oxygen-equilibrated bacterial cell suspension in a sealed vessel produces breaking survival curves similar to those observed for cells irradiated as a thin layer at ultra-high dose-rates . A mathematical model previously derived by this laboratory, which also yields breaking survival curves, is compared with the surviving fraction measured at both dose-rates . This comparison yields, at the two extremes, determinations of the rate of radiochemical depletion of dissolved oxygen and of the factor K, the oxygen concentration at which the radiosensitivity is halfway between the anoxic and fully oxygenated values . It is found that there is a dose-rate dependence in K. For the sealed vessel, the determination of the rate of depletion is in essential agreement with that measured directly with an oxygen electrode . The agreement supports the assertion that the breaking behaviour is due to oxygen depletion .

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