[DEPENDENCE OF THE RADIOSENSITIZING EFFECT OF OXYGEN ON ITS CONCENTRATION AND THE LIFETIME OF ITS FREE RADICALS].

The dependence of the radiosensitizing effect of oxygen on its concentration is explained in a mathematical development that compares the impacts of oxygen molecules with the same biomolecule and the lifetime of free radicals originating in the irradiation. In a living cell the shortlived free radicals are considered to react with oxygen only when they come in contact with it. The probability of this reaction is determined by the lifetime of the radicals and the time between two collisions with the same radiosensitive substrate. Obviously, at higher oxygen concentrations in the medium (which are dependent on the concentration in the atmosphere above the medium), the intervening time is less. The time interval calculated for aqueous solutions of oxygen in equilibrium with 2% oxygen in the gaseous phase, i.e., the concentration of oxygen at which radiosensitivity is approximately maximum, is 4 x 10/sup -3/ sec. Since the radiosensitivity is maximum when this time is equal to the lifetime of the radicals, the life of the latter should be easily calculated. However, this is not ertirely true. Actually, the effective cross section of the reaction of the radicals with oxygen within the limits in which the radicals and oxygen can react ismore » not entirely coincident with the cross sections of the radicals and oxygen. (TTT)« less