The LD50 in mice due to gastrointestinal radiation injury from exposure to either60 Co gamma rays or 250 kVp x-rays has been measured for exposure rates ranging from over 100 R/minute down to less than 1 R/minute. The LD50 for60 Co gamma rays was greater than the LD50 for x-rays, but at all exposure rates there was a nearly constant ratio of LD50 for the two kinds of radiation. Measurements of rate of recovery of radiation injury were made with x-rays, using the split-dose technique. The recovery was exponential, with a half-time of 23 minutes. The amount of repairable injury was about half of the exposure given for exposures up to 470 R, but became nearly constant and independent of exposure between 470 and 700 R. A model for repair of injury during exposure was constructed on the basis of the measured recovery kinetics. The increase of LD50 with decreasing exposure rate of either type of radiation was shown to be reasonably accounted for by the model for repair of radiation injury.

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