INJURY ACCUMULATION AND RECOVERY IN SHEEP EXPOSED TO PROTRACTED CO60 GAMMA RADIATION

A modified split-dose technique was used to measure the accumulation of injury during protracted exposures of the domestic sheep to Cobalt-60 gamma radiation. An initial or conditioning exposure of 165 R was given at 0.5, 0.95, 1.85, and 3.94 R/hr, and the acute exposure LD50/60 was determined immediately thereafter. These studies indicate that there is a dose rate threshold for injury accumulation under these conditions or protracted irradiation and that the threshold lies between 1.0 and 1.5 R/hr. Recovery from injury accumulated during protracted exposures has been studied by determining the LD50/60 at various times after protracted exposures. These studies indicate a rapid recovery without a lag period that proceeds to a transient acquired radioresistance 15 days after the conclusion of the protracted irradiation. (Author)