The thresholds for permanent sterility in men for an acute exposure and prolonged exposure conditions are

Studies of patients receiving radiation therapy indicate that permanent sterility can result from 2.5 to 3 Gy (250-300 rad) in a fractionated regime over 2 to 4 weeks. The induction of sterility by radiation in human males does not produce significant changes in hormone balance, libido, or physical capability. The original single-dose data came from the irradiation of prisoners, which showed that a dose in excess of 6 Gy (600 rad) is needed to result in permanent sterility. In contrast to most organ systems, where fractionation of dose results in sparing, fractionated courses cause more gonadal damage than a single doseReference: Eric J Hall Textbook of Radiobiology for Radiologist; 5th edition