Samples of soil collected from the Kennedy Space Center near the spacecraft assembly facilities were found to contain microorganisms very resistant to conventional sterilzation techniques. The inactivation kinetics of the naturally occurring spores in soil were investigated by using dry heat and ionizing radiation, first separately and then simultaneously. Dry-heat inactivation kinetics of spores was determined at 105 and 125 C; radiation inactivation kinetics was determined for dose rates of 660 and 76 krads/h at 25 C. Simultaneous combinations of heat and radiation were then investigated at 105, 110, 115, 120, and 125 C, with a dose rate of 76 krads/h. Combined treatment was found to be highly synergistic, requiring greatly reduced radiation doses to accomplish sterilization of the population.
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