An ultrafast mixing technique has been developed to investigate the kinetics of rapid radiobiological processes. The technique has been applied to the study of the time scale of the oxygen effect in irradiated Serratia marcescens. The bacteria were held on a Millipore filter inside a chamber which was usually flushed with humidified nitrogen and were exposed to an explosion of oxygen released into the chamber through a fast-acting solenoid-operated valve. The bacteria were irradiated with a single 2-μsec pulse of 1.8 MeV electrons from a linear accelerator. The arrival of oxygen at the bacteria was timed to occur at a preset interval before or after the electron pulse. The time resolution was 100 μsec. After fixed doses of 10, 19, and 28 krads per pulse, the bacterial survival was measured as a function of the oxygen contact time. It was found that, when oxygen contact occurred at any instant before or up to the time of the electron pulse, full sensitization was obtained. However, bacteria exposed to oxygen at times after the electron pulse showed decreasing sensitization as the time interval increased, the survival approaching the anoxic control value at times greater than 2 msec. The data show that, in Serratia marcescens irradiated under anoxic conditions, oxygen-dependent damage decays approximately exponentially with a half-life of about 500 μsec.

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