The oxygen effect in cultured HeLa cells exposed to single short (3 nsec) pulses of electrons from a field emission source has been investigated. Cellular radiosensitivity as measured by the ability of surviving cells to form colonies has been determined experimentally over a range of concentrations of oxygen with which the cells were initially equilibrated. Cells attached to a Mylar disc were irradiated in a Petri dish from which the medium had been removed. Prior to irradiation it was found essential to transfer the disc from the dish in which plating originally took place to a new dish; this served to avoid any possible influence on experimental data by migrant cells shielded from the low energy electron beam at the edge of the dish. A family of breaking survival curves for the various initial oxygen concentrations was observed. These curves are similar to those previously measured for E. coli B/r by this laboratory; however, because of the higher radiosensitivity of mammalian cells, the curves break at values of absorbed dose and oxygen concentration much lower than for bacteria. The data appear to be reasonably consistent with a physicochemical mechanism involving the radiochemical depletion of oxygen previously invoked for the pulsed irradiation of bacteria.

The oxygen effect has been studied in Escherichia coli B/r irradiated with high-intensity pulsed electrons as a function of oxygen concentration. The dose to the cells was delivered in single pulses of time duration 30 nanoseconds. As was observed previously at low oxygen concentrations, survival measured under pulsed conditions for higher concentrations is greatly different from that obtained under relatively low-dose-rate conditions. Typically, the survival curve for cells which are pulse-irradiated follows the oxygenated response to a point dependent on concentration before breaking away to parallel the slope of the survival curve measured under conditions of pure nitrogen. A plot of the "break-point dose" versus oxygen concentration appears linear. The data can be interpreted as further support for a hypothesis based on oxygen depletion within the cell by the first part of the electron pulse previously proposed for Serratia marcescens pulse-irradiated at low oxygen concentrations. The data can also provide a basis for the direct measurement of oxygen diffusion time in cells.