Cell survival curves have been established from mouse jejunum after irradiation with 6-MeV neutrons at 50-100 rads/min from the Hammersmith Cyclotron, 7-MeV electrons at 6 krad/min, and 7-MV x-rays at 100 rads/min given in single doses or two fractions. After an initial shoulder region all curves show an exponential region between surviving fractions of$10^{-1}-10^{-8}$. The${\rm LD}_{50(5)}$ values for all types of irradiation fall at a cell surviving fraction of 10-2. The relative sensitivities of these radiations obtained from a comparison of D0 values are for neutrons relative to 7-MV x-rays at 100 rad/min 2.0; for neutrons relative to 7-MeV electrons at 6 krad/min 1.5; for 7-MeV electrons at 6 krad/min to 7-MV x-rays at 100 rad/min 1.3. The relative sensitivities obtained from a comparison of$D_{Q}$ values are similarly 4.7, 3.6, and 1.3. Thus, the effect of a change in dose rate in the low LET radiation has almost as big an effect on D0 as a change in LET but the effect on shoulder size is much less than is seen with a change in LET.

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