186 



a. W. BARENDSEN 



RESULTS 



Survival curves 

 In Fig. 1 the fraction of cells which after irradiation have retained 

 the capacity for clone formation is plotted as a function of dose. Doses 

 of a-, /3- and X-radiation are given in rads. The main features of the 



o 



V 

 en 



a 



Fig. 1. — Effects of a-, j8- and X-radiation on the capacity for clone formation. 



1, Curve obtained with a-radiation. 



1', Curve 1 corrected for cells, not adliering to the bottom of tlie dishes (see text). 



2, Curve obtained with ^-radiation. RBE S:: 0-85. 



3, Curve obtained with 200 kV X-radiation. 



3', Theoretical curve nino = e-""-*^ ( 1 +7>/ 135). (see text). 



4, Curve obtained with 20 kV X-radiation. RBE X 1 • 1 ;">. 



curves are the exponentially decreasing survival for /3-radiation 

 and the less simple shape obtained with X- and /3-radiation. 



At doses of a-radiation of more than 150 rad the survival curve 

 deviates from the exponential. This may be explained by the assump- 

 tion that a small percentage of the cells, about 0-5 per cent, has not 

 attached projjerly to the Melinex bottom of the cidture dish. As the 

 range of a-particles after penetrating the Melinex bottom is only 

 about 20 fi, cells which are still in suspension at the time of irradiation 

 will not be irradiated at all. When a constant fraction of 0-5 per cent is 

 subtracted from the surviving fraction at all dosages, an exponential 



