726 



RADIATION BIOLOGY 



Although the majority of the X-ray-induced interchanges appear to 

 result from two separate hits, a certain fraction has been shown to 

 originate as a result of single hits. This proportion has been investigated 

 for chromatid exchanges, by Catcheside, Lea, and Thoday (1946b) in 

 experiments in which two similar dosage curves were obtained at con- 

 stant, but widely different, exposure times. The results indicate that 

 one-hit exchanges are of relatively minor consequence at high intensities, 

 but make a substantial contribution to the total aberration yield at low 



lOO-i 



d 



w 

 o 

 o 

 o 



Q. 



ZSO- 

 o 



y- 

 <. 



K 



cc 



Ul 



m 



< 



100 



300 

 DOSE OF XRAYS, r 



500 



J 160 r/min 



INTENSITY A 20 r/min 



O 2.7 r/min 



Fig. 10-4. Effect of dosages of X rays at different intensities upon the yield of chromo- 

 some exchanges. (Data from Sax, 1941.) 



intensities. Similar conclusions have been arrived at by Sax for chromo- 

 some exchanges (1950b). 



Rick (1940) has shown that the yield of interstitial deletions increases 

 as the 1.5 power of the dose when irradiations are performed at constant 

 time and at relatively high intensities, and has concluded that these 

 types represent a mixture of one- and two-hit aberrations. 



With fast neutrons (Giles, 1940; 1943; Giles and Conger, 1950; Thoday, 

 1942) and a particles (Kotval and Gray, 1947), all types of exchange 

 aberrations increase linearly with dose. As indicated previously, these 

 results are compatible with the view that such two-break aberrations are 

 produced by the passage of a single ionizing particle. The special case of 

 slow-neutron effects will be considered later. 



