724 



RADIATION BIOLOGY 



including chromatid and chromosome exchanges, were linearly related to 

 dose (Fig. 10-3). Further, when equal doses of X rays and neutrons were 

 compared, neutrons were found to be much more efficient than X rays in 

 producing aberrations. These results were interpreted as indicating that 

 with fast neutrons all aberrations are produced by single events, and that 

 the passage of an ionizing particle, rather than the occurrence of a single 

 ionization, constitutes the event, or hit. 



30 -i 



(/) 



_i 

 _i 



LU 

 O 



Q 



O 



tr 



LU 

 Q. 



en 



O 



h- 

 < 



20 



LU io- 





50 100 



X-RAY DOSE, r 



O ISOCHROMATID BREAKS 

 • CHROMATID EXCHANGES 



Fig. 10-2. Relation between X-ray dosage and frequency of isochromatid breaks 

 (one-hit) and chromatid exchanges (two-hit). Time of exposure constant. (Data 

 of Sax, 1940. Dosage values multiplied by correction factor, cf. Catcheside, Lea, and 

 Thoday, 1946.) 



These observations on X-ray and fast-neutron effects on Tradescantia 

 chromosomes have been subsequently confirmed and extended in numer- 

 ous independent experiments. The following general conclusions relating 

 the frequency of various aberration types to dose can be made. Simple 

 (one-hit) break types, i.e., chromosome and chromatid breaks and iso- 

 chromatid breaks, increase in approximately linear proportion to dose for 

 all types of radiations tested — X and y rays (Sax, 1938; 1940; 1941; 

 Thoday, 1942; Newcombe, 1942; Rick, 1940; Catcheside et al, 1946b), 

 neutrons (Giles, 1940b; 1943; Thoday, 1942), and a particles (Kotval and 



