444 



RADIATION BIOLOGY 



Comparison of Different Ionizing Radiations. The dis.sipatioii of energy 

 along the paths traversed by ionizing particles, rather than randomly 

 throughout the material, is undoubtedly the major reason tor dilTerences 

 in biological etTectiveness of physically equivalent doses of ionizing radi- 

 ations of different quality (Lea, 1947). Some studies on the comparative 

 etTectiveness of different ionizing radiations are summarized in Table I l-li. 

 With A. terreus as test organism, the interesting observation has been 

 made that densely ionizing radiations (neutrons and a rays) were more 

 effective in killing but less effective in mutating than 7 and X rays 

 (Stapleton and Martin, 1949). A further difference has been ol)served in 

 that survival followed a sigmoidal curve whereas mutation production 

 was linear with dose (Stapleton and Martin, 1949). Aspergillus niger 

 spores behave similarly to .1. terreus, as regards comparative lethal effec- 

 tiveness of densely ionizing and more disperse radiation, while .S. cere- 



Table ll-C. Relative Effectiveness of Ionizing Radiations in Causing 



Killing and Mutation in Fungi 



visiae showed only a slight effect, and Escherichia coli a complete reversal 

 of effectiveness (Zirkle, 1940). There appears to be some contradiction 

 in the data on yeast, since a rays have been reported to be more effective 

 (Tobias, 1952) and less effective (Holweck, 1930) than X rays; and 

 longer X rays (8 A) to be less efficient in killing than shorter radiation 

 (2 A) (Holweck and Lacassagne, 1930b). At the point of 50 per cent 

 lethality, twice the cathode-ray dose, three times the X-ray dose, and six 

 times the ultraviolet exposure are required for Rhizopus nigricans spores, 

 as compared to yeast cells (Luyet, 1932). In general, such differences 

 between the effectiveness of radiations of different quality are indicative of 

 a rather close localization of the effects to points along the paths of ion- 

 izing particles. If the effects were not localized, it would be impossible 

 to explain the results in terms of the spatial relations of the biological 

 targets and the nonrandom distribution of elementary processes. Present 

 knowledge provides no reasonable alternative. 



Dose-effect Curres. Survival curves have been obtained with various 

 fungi, a few of which are listed in Table 11-7. The survival curves 



