136 EADIATION BIOLOGY 



potency of each ionizing radiation is a negligible factor. Ionizing radia- 

 tions produce, in general, chemical and biological effects of comparable 

 order of magnitude, at parity of energy dissipation. On the other hand, 

 light has only Kmited chemical potency. Accordingly, ultraviolet Ught is 

 much less effective than ionizing radiation, and the effectiveness decreases 

 greatly as we proceed to visible and infrared light. Radiation of still 

 lower potency, i.e., radio waves, acts only indirectly through the genera- 

 tion of heat, with still much lower efficiency. 



Different radiations may display a different comparative effectiveness 

 in the production of different effects. Therefore even the qualitative 

 picture of the various effects produced by different radiations may differ. 

 For example, the proportion of the genetic effects produced in fruit flies 

 which is associated with chromosomal breaks is much greater for ionizing 

 radiations than for ultraviolet light (see Muller, 1941, p. 160). [Inci- 

 dentally, the effect of chemical mutagens of the mustard group indicates 

 a potency intermediate between that of ultraviolet and that of ionizing 

 radiation (Auerbach and Robson, 1947).] By and large, ionizing radia- 

 tions duplicate all the effects of less potent radiations, as expected, but 

 the reverse is not true. The experimental observations bearing on this 

 point agree with expectation but do not contribute much new information. 



Different ionizing radiations differ from one another in their action, 

 depending on the average interval of collisions along the tracks of ionizing 

 particles. Data on the comparative effectiveness of different ionizing 

 radiations bear on the mutual influence of the aftereffects of successive 

 collisions. 



X rays and fast electrons of energy greater than about 10 kev, as well 

 as heavy particles with energies in the 100-Mev range, have essentially 

 equal effectiveness for equal energy dissipation. This means that the 

 aftereffects of successive colHsions do not influence each other if the 

 colhsions occur sufficiently far apart. Other, lower energy, radiations, 

 which give rise to more closely spaced collisions, seem to be a few times 

 more effective in most instances, but occasionally also less effective.^ 



5-5a. Equal Effectiveness. Let us consider first the phenomena which 

 follow exponential dose-effect curves and in which different ionizing radia- 

 tions produce equal effects for equal energy dissipation. The rates of 



6 Note added in July, 1952. Evidence accumulated in recent years has progressively 

 disproved the statement "X rays and fast electrons of energy greater than about 10 

 kev . . . have essentially equal effectiveness." By and large the effectiveness of 

 these radiations appears to decrease steadily, by a factor of the order of 2, as the 

 energy of the electrons (whether primary or secondary generated by X rays) increases 

 from 10-30 kev to about 0.5-1 Mev. Beyond this energy the average distance of 

 collisions along electron tracks no longer decreases and no further change of effective- 

 ness should be expected. 



The newer evidence consists mostly of experiments on the production of complex 

 effects in higher organisms (see Chase et al., 1947). Possibly because of this reason the 



