750 RADIATION BIOLOGV 



modifying X-ray-induced chromosome aberration frequencies does not 

 arise as a result of general metabolic disturbances within cells produced 

 by the presence or by the absence of this gas. Further, the evidence 

 favors the view that the effect of oxygen is not on the recovery mecha- 

 nism, involving the restitution or reunion of broken chromosome ends. 

 It thus appears that the increased aberration frequencies observed in 

 the presence of oxygen probably result from an increased frequency of 

 radiation-induced chromosome or chromatid breaks when this gas is pres- 

 ent during irradiation. Consequently, the problem remains to determine 

 what mechanism or mechanisms can explain this increased frequency of 

 chromosome breakage by radiation, especially by X radiation, in the pres- 

 ence of oxygen. Further, consideration must be given to the way in 

 which the results of the oxygen-effect studies can best be fitted into an 

 over-all interpretation of the mechanism of chromosome aberration pro- 

 duction by ionizing radiations. 



During the past few years, increasing evidence has accumulated which 

 indicates that many radiation effects in aqueous systems are largely 

 indirect, mediated by active substances resulting from the radiodecom- 

 position of water. Weiss (1944) concluded that these initial substances 

 are OH radicals and H atoms and has discussed their mode of origin in 

 irradiated water. Weiss (1944, 1947) and others (e.g., Allsopp, 1944; 

 Lea, 1946; Allen, 1948; Bonet-Maury and Lefort, 1948; Sparrow and 

 Rubin, 1952; Gray, 1952) have discussed the subsequent interactions of 

 these primary products with one another and with dissolved substances, 

 especially with oxygen, when water is irradiated with various radiations. 



When pure water is irradiated the following reactions are believed to 

 occur: 



radiation 



H 2 . . . H 2 > H 2 0+ . . . H 2 0- (1) 



(ionization) 



H 2 0+ > H+ + OH (2) 



H 2 0- > H + OH" (3) 



The result of the electron transfer in reaction (1) and the subsequent 

 decompositions in reactions (2) and (3) are to produce H atoms and OH 

 radicals. The relative magnitude of the various reactions which follow 

 depends upon the type of radiation and the resulting geometric distribu- 

 tion of these primary products. The following reactions are all thought 

 to occur (Allen, 1948) : 



H + OH^ H 2 (4) 



OH + OH -> H 2 2 -> H 2 4- O (5) 



H + H -» H 2 (6) 



OH + H 2 -> H 2 + H (7) 



H + H 2 2 -> H 2 + OH (8) 



