MECHANISMS OF SPECIFIC IONIZATION EFFECTS 403 



radiated animals to have died). On the other hand, similar animals re- 

 ceiving 11 r/day did not exhibit any definite cataracts. 



TABLE 1 



Ch.\nge in Ratio of Dose in r to Do.se in n under Different Conditions 



IN Mice (13) 



(Accumulated exposure at LTso) 

 (Accumulated exposure at LTso) 

 (Accumulated exposure — anestrus) 

 (Accumulated exposure — sterility in males) 

 (Frequency of cataract after LTso) 



Discussion of Possible Mechanisms of the Specific 

 Ionization Effects 



The final injury produced by either type of radiation (when this injury 

 is of the same degree) does not disclose any characteristic difference 

 which would allow one to determine whether the effect had been pro- 

 duced by neutrons or by x-rays. The changes taking place in the inter- 

 action of radiation and matter are of the same general type for the two 

 kinds of radiation. The first step is the production of ionizing particles ; 

 the second is the production of ion pairs. Then follow the excitation of 

 atoms and activation of molecules with production of active radicals, 

 and finally absorption of energy in producing thermal changes. It is 

 possible because of the great difference in the specific ionization of the 

 proton and the photoelectron that the greater portion of energy loss of 

 neutrons may be expended in production of active radicals, whereas 

 some other radiation might expend a greater proportion of energy in 

 ionization. There seem to be two lines of evidence at the present time 

 to indicate that x-rays preferentially injure cells either beginning division 

 or preparing for mitosis; whereas neutrons appear to damage mitotically 

 inactive as well as dividing cells. One line of evidence comes from 

 studies of cells irradiated in different stages of mitotic activity. Mar- 

 shak (48) obtained evidence which he interpreted as indicating that 

 cells in early prophase were more severely damaged by x-irradiation. 

 Neutrons, on the other hand, had a more general effect on all stages, 

 including mitotically inactive cells. Gray, Read, and others (19, 21, 22, 

 23, 24, 25, 26, 56, 61, 62, 65) have made extensive studies of cellular 

 damage after irradiation with neutrons and with x-raj^s. The x-ray ef- 

 fects were primarily inhibition of prophase with later resumption of cell 



