140 RADIATION BIOLOGY 



should be possible to determine the trend of variation of effectiveness in 

 detail. Only preliminary results in this direction are available at the 

 time of this writing (see Table 1-12). 



5-5c. Saturation Effect. It was mentioned in Sect. 3-6 that a com- 

 bination of the aftereffects of different collisions may conceivably lead 

 to at loss rather than to a gain of effectiveness. This effect seems to 

 occur in a number of radiochemical reactions produced by a particles. 

 (See, however, footnote 6, Sect. 5-5.) Molecules that are activated as 

 a result of successive collisions a short distance apart are believed to 

 neutralize one another as they meet while diffusing away from their 

 point of origin (see, for example. Chap. 4). 



Another effect may contribute to reduce the effectiveness of densely 

 ionizing radiations. If the rate of energy dissipation along the track of a 

 particle is sufficiently high, it may be imagined that this energy dissipa- 

 tion suffices to achieve whatever effect it was intended to produce. A 

 further increase of the rate of energy dissipation would be wasteful. 



According to this argument the effectiveness of ionizing radiations 

 should eventually decrease provided the rate of energy dissipation along 

 the particle tracks attains a sufficiently high value. This loss of effec- 

 tiveness is generally called the "saturation effect." 



Under conditions of high saturation the action of radiation is visual- 

 ized as definitely concentrated around the tracks of ionizing particles. 

 The effectiveness of the radiation depends on the effective radius of 

 action of the intense disturbance caused by the passage of a particle. 

 Since the chance of production of an effect at a certain position depends 

 primarily on the passage of a particle at a sufficiently close distance, the 

 target theory picture of a "sensitive volume" or "sensitive area" becomes 

 more definitely realistic. 



The effectiveness of neutrons of a few Mev for the production of sex- 

 linked lethal mutations in fruit flies is slightly (about 30 per cent) lower 

 than the effectiveness of X rays. This effect is generally regarded as an 

 example of incipient saturation. Catcheside and Lea (1943) have 

 reported a similar drop of effectiveness for the X-ray production of 

 chromosome breaks in a spiderwort as the X-ray energy drops from 3 to 

 1.5 kev. (The same effect increases as the energy drops from 10 to 3 kev; 

 see Sect. 5-5b). General indications are that heavy particles produce 

 chromosomal breaks in this material under conditions of high saturation, 

 i.e., that a break arises whenever a heavy particle smashes through a 

 chromosome. 



Target-theory models lead to quantitative predictions regarding the 

 decrease of radiation effectiveness as a function of the increasing rate of 

 energy dissipation. For example, the probable "waste" of radiation 

 energy due to the accidental occurence of two or more ionizations within a 

 single sensitive volume can be calculated. Such predictions have been 



