402 INFLUENCE OF EXPOSURE FACTORS 



ation. Although it has been difficult in the past to demonstrate a dif- 

 ference in biologic effectiveness of different wave lengths of x-rays in 

 the usual therapeutic range, evidence is mounting to indicate that (for 

 tissue effects) low-energy x-radiation is more effective than extremely 

 high-energy x-rays and gamma rays. 



When the incident beams consist of ionizing particles, a difference in ef- 

 fectiveness related to specific ionization is very evident. With particles 

 of large size (such as protons) and with a double positive charge (such as 

 alpha particles) the ion track is very dense as compared to that of beta 

 particles. The penetrability of alpha particles is very low, and thus a 

 comparison of their effectiveness in tissue with those of beta particles 

 and photons presents technical difficulties. Such studies are usually 

 done by injecting, or by having the animals inhale, some alpha-emitting 

 radioactive material (38, 54). Fast neutrons, on the other hand, have a 

 penetrability in tissue somewhat comparable to that of 200-kv x-rays 

 and gamma rays. They produce ionization in tissue primarily through 

 the production of recoil hydrogen nuclei. These protons, although not 

 ionizing as densely as alpha particles, have a specific ionization much 

 greater than that of beta particles or photoelectrons. 



It may be said that in general, for tissue effects, alpha particles and 

 neutrons are more effective than x-rays or gamma rays per roentgen 

 equivalent physical (rep). This holds for a single exposure and is even 

 more marked for protracted and fractionated exposures (7, 2, 13, 18, 

 26, 31, 38, 49, 54). 



Certain organs and tissues have been found to be especially sensitive 

 to neutrons as compared to x-rays and gamma rays. Complete destruc- 

 tion of germinal tissue in the testis of the rabbit without irreparable 

 damage to the overlying skin has been reported for neutrons (40). This 

 is different from the findings with x-rays in a similar single exposure. 



Effect of Peotraction and Fractionation on the 

 r/n Dose Ratio 



Protraction and fractionation (13, 31, 49) have been found to increase 

 the r/n dose ratio. In other words, animals recover from effects of 

 whole-body x-irradiation more rapidly than they do after exposure to 

 alpha or neutron radiation. Examples are shown in Table 1 (for mice). 

 The gonads appear to be especially sensitive to small daily exposures 

 of fast neutrons. The selective action of neutrons in producing cata- 

 racts is apparently even more marked. In several experiments, expo- 

 sures to 1.4 n/day produced cataracts in about 50 per cent of the eyes 

 in the animals surviving the LTgo (time required for 50 per cent of ir- 



