1030 RADIATION BIOLOGY 



Desjardins (1932), Aubertin and Beaujard (1905), Clarkson et al. (1938), 

 Mottram and Russ (1921), Czepa (1923-1924), and Linser and Helber 

 (1905). Because of the fact that dosage measurement was neither 

 accurate nor accurately reproducible and because of other technical 

 difficulties, much of the early research has been repeated in order to cor- 

 relate dose with effect. In addition, new types of radiation have become 

 available which have made comparative studies on the biological effects 

 of various radiations desirable. This chapter is based largely on studies 

 conducted by various investigators during the past ten years. 



COMPARATIVE EFFECTS OF IONIZING RADIATIONS 



In considering the irradiation of the whole body with penetrating radia- 

 tions of external origin, those of significance in so far as direct effects on 

 the blood and blood-forming tissue are concerned are X rays, y rays, fast 

 neutrons, and slow neutrons. The rays and a particles that emanate 

 from radioisotopes can, in general, be disregarded because penetration in 

 tissue is only a few millimeters for /3 rays and a fraction of a millimeter for 

 a particles. Studies on mice and rabbits by Raper and Barnes (1951), 

 Raper, Zirkle, and Barnes (1951), Raper, Henshaw, and Snider (1951), 

 and others have demonstrated that, although /3 rays may penetrate the 

 skin, the hematologic effects, if any, are minimal and are secondary to 

 other pathologic changes such as ulceration of the skin. No studies are 

 yet available on the hematologic effect of exposure to /3 rays from a 

 betatron source. 



Studies on the effect of acute exposure of animals to a slow-neutron 

 flux have been carried out by Zirkle (1945) and by Zirkle et al. (1947). 

 This slow-neutron source was contaminated with fast neutrons, and the 

 effect on the hematopoietic system, as reported by Raper, Henshaw, and 

 Snider (1951), is therefore difficult to interpret. According to Zirkle 

 (1947, 1950), however, it is not the slow neutron that produces biologic 

 effects but rather the 7 rays and fast atomic nuclei emitted upon nuclear 

 capture of the slow neutrons. It is likely, therefore, that exposure of 

 experimental animals to a pure slow-neutron flux would produce hemato- 

 logic effects comparable to those induced by 7 rays plus fast neutrons. 

 Studies using a 7-ray source for single total-body exposures delivered in a 

 few minutes, as reported by Henshaw et al. (1946) and by Henshaw, 

 Riley, and Stapleton (1947), indicate that the effect on the blood and 

 blood-forming tissue is qualitatively the same as that produced by X rays, 

 and therefore no special category of discussion is devoted to this subject. 



Data on single exposures of rabbits to fast neutrons in about the 30-day 

 LD50 range (Jacobson and Marks, 1947) indicate that the degree of 

 depression of the formed elements in the circulating blood and the time 

 required for recovery are largely comparable to those observed after a 



