L. H. GRAY 



will be -y/SS — (6). There will thus be appreciable and possibly important 

 differences between the quantities of energy dissipated in each nucleus, but 

 the chance that any given nucleus altogether escapes ionization is e"^^, which 

 is quite negligible. Suppose, on the other hand, that the total dose w-ere that 

 corresponding to the maximum permissible weekly exposure, namely 0-3 



0-3 



rad, the mean number of particles will then be 



0-67 



0-45 and no energy 



at all will be deposited in a fraction e -o-*^^ or 64 per cent of the nuclei. 

 Again, if the total dose were 25 rad but the radiation were a radiation instead 

 of X radiation, the nuclei would be traversed by an average of one particle 

 and e"^, or 37 per cent of the nuclei would be unaffected. Thus, whether or 

 not the statistical aspects of dose are important in any given case w^ould be 

 dependent on: 

 {1) the size of the element of the tissue under consideration, 



(2) the magnitude of the dose, and 



(3) the type of radiation. 



«5 

 °c 



0; 

 to 



E 



E 



£ 

 n 



a. 



c 

 o 



A 



a. 



m 



vn 



>< 

 en 



(U 



c 

 111 



Organs of 

 a mouse 



200 i^-^^y^ 



L 



L 



loot 



I- 



r 



L 



OL 



Cells 

 (10 /J.) 



Mitochondria Enzyme 



(111) Granules molecules 

 '^ ' (0-3fx) 



2Q0 ^Neutrons I 

 r 



r 



100^ 



h 

 |- 

 r 



200 f P^'"''*^'" 

 L ! 



I- 



100 [ 



h 



r 



550 



■■>••■■■•• 



Figure 3 



Figure 3 shows the actual estimated amount of energy deposited in each of 

 10 elements of tissue of the same size when exposed to 100 rad of X-rays, 

 neutrons, or a-particles respectively. It is evident that in general 10 mito- 

 chondria or smaller particles could be selected at random from a group 

 exposed to 100 rad of a-particles, and none ^\•ould have been ionized. In the 

 case of objects exposed to neutrons, a similar situation obtains for particles 

 of 0-3 [J. in diameter and less. In the case of X-rays, the critical size in rela- 

 tion to the dose of 100 rad occurs at the level of macromolecular dimensions. 

 The above statements all refer to the direct deposition of energy by ionizing 

 particles within the objects under consideration. Some of the energy 

 2 5 



