318 EADIATION BIOLOGY 



cally 100 per cent photoelectrons. In the intermediate range 

 (0.05 to 0.4 A, roughly), where both the photo- and Compton 

 electrons make significant contributions, the situation is complex 

 (Mayneord, 1934; Lea, 1947a). The mean LET passes through 

 a maximum in the vicinity of 0.1 A, and then, with increasing 

 wave length, decreases to a nearly constant value which is main- 

 tained between 0.2 and 0.4 A. Values of mean LET which have 

 been obtained with X rays in pertinent experiments vary from 

 about 0.5 kev/fjL (highly filtered 1000 kv radiation) to 15 kev/n 

 (8.3 A radiation). 

 B. Fast atomic nuclei or radiations which produce them. 



4. Fast neutrons. These rays activate molecules preponderantly 

 •by means of light nuclei which they eject from atoms of the bio- 

 logical material. Since the predominant atoms in soft tissues 

 are hydrogen, oxygen, nitrogen, and carbon their nuclei are the 

 ones of interest. Moreover, since, in a soft tissue of a\^erage 

 water content, the hydrogen atoms outnumber the carbon, nitro- 

 gen, and oxygen, taken together, by a ratio of about 1.7:1, and 

 since the average energy transfer from a fast neutron to a hydro- 

 gen nucleus is far greater than to any of the other nuclei 

 mentioned, the proton takes a place of special importance. The 

 individual proton tracks are unequal in length and therefore in 

 mean LET. The mean LET of the whole population of tracks 

 has varied in different investigations from about 8 to 43 kev/^i, 

 depending on the neutron energy spectrum. 



5. a Particles. To date only the a particles from polonium and 

 from radon -|- RaA -|- RaC have been used in pertinent inves- 

 tigations, although other good a emitters exist (e.g., plutonium). 

 For all a particles, the maximum LET is about 260 key/ fi. The 

 minimum for those from polonium is 95, and the mean is 135. 

 The mean for those from Rn -\- RaA + RaC is 120. 



6. Accelerated deuterons and helium nuclei. These have been used 

 to obtain, by the track-segment method, values of LET ranging 

 from 0.6 kev/^t (190 Mev deuterons) to 44 kev/fx (relatively slow 

 helium nuclei). (Higher values of uniform LET, up to 260 

 kev/n, could be obtained by using initially less energetic helium 

 nuclei.) 



ESTIMATION OF DOSE 



At present the most acceptable method of expressing doses of all the 

 various types of ionizing radiations is in terms of energy absorbed per 

 unit mass of biological material. 



X and Gamma Rays. The commonly accepted method of estimating 

 the amount of energy absorbed per unit mass of tissue from the electrons 



