Chapter III 



PHYSICAL AND RADIOLOGICAL CONSIDERATIONS 



The hazards of exposure to radionuclides depend greatly on their physical and 

 chemical properties as well as the chemical form in which they are encountered. 

 These characteristics influence the rate of entry into the body, distribution to various 

 organs, and retention. The duration of radiation exposure is determined by the factors 

 that influence retention and by the physical half-life of the radionuclide. In some in- 

 stances the situation is complicated by decay chains and the biological redistribution 

 of the radioactive daughters. 



The nature of the emitted radiations determines the volume of tissue exposed 

 and hence the pattern of injury. For example, an energetic alpha ray penetrates 

 no further than 0. 07 mm in tissue; beta rays deposit their energy largely within the 

 organ of deposition; and gamma ray energy is absorbed in a much larger volume of 

 tissue, some escaping the body altogether. In addition, the degree of biological damage 

 depends to some extent on the concentration of ions produced along the path of ioniza- 

 tion. In general, ionization density is greatest in the case of those emissions with the 

 shortest range; i. e. , alpha rays. 



In recent years, attention has been centered upon the long-term hazards of radio- 

 nuclides of long half-life, including Ra'^'^", Sr^", andCs^'. However, intermediate 

 and short-lived radionuclides may be of comparable importance. In the event of ac- 

 cidental discharge of radioactive material from reactors and after nuclear detonations, 

 a large variety of fission products and some activation products may contaminate local 

 areas. Even in the case of global fallout from thermonuclear tests, some of the de- 

 posited fission products of intermediate half-life are the source of gamma irradiation 

 which, for a few months after detonation at high latitudes, has exceeded that from 

 Csl37^ 10 When the time between the release of fission products and human exposure 

 is a few weeks or less, I^^-'- is a principal contributor (see Sect. VII. D). 



Ru 



106 



The important radionuclides of intermediate half-life include Ba^^O^ j^^lOS^ 

 , Ce''^^ , Ce^'*'*, Y^l, Zr^^, and Sr^^. Some of these are so poorly absorbed 

 that for practical purposes their only effect is to irradiate the gastrointestinal tract 

 as they pass through it. Ba-'-'*'^ and Sr^^ are absorbed and must be considered along 

 with Sr^O as contributing to the skeletal dose-^-^. 



Detailed studies of local fallout patterns after tests at the Nevada test sitel2 have 

 shown that the geographical distribution of these radionuclides depends on a number of 

 factors, including the altitude of explosion and the nature and amount of surrounding 

 material. Size and solubility of radioactive particles, both of which obviously affect 

 the degree of exposure, vary with distance from point of release, prevailing meteor- 

 ological conditions, and other factors. Each nuclear accident is likely to produce a 

 unique hazard pattern. 



