sparrow: cytogenetic effects of ionizing radiations 57 



tions. The cosmic rays are really a complex of radiations which are 

 derived from interactions of very highly energetic particles. The less 

 energetic particles interact with the atmospheric atoms and various 

 reactions occur, including the production of mesons which in turn 

 decay with the production of electrons, positrons, gamma rays, sec- 

 ondary pair production, and the associated showers or avalanches of 

 low-energy electrons. The amount of cosmic radiation varies with the 

 altitude and with the position on earth with respect to the magnetic 

 poles (112). 



With the rapid developments in space science, it is now possible 

 to send seeds or spores into outer space and recover them. Since labo- 

 ratory experiments using man-made radiations similar in nature to 

 (but perhaps less energetic than) the particles from outer space are 

 now under way, it will be possible to test the biological effects of both 

 cosmic radiation and other ionizing radiations in the Van Allen belt. 

 Reports of such experiments will appear shortly in the literature and 

 should show significant effects if the predicted doses (up to 3 X 10 4 r 

 per hour) really exist (112). 



The characteristics of the various ionizing radiations are of con- 

 siderable interest to physicists and some of their properties should be 

 understood by radiobiologists. A partial list of the ionizing radiations 

 and some of their more important properties are summarized in 

 Table 1. This table may appear, at first sight, to be unnecessarily 

 complicated. However, the minimum amount of information is given 

 which, in my opinion, will allow the average biologist to use these 

 radiations with some understanding of their practicality and the 

 reason for differences in their relative biological effectiveness. 



It should be clearly understood that ionizing radiation generally 

 produces an effect proportional to the energy absorbed in the tissue 

 in question. The fraction of energy which passes right through a cell 

 or tissue produces no effect nor does the energy absorbed in the air or 

 medium surrounding the object in question (except for very small 

 structures). Thus, in general terms, it is obvious that a radiation 

 must have sufficient energy to penetrate to the position where its effect 

 is sought and not sufficiently penetrating that most of it passes right 

 through. The penetration of the different radiations is thus a major 

 factor in deciding which radiations should be used in many experi- 

 ments, and it also has an important bearing on their relative hazards. 



