Appendix D 



Ionizing Radiations 



The biological effects of ionizing radiations were discussed in Chapters 

 10 and 16. Some of the physical properties of these radiations are 

 outlined here. 



Ionizing radiations may have a number of different origins. So-called 

 "naturally radioactive materials" give off a, |8, or y rays. Artificially 

 produced radioactive isotopes emit positive and negative jS's and y's. 

 (Artificially produced isotopes include the products of nuclear fission.) 

 In addition, neutrons may be obtained from a nuclear reactor. High 

 energy protons, deuterons, alpha particles, and electrons can be pro- 

 duced with the appropriate types of accelerators. The different types 

 of particles are listed in Table I on page 611, along with their properties. 



All of these radiations, when striking an atom, impart energy to it by 

 either exciting its electrons to a higher energy state or knocking one (or 

 more) electrons from the atom. For each type of radiation, the energy 

 loss per centimeter, the shape of the path, and the stopping distance, 

 all have characteristic values. 



Alpha particles have the greatest rest mass of the various types of 

 radiation considered here. The alpha particles are helium nuclei. 

 Each has an atomic weight of 4 and a positive charge equal to twice the 

 magnitude of the charge of an electron. Alpha particles interact 

 strongly with electrons, losing their excess kinetic energy in a matter of a 

 tenth of a millimeter of tissue or less. They are only harmful to bio- 

 logical cells if the a-emitter is incorporated in the cell or in a neigh- 

 boring one. 



Other massive positively charged particles are the deuteron and the 



610 



