XVII. RADIOACTIVE TRACERS 601 



could learn the technit^ues involved. It is hoped that this chapter 

 will serve as an introduction to the method and to its rather extensive 

 literature for such persons. 



B. TERMINOLOGY OF RADIOACTIVITY 



In the use of radioactive tracers there tue certain ideas and terms 

 of such basic importance that some time should be spent in discussing 

 them before proceeding to the methods of applying tracers to bio- 

 logical problems. For further information see one of the books on 

 this subject U~10). The term isotope itself has been used with sev- 

 eral different meanings and for clarification a new term "nuclide" 

 had been introduced by Kohman {13) to mean any particular nuclear 

 species, with given atomic number, Z, and mass number, A. The 

 term isotope is then reserved for its original meaning, that of nuclides 

 with the same Z and different A. Other terms frequently met are 

 isobars, nuclides with different Z but the same A, and isomer (in the 

 radiochemical sense), meaning "with the same Z and A but having 

 different nuclear properties." 



1. Types of Radiation and Decay Processes 



Radioactive atoms are characterized by the emission of ionizing 

 radiation. This ionization is capable of being extremely harmful in 

 large doses, but the point at which the radiation may affect the bio- 

 logical sj^stem is not well defined. Since, in tracer studies, it is neces- 

 sary to avoid affecting the system by the radiation, a consideration 

 of this subject is essential in planning the experiments. The relative 

 effects of the various radiations is considered briefly below along with 

 their other properties, but a much more complete discussion of some 

 of these points is given in Chapter XV. 



The radiations by the emission of which radioactive elements de- 

 caj'^ are: a particles or helium nuclei; (3 particles, which are elec- 

 trons if negative in sign and positrons if positive; and y rays, which 

 are similar to X rays. 



a rays are highly energetic, most of them carrying an energy of 

 4-6 m.e.v. (One million electron volts, 1 m.e.v., is that energy an 

 electron has after being accelerated by 10*' v. It is equivalent to 1.6 

 X 10~^ erg.) The "range" or maximum penetration of these parti- 

 cles is 3-5 cm. of air or roughly 30-50 n of tissue. Since a-emitting 



