164 



ISOTOPIC TRACERS 



tegrating in a unit of time. From this equation it can be seen that half 

 of the atoms will disintegrate in some certain time. The same length 

 of time is required for half of the remaining atoms to disintegrate. This 

 disintegration rate can be seen more easily in Fig. 12T. The time required 

 for half the atoms to disintegrate is called the half-life of the isotope, 

 designated as ty.. Since the rate of disintegration of any isotope is a dis- 

 tinctive prop>erty of that isotope, each isotope has a characteristic half- 

 life. Biologists have become so accustomed to the term that they now 

 facetiously speak of the half-life of their paychecks and of other items 

 to which the reasoning is not strictly applicable. 



0.5 



2 2 2 2 2 



Fig. 12-1. Curve describing decay of a radioactive isotope. 



The half-lives of isotopes influence the selection of tracer isotopes. 

 Obviously the half-life must be some convenient period of time. If half 

 the isotope disintegrates in 0.01 sec, any experiment must be done in an 

 impossibly short time. The only radioisotopes in common use as tracers 

 have half-lives of at least several days; C^^ decays so slowly that half of 

 it still remains after almost 6000 years. Thus in case of a choice among 

 several radioisotopes of the same element, some always have more favor- 

 able half-lives than others. 



A second major consideration in choosing radioisotopes is largely a 

 matter of laboratory safety. Alpha-emitters are not commonly used as 

 biological tracers. There is often a choice, however, between (3- and y- 



