DISINTEGRATION (DECAY) 



115 



fast neutrino; high, corresponding to a slow neutrino; and reaching the 

 largest, or maximum value when the velocity of the ejected neutrino is zero. 

 The spectra are represented in Fig. 5-8 for pure emitters. The areas under 

 the curves for each type represent the total emission. Table 5-2 gives the 

 energies of the emanations from some unstable isotopes of biological interest. 



en 



01 



i 



c 

 o 



alpha 



gamma 



Energy E 



Figure 5-8. Energy Spectra of Three Emanations, Each from a Pure Source. Alphas and 

 gammas are monoenergetic; betas come off with a range of energies (i.e., speeds). 



Many biologically active chemical elements have unstable isotopes, of 

 which the type, the speed, and the length of time over which the emanation 

 is given off (i.e., the rate of decay) vary widely. There are now over six 

 hundred isotopes known. Only about twenty of these satisfy the chemical, 

 the energy, and the half-life requirements sufficiently well to be useful in 

 biology. Of these, the uses of P 32 , I 131 , C 14 , and Co 60 are the most advanced. 



TABLE 5-2. Some Isotopes Used as Biological Tracers" 



*From "Radiological Health Handbook," National Bureau -I Standards, Washington I > < ' 



