602 



ADOLF F. VOIGT 



isotopes are confined to the elements with atomic numbers of 83 and 

 greater, they are not of wide interest as biological tracers. 



Negative 13 rays are electrons ejected from the nucleus. A nuclide 

 with a greater number of neutrons than its stable isotopes tends to 

 transform itself, by emitting negative /3 rays, into its isobar of the ele- 

 ment of next higher atomic number, corresponding to the hypotheti- 

 cal process : 



• » iHi + 



on"- 



i^» 



or conversion of a neutron into a proton. In general the energy of 

 j8 rays is considerably less than that of the a particles, those from the 

 useful tracers ranging from 0.05 to 2 m.e.v. The /3 rays from a given 

 substance do not all have the same energy but have an energy distri- 

 bution, whose maximum value is usually listed in tables, the average 

 energy being 0.3 to 0.4 of this maximum. A discussion of this, of the 

 loss of energy by electrons in traversing matter, and of the distribu- 

 tion of ions along the tracks of /S particles is given in the chapter by 

 Gray. 



en 



CO 



UJ 



X < 



< - 



=1 1.0 2.0 3.0 



f'^XIMUM BETA-RAY ENERGY,m.e.v. 

 Fig. 1. Absorption of /3 rays 



in aluminum — energy of /3 rays 



vs. range in aluminum. 



0.4 0.8 1.2 1.6 2.0 2.4 

 GAMMA -RAY ENERGY, m.e.v. 



Fig. 2. Absorption of y rays in 

 lead— energy of y rays vs. half- 

 thickness in lead. 



jS particles from a given radioisotope are considered to have a 

 range, but this is much less well defined than for a particles. The ab- 

 sorption curve for j3 rays (the fraction passing through a given thick- 

 ness plotted against that thickness) is roughly exponential but falls 

 off more rapidly than exponentially as the absorber thickness is in- 

 creased. This range is usually listed in terms of surface density or 

 grams per square centimeter (thickness times density), since in such 

 units it is nearly independent of the nature of the absorber. Figure 1 



