XVII. RADIOACTIVE TRACERS 003 



is a curve of /S-ray energy (maximum) versus the range of the (3 rays 

 in aluminum (16). 



7 rays are photons with energies that range from 0.01 to 2 m.e.v. 

 for the important tracer nucUdes. They have greater penetration 

 than the usual X rays though the newer high potential X-ray instru- 

 ments can duplicate and even exceed these energies. Their penetra- 

 tion is very much greater than that of the jS rays and consequently 

 their effects are not localized near the source but appear throughout 

 the entire system, y rays produce effects by means of the secondary 

 electrons to which they give part of their energy, principally by means 

 of the Compton process, or collision between a 7 ray and an electron, 

 in which the electron takes part of the energy of the 7 ray as kinetic 

 energy and the 7 ray is scattered with the remainder. The absorp- 

 tion of 7 rays in matter is not characterized by a range but is exponen- 

 tial and can be considered in terms of the absorption coefficient, de- 

 fined as ju in the equation : 



/ = he-'"' 



in which h is the incident intensity and / the intensity after passing 

 through a thickness d. The half-thickness, the layer necessary to cut 

 the intensity to half its original value, is given by the relation: 



d,/, = 0.693/)u 



The thickness, d, is usually multiplied by the density and expressed 

 in grams per square centimeter and m is similarly given in square 

 centimeters per gram. Figure 2 shows the relation between the ab- 

 sorption half -thickness in lead and the energy of the 7 ray (16). 



7 rays, in particular those of low energy, frequently are absorbed 

 in the atom in which they originate and their energy is transferred 

 to an electron as kinetic energy. This process, known as "internal 

 conversion," results in an electron with energy less than that of the 

 original 7 ray by an amount equal to the binding energy with which 

 the electron was held in its place in the atom. The electron is ac- 

 companied by X rays given off when other electrons drop in to fill its 

 place and usually by some fraction of the original 7 rays that escapes 

 conversion. 



Other processes by which radioactive atoms decay are positron 

 (or positive /3-ray) emission and K electron capture. These processes 

 occur in nuclides with fewer neutrons than their stable isotopes and 

 correspond to the hypothetical conversion of a proton into a neutron. 



h. 



