RADIOACTIVITY METHODS 991 



gammas are very penetrating, but their absorption is usually underesti- 

 mated. It is well to remember that one foot of rock will absorb about 99 

 per cent of the gammas from the uranium series. Gamma rays are absorbed 

 in several different ways ; but photoelectric absorption and Compton scat- 

 tering are particularly important in this discussion. 



When a gamma ray is absorbed by the photoelectric effect, it transmits 

 its energy to an electron. It effectively tears the electron away from its 

 atom and propels it through the absorber. The photoelectron acts much 

 like a (3 and is soon absorbed. Therefore, the simple absorption law 



describes the photoelectric absorption quite accurately. lo and / are the 

 initial and final y intensities, /x is the photoelectric absorption coefficient and 

 X is the thickness of the absorber. 



The second important effect is Compton scattering where a y, acting 

 this time just like a particle, collides with an electron and bounces off, 

 transferring some of its energy to the recoiling electron. The y does not 

 disappear, but continues on its way with diminished energy and in a new 

 direction. Consequently, we may not represent this type of "absorption" 

 by the simple exponential relation. Instead we must write 



I = Io{e-^^ + C) 



where n is the Compton absorption coefficient and C is the proportion of 

 quanta scattered back into the detector after the first Compton collision. 

 C will be determined by the geometrical disposition of the source, absorber, 

 and detector, as well as by the atomic number and density of the absorbing 

 material. The analytical evaluation of C is difficult, even for simple geom- 

 etries. 



Photoelectric absorption increases with the fourth power of the atomic 

 number Z, whereas Compton scattering depends only on the electron den- 

 sity in the absorber, or roughly the first power of Z. Therefore photo- 

 electric absorption will predominate in heavy elements but the Compton 

 effect will be more important for the lighter atoms. In light elements such 

 as aluminum, for instance, almost all first interactions of 1-Mev y's will be 

 of the Compton type. It should be kept in mind that common rocks consist 

 of light elements almost exclusively. 



Natural Radioactive Elements 



Most of the naturally occurring radioactive elements belong to three 

 major families, each of which comprises isotopes of most of the elements 

 on the heavy end of the periodic table. The parent elements of the three 

 famines are uranium (92^-^^ and 92^"^^) and thorium {^qTIi-^^) , and the 

 daughter elements include isotopes of protoactinium, actinium, radium, 

 francium, radon, polonium, bismuth, lead, and thallium (in order of 

 decreasing atomic weight). The decay cascade of the three families 



