34 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 2 



the particular absorber. One half-value layer is the quantity of material, 

 either in grams per square centimeter or in centimeters, required to reduce the 

 intensity of gamma rays of a particular energy to one-half its initial value at 

 the surface. In terms of the absorption coefficient in corresponding units 

 the half-value layer is T^ = 0.693 //x. The gamma-ray intensity after tra- 

 versing n half- value layers is then I = I 2~~ n . 



Table 4. Mass-absorption Coefficients for Gamma Rays 

 Data taken from "Handbook of Chemistry and Physics," 30th ed., Chemical Rubber 

 Publishing Company, Cleveland, by permission of the publisher. 



It should be noted that the coefficients, cross sections, and half-value 

 layers are constant only for a particular energy and absorbing atom. When 

 the absorber consists of atoms with only one atomic number but the gamma 

 radiation contains photons of several different energies, the intensity at any 

 depth in an absorber is given by the sum of exponentials for the components. 



/ = her™ + I 2 e-^ x + 



+I s e- 



HsX 



where Ii, la, • • • In are the intensities of the 5 components at the surface 

 and fj.1, /x 2 , . . . Us are the corresponding absorption coefficients. 



In practice, the experimental conditions involved in gamma-ray absorption 

 are often more complicated: the energy may contain many components or 

 may even be continuous in distribution, the absorber may contain several 

 atomic species, and the geometry may be confused by a divergent beam. 



