102 



RADIATION BIOLOGY 



Therefore the relation between the number N of electrons transmitted and the 

 thickness ^ of a foil may be represented by the empirical equation 



N{t) ~ N{0)e-'^^ 



(37) 



The coefficient n characterizes the slope of the plot in Fig. l-63b, which is de- 

 scribed by the equation 



In Nit) « In NiO) - tit (38) 



This coefficient ju is called the "absorption coefficient" by analogy with the 

 similar coefficient which appears in the law of X-ray penetration (see Sect. 4-3a). 

 However, the coefficient /x serves merely to characterize an approximate empirical 

 relation for j8-ray absorption, whereas it has an intrinsic physical significance for 

 the absorption of X rays. 



The absorption coefficient n may be related to the average energy or, even 

 more conveniently, to the maximum energy Em of the electrons of each jS-emitting 

 substance by means of the approximate empirical equation 



m/p = 22E 



1.33 



(39) 



where p is the density of the absorber in g/cm^ and Em is expressed in million 

 electron volts. Alternately, and more frequently, the abscissa of the point 



MAXIMUM 

 RANGE 



L 



FOIL THICKNESS 

 Fig. 1-63. Diagram indicating the reduced transmission of /3 rays through foils of 

 increasing thickness. 



in Fig. 1-63 at which the curve fades into the background is considered as "maxi- 

 mum range," Rm- Figure 1-62 gives the experimental values of Rm vs. Em for 

 aluminum. 



4-3. X RAYS 



The penetration of X rays into matter is limited by the same processes 

 which serve to distribute the X-ray energy to matter, namely, photo- 

 electric effect, Compton scattering, and pair, production. As seen in 



