Sec. 2.6] GAMMA RAYS 45 



Np e -/Np „ 2/ 



Nfiy/N y "/ f €(T 



(l + /-'f) ! 



where / = total conversion coefficient 



N$ e - = number of (/3e _ ) coincidence counts 

 Np = number of beta counts 

 iV/3-y = number of (@y) coincidence counts 

 N y = number of gamma counts 

 a = geometrical efficiency (per cent solid angle) of counter B 

 e = beta particle efficiency of counter B. 

 If/ and <r are small, the term/tre/2 can be neglected. The ratio Np e -/N$ is in 

 general a rapidly changing function of absorber thickness, the shape of the 

 curve depending upon the relative values of the electron energy and the beta 

 maximum energy. For this reason the effect of air and window absorption 

 must be taken into account. In practice, the value of the ratio Np e -/Ng 

 used to calculate/ is determined by plotting the observed ratio as a function 

 of absorber thickness and extrapolating, from the limiting value which the 

 curve approaches, back to zero absorber. 



If two cascade gamma rays are converted, the conversion coefficient 

 determined by the above method is the sum of the two coefficients, but the 

 separate values are not uniquely determined. 



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