82 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 4 



An approximate value of the effective nuclear charge at the K orbit may be 

 obtained by using Slater's [28] screening constant of 0.3 for Is electrons; 

 hence, Z e g = Z — 0.3 where Z is the actual atomic number. At very low 

 energies the K electrons, except in very light elements, do not contribute to 

 the stopping and B K — -> 0. At high energies where 17 ^>> 1, C K — » and there- 

 of 



(a) 



10 20 30 40 50 60 

 ATOMIC NUMBER 



70 80 90 100 



(b) 



1.0 

 0.9 

 0.8 

 0.7 

 0.6 

 0.5 



0.4 



10 20 30 40 50 60 70 80 90 100 



ATOMIC NUMBER 

 Fig. 16. Effective contribution of each K electron (a), and each L electron (b) to atomic 

 number as a function of Z. [From H. Ilbnl, Z. Phys., 84, 1 (1933).] 



fore can be neglected. Consequently at high energies (> ~ 10 mev) the 

 simple stopping formula is valid. 



It has also been pointed out by Bethe [6] that in medium and heavy 

 elements all electrons in deep lying shells with ionization potentials greater 

 than mE/M have little probability for excitation by a passing particle. 

 These electrons (K, L, M, and possibly N) therefore contribute less than one 

 charge unit per electron to the atomic number used in the stopping formula. 

 The contributions, or oscillator strengths, of such electrons have been calculated 

 by Honl [14] for the K and L electrons and are plotted in Fig. 16. The 

 effective value of Z in the stopping formula is then Z =f K n K -^-JlUl + 

 remainder of electrons, where /is the oscillator strength of an electron in the 



