Sec. 2.6] 



GAMMA RAYS 



43 



Nuclei with complex decay schemes usually undergo several isomeric 

 transitions to reach the ground state. Such multiple transitions may occur 

 in cascade and as alternative branches, and each transition involves a different 

 amount of energy. When gamma rays from several of these transitions 

 undergo appreciable internal conversion, the energy spectrum of conversion 

 electrons often exhibits considerable complexity since it may then consist of 

 electron groups with kinetic energies determined by the various values of 

 both hv and /. The relative intensities of the different groups show great 

 variation. In most instances the 

 strongest conversion occurs in the K 

 shell followed by successively smaller 

 contributions from theL, M, N, and, 

 in a few cases such as ThC and RaB, 

 the shell. Among the lighter ele- 

 ments and for gamma-ray energies 

 less than m c 2 , the strongest conver- 

 sion occurs in either the K or the L 

 shell depending upon the energy and 

 multipole order of the gamma quaji- 

 tum emitted in the transition. While 

 K conversion is in most instances the 

 strongest, L conversion in light ele- 

 ments, Z < 40, for gamma rays of 

 low energy, <3C m Q c 2 , and high multi- 

 pole order tends to become more 



10 15 



Z 2 /E in KEV" 1 



Fig. 5. Ratio of K to L conversion in ele- 

 ments for Z < 40 and for E < M c 2 . N K 

 and Nl are the numbers of K and L con- 

 version electrons, respectively, I is the 

 change in angular momentum of the nucleus 

 in the transition, and E is the gamma-ray 

 energy. [From M. H. Hcbb and E. Nelson, 

 Phys. Rev. 58, 486 (1940).] 



0.1 to~ 10 [18], 



important. The ratio of K to L con- 

 version under these conditions may have a value from 

 as shown in Fig. 5. 



The ratio a — N e /N y , the total number of conversion electrons divided by 

 the number of gamma quanta associated with a given isomeric transition, is 

 referred to in modern literature as the conversion coefficient. This form of 

 the coefficient is to be preferred on physical grounds to the erroneous older 

 expression N e /(N e -f- N y ), designated here by the letter/. Nevertheless, the 

 value of / is still frequently used and in some instances is convenient since 

 it expresses directly the fraction of transitions in which conversion electrons 

 are emitted and the fraction 1 — / in which gamma quanta are emitted. In 

 complex decay schemes where branching occurs, the number of electrons and 

 the number of gamma quanta emitted per disintegration depends also upon 

 the transition probability p (< 1) as given in the decay scheme. Hence the 

 number of electrons per disintegration is fp or ap/(a + 1) and the number of 

 quanta, (1 — f)p or p/(a -f- 1). 



Theoretical investigations of internal conversion lead to a complicated 



