42 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 2 



of the absorber, or more directly, on the separate absorption coefficients /j. and 

 Ho as expressed by the relation 



J. = /J " > 



I (X — Ho 



where I, I = intensities of secondary and primary radiations, respectively 



H, fi = absorption coefficients of secondary and primary radiations, 



respectively 



It is seen from the expression above that if the beam of gamma rays enters 



a second medium more dense than the first, e.g., from air into lead, the 



intensity of the secondary radiation will make a transition to a higher relative 



intensity level before coming to equilibrium. The transition region is 



characterized by a very rapid increase in electron emission and reaches a 



maximum value at a depth in the absorber approximately equal to maximum 



range of the recoil electrons. From the surface of the second absorber (see 



Fig. 4) the variation in the secondary radiation intensity I with depth x is 



given by the equation 



I = I M ° (<?-"•* - e-» x ) 

 M - Mo 



where I = gamma-ray intensity at surface of absorber 

 Ho = absorption coefficient for gamma ray 

 H = absorption coefficient for secondary radiation 



2.6. Internal Conversion. An isomeric transition in an excited nucleus is 

 accompanied by the emission of either a gamma quantum or an electron. 

 When a gamma quantum is emitted, it has its origin in the nucleus and 

 possesses an energy hv equal to the difference in energy between the initial 

 and final states of the nucleus. When an electron is sometimes found to 

 accompany an isomeric transition, the gamma ray is said to undergo internal 

 conversion. In such instances a gamma quantum is not emitted, but instead 

 an electron is ejected from one of the innermost electron shells, K, L, . . . 

 shells, of the same atom in which the isomeric transition occurs. 



The kinetic energy of the conversion electron is exactly equal to the energy 

 of the gamma ray with which it is associated minus the binding energy of the 

 electron in the atom, E = hv — I. In general, for a large number of radio- 

 active nuclei of the same species the conversion electrons associated with a 

 particular isomeric transition are observed to fall into one or more mono- 

 energetic groups with kinetic energies of E = hv — 7 K , E = hv — 7l, 

 E = hv — 7m , etc., where Ik, 7l, Iu, . . . are the ionization potentials of 

 electrons in the K, L, M, . . . shells of the atom in which the transition 

 occurs. It is important to note that if beta decay precedes the isomeric 

 transition the binding energies to be used are those of the daughter isotope 

 and not the parent. 



