44 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 2 



dependence of a on the type of gamma radiation, which may be either electric 

 or magnetic multipole, the multipole order of the radiation (dipole, quadru- 

 pole, . . .)> the energy involved in the transition, hv, and the atomic num- 

 ber of the nucleus. For heavy nuclei and for gamma rays with hv > m c 2 , 

 calculations of a [9,10,11] indicate stronger conversion for magnetic dipole 

 than for electric dipole radiation and an increase in a with multipole order. 

 The most probable conversion under these conditions occurs in the K shell, 

 while L, M, . . . conversion occurs with successively smaller probability for 

 any one value of Z and E y . Conversion in lighter elements, Z < 40, and for 

 low-energy isomeric transitions has been calculated for both the K shell [17] 

 and the L shell [18]. The results of these investigations indicate that, for 

 decreasing gamma-ray energy, conversion increases rapidly with multipole 

 order. For any one energy and kind of radiation conversion varies approxi- 

 mately as Z z . 



Direct experimental evaluation of conversion coefficients usually has been 

 obtained by beta spectrograph analysis of the beta spectrum. The line 

 spectrum of monoenergetic conversion electrons appears as a set of peaks 

 superimposed on the continuous background of the decay electrons when 

 these are present. The peak positions in units of Hp indicate the energies of 

 electrons from various shells, and the area under the peaks indicate the 

 relative intensities of the electrons. For a radioisotope with a simple decay 

 scheme and one strongly converted gamma ray the conversion coefficient is 

 given unambiguously by the ratio of the number of electrons in the line 

 spectrum to the number in the continuous portion. However, when the 

 decay scheme is complex, the method is difficult and the results often uncertain. 



A second but less accurate method for determining conversion coefficients 

 involves counter measurements of the coincidences (I3e~) and ((87) [12,14]. 

 A sample of the isotope for which internal conversion is to be measured is 

 placed between two thin window counters one of which, counter A, registers 

 single events as well as twofold coincidences with counter B. With sufficient 

 aluminum absorber placed in front of counter A to stop all electrons, single 

 gamma events are registered in A and {J5y) coincidences by counters B and A . 

 In the same way, but with the absorber removed, single beta events are now 

 registered in counter A and ((3e~) coincidences by counters A and B. The 

 recorded number of (j3e~) coincidences, Np e -, must be corrected for (J3y) and 

 (77) coincidences as measured above. These corrections are normally small, 

 however, since the gamma-ray efficiency of counters is ~ 0.01 whereas for 

 beta particles the efficiency is ~ 1.0. 



If the decay scheme of the isotope is simple and only one gamma is con- 

 verted appreciably, the conversion coefficient is calculated from the counting 

 measurements above by the expression 



