REFERENCES 51 



(19) report that, when trivalent La^^^ decays to Ce^'*^, 60 per cent of 

 the Ce is found in the tetravalent state; hkewise when Se^^ in Se^^Oa^ 

 decays to Br^^, 35 per cent of the Br^^ is found as BrOs". In view of 

 the electronic excitation of the recoil atom that is caused by the beta- 

 decay process, these orderly oxidations seem surprising and may very 

 well be the final state of a complex molecular reorganization. 



The recoil energy that follows the gamma emission accompanying 

 isomeric transition can be calculated from the expression used for 

 obtaining the in, y) recoil energy, and the fraction available for breaking 

 bonds can be obtained from the expression above. In some cases the 

 gamma-ray energy is sufficient to break chemical bonds directly, 

 causing Szilard-Chalmers reactions as already discussed. In others, as 

 for example Br^°, the 48.9-kev gamma ray imparts a recoil energy of 

 no more than 0.016 ev. The percentage of the recoil energy available 

 for bond rupture is only 1.2 per cent in the case of Br^^ in HBr, compared 

 with 26.7 per cent for Br*° in C2H5Br. The subsequent rupture of the 

 chemical bond must therefore be due to internal conversion of the gamma 

 ray, with the consequent emission of electrons in the K or L shell. This 

 process coupled with the Auger effect leads to the loss of many electrons ; 

 for Br^°, calculations predict that 60 per cent of the recoil atoms will 

 lose four or more electrons. 



Seaborg, Friedlander, and Kennedy (20) have shown experimentally 

 that, in the case of zinc and tellurium, internal conversion is necessary 

 for bond rupture. The gamma ray emitted in the isomeric transition 

 of Zn*^^ is unconverted and more energetic than the gamma rays emitted 

 in the isomeric transition of Te^^^ and Te^"^. However, the tellurium 

 gamma rays are largely converted, and Seaborg et al. were able to observe 

 bond rupture in radioactive tellurium diethyl, whereas they could find 

 none in zinc diethyl observed under the same conditions. 



In sum, we can see that recoil effects, whether from beta decay or 

 gamma emission, almost inevitably disrupt chemical bonds and increase 

 the disorder of the system. In the whole spectrum of such effects re- 

 sistance to rupture or recombination must be viewed as a rare and 

 occasional process. 



REFERENCES 



1. Melkonian, E., Phys. Rev., 76: 1750, 1949. 



2. Engelkemeier, A. G., W. H. Hamill, M. G. Inghram, and W. F. Libby, Phijs. 

 Rev., 75:1825, 1949. 



3. Kruger, P. G., Phys. Rev., 51: 250, 1940; Proc. Natl. Acad. Sci., 26: 181, 

 1940. 



4. Zahl, P. A., F. S. Cooper, and J. R. Dunning, Proc. Natl. Acad. Sci., 26: 589, 

 1940. Zahl, P. A., and F. S. Cooper, Science, 93: 64, 1941. Zahl, P. A., and F. 



