Sec. 7.11] RADIOACTIVITY \" 



REFERENCES FOR DECAY SCHEMES (SEC. 7.10) 



1. Bleuler, E., and W. ZuNTi: Helv. Phys. Acta, 20, 195 (1947). 



2. Siegbahn, K.-.Phys. Rev., 70, 127 (1946). 



3. Siegbahn, K., and N. Hole: Arkiv. Mat. Ast. Fysik, 33A, Xo. 9 (1946). 



4. Bleuler, E., W. Boltman, and W. Zunti: Helv. Phys. Acta, 19, 419 (1946). 



5. Gleditsch, E., and T. Graf: Phys. Rev., 72, 640 (1947). 



6. Siegbahn, K.: Arkiv. Mat. Astron. Fysik, 34A, Xo. 10 (1946); 34B, Xo. 4 (1946). 



7. Peacock, C., and R. G. Wilkinson: Phys. Rev., 72, 251 (1947). 



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7.11. The Natural Radioactive Series. 



The three radioactive chains (see Figs. 50, 51, and 52) growing out of the 

 long-lived parent isotopes Th 232 , AcU 235 , and UI 238 , contain those radioactive 

 isotopes that, together with K 40 and Rb 87 , comprised the only radioactive 

 isotopes known before the discover}' in 1932 of artificially induced radio- 

 activity. The members of these series are found to occur in natural sources 

 and in this sense are actually the only ''natural" radioactive series. In 

 recent years, however, numerous short side branches have been added to 

 these series as a result of the discovery of many new isotopes produced 

 artificially. All of these branches consist of isotopes with relatively short 

 half-lives and therefore do not occur naturally. The longest of these 

 branches, as shown in Fig. 50, contains five members, and, although it runs 

 into the uranium-radium series, it has been separately named the protacti- 

 nium series after Pa 230 , its longest-lived member. 



A new series, completely independent of the three naturally occurring 

 series, has also been found whose leading members were produced artificially. 

 Although the series starts with Pu 241 , it has been named the neptunium series 

 after its longest-lived member Np 237 . This chain would appear to complete 



