592 BELL SYSTEM TECHNICAL JOURNAL 



states and energy-levels, to assume further that hv-vaXnes and 

 {Ui — C/y)-values are alike the differences between these energy-levels, 

 and to ascribe apparent defects of correlation to special circumstances 

 by virtue of which certain gamma-rays and certain alpha-rays are 

 too feeble to be detected. Should these ideas prove untenable, we 

 shall probably have to suppose that the nucleus is even more different 

 from the extra-nuclear world than we have hitherto admitted. 



Now arises the important question: when alpha-particles and 

 photons both are emitted in the course of the complete transformation 

 of one nucleus into another, which comes first? Despite the im- 

 measurable shortness of the times which are involved, this is in 

 principle an answerable question. For as I have mentioned already, 

 gamma-rays are detected and their photon-energies are measured by 

 examining the spectrum of the electrons which they eject from the 

 orbital electron-layers of atoms, chiefly from the layers surrounding 

 those very nuclei whence the photons themselves proceed. Now if 

 the photons come before the alpha-particles, say for example in the 

 transformation of ThC into ThC", these electrons will come from the 

 electron -layers of ThC atoms; in the contrary case, from the layers 

 of ThC". It is possible to distinguish from which they do come, 

 even when the energy of the photons is not independently known and 

 must itself be derived from the same data.^" 



The classical and crucial experiment of this type was performed 

 about ten years ago by Meitner, and it proved that the gamma-rays 

 emitted during the transformation of RdAc into AcX and during that 

 of AcX into An spring forth after the alpha-particle has departed and 

 the nucleus has become that of the daughter-element. These are 

 two of the cases in which the alpha-ray spectrum exhibits fine- 

 structure; and it is now generally supposed that the rule extends to all 

 such cases. The stationary states or energy-levels deduced from the 

 {Ui — f/y)-values and the /zj^-values then would pertain to the "final" 

 or daughter nucleus. In the instances where all the alpha-particle 

 groups except the main one are designated "long-range groups" — 

 RaC and ThC (the quotation above from Rutherford, Lewis and 

 Bowden refers to the former of these) — Gamow argues that the gamma- 

 rays are emitted before the alpha-particles; the energy-levels deduced 

 from the alpha-ray and the gamma-ray spectra would then pertain to 



^^ See the previously-cited article "Radioactivity," pp. 94-96. I mention in 

 passing that sometimes the "internal conversion" of photons whereby electrons are 

 ejected is apparently so much the rule, that no appreciable fraction of the gamma- 

 rays of some particular energy (or energies) escape from the atoms at all; in which 

 cases it becomes expedient to speak not of gamma-rays at all, but of an immediate 

 transfer of energy from the nucleus to the orbital electrons (a policy which may be 

 applied to all cases of internal conversion). 



