CONTEMPORARY ADVANCES IN PHYSICS 97 



it to the binding-energy, and dividing the quotient by //. This is in a 

 sense the reverse of the usual process of ascertaining which is the 

 element from which the beta-ray line-spectrum proceeds; and as a 

 matter of fact the two have frequently been carried out as parts of one 

 single investigation. The line-spectrum is photographed and its lines 

 are measured, and then the student works over the data until he suc- 

 ceeds in setting up a hypothetical gamma-ray spectrum in which not 

 too small a fraction of the beta-ray lines are explained by .the action of 

 not too great a number of gamma-ray frequencies upon the K and L 

 and M classes of electrons, and reversely there is no too obtrusive 

 case of a beta-ray line being predicted from his hypothetical gamma- 

 spectrum and failing to appear. 



For illustration I will quote some actual results. Meitner and Hahn 

 located forty-nine lines in the beta-ray spectrum of radioactinium ; 

 among these, thirty-seven could be attributed to the action of one or 

 another of twelve gamma-ray frequencies upon one or another of nine 

 classes of electrons. In the spectrum of actinium X they observed 

 twenty-nine lines and explained fourteen of them by postulating seven 

 frequencies. Happily there are much more perspicuous cases. The 

 spectrum of radium D consists of only a few lines — five, according to 

 L. F. Curtiss,^^ whose measurements show that four may be supposed 

 to consist of electrons ejected from the Li, Lu, Mi, and Ni shells by a 

 single gamma-radiation of wave-length 0.26A, while the energy of the 

 electrons forming the fifth line is not perceptibly different from the 

 quantum-energy hv of the rays themselves. These last electrons may 

 have been extracted from the outer layers of the atoms, where the 

 binding-energy is so small that it makes but an insignificant deduction 

 from the energy transferred to the electron. Another instance is that 

 of radium itself, of which the three lines composing the beta-ray 

 spectrum may be ascribed to a single gamma-ray of wave-length 

 0.066A expelling electrons from the K group, the Li and the Mi group. 

 Such cases as these are so simple that the theory in general and the 

 wave-lengths calculated for the gamma-rays in particular are almost 

 beyond all question. 



Certain of the gamma-ray frequencies thus determined, and some 

 which are directly measured with the crystal spectroscope, are found 

 to agree with characteristic X-ray frequencies of the atoms whence 

 they come. This is true of the solitary gamma-ray which is necessary 

 and sufficient to explain the beta-ray spectrum of UXi, and of two of 

 the rays postulated by Meitner to account for the spectrum of RdAc 



33 L. F. Curtiss: Phys. Rev. (2), 27, pp. 257-265 (1926). A previous investigation 

 by L. Meitner (ZS.f. Physik, 11, pp. 35-54; 1922) had led to substantially the same 

 conclusion regarding the gamma-ray spectrum. 

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