98 BELL SYSTEM TECHNICAL JOURNAL 



and two of those for AcX. This is precisely what was to be expected; 

 for in the contemporary atom-model, the characteristic X-rays of an 

 atom are conceived to arise from its circumnuclear electron-family, and 

 to arise after and because an electron has been evicted from the family 

 — a cause which the primary gamma-rays, or the alpha-rays or the 

 electrons coming out of the nuclei, can themselves supply. The elec- 

 trons expelled by these "secondary" gamma-rays or X-rays (the latter 

 term is now preferred, in all cases where the identification can be surely 

 made) are ejected as the fourth stage of a complicated process: first, the 

 primary quantum or particle departs from the nucleus, then a tightly- 

 bound electron is ejected from the electron-family, then a rearrange- 

 ment of the remaining electrons brings about the emission of an X-ray, 

 which in turn expels the loosely-bound electron. (It seems unlikely, as 

 I intimated before, that the four stages are really separate; probably 

 the passage from the initial state to the final takes place in a single 

 operation, in a flash; but one does not see how to conceive that single 

 operation without resolving it into four.) Since even the primary 

 gamma-rays are emitted after the transmutation, the secondary X-rays 

 a fortiori must come from the atoms of the daughter-substance; and 

 this they do.^^ 



The gamma-ray spectra thus far mapped out consist of from one to 

 fourteen frequencies, not counting the secondary X-rays; the palm, in 

 this respect, is awarded to radium C. The highest frequency thus far 

 recorded is 5.4-10-'', corresponding to a wave-length of 5.57X (5.57 

 X 10~^^ cm) and a quantum-energy amounting to 3.54- 10"'^ erg or 2.22 

 millions of equivalent volts; it has twenty times the frequency of the 

 highest X-ray known, and twenty times as great an energy in each 

 quantum as is required to tear the most tightly-bound electron from the 

 family of the most massive atom. It emerges from the nuclei of atoms 

 which have just transmuted themselves out of radium C into radium 

 C. The fastest electrons forming a definitely-known line in a beta- 

 ray line-spectrum occur in that of thorium C" ; their speed amounts to 

 0.986 of that of light, their energy to almost 2.5- 10« equivalent volts; 

 but there are still faster ones in the continuous spectrum of radium C, 

 which extends at least as far as to 0.998 of the speed of light. The 

 energy of the alpha-particles of the various substances which emit them 



3* The strongest single piece of evidence is the measurement upon two radiations 

 of radium B, performed with the crystal spectroscope by Rutherford and Wooster 

 {Proc. Camb. Phil. Soc, 23, pp. 834-837; 1925) who found that the difference between 

 the angles at which they were diffracted from the crystals agreed closely with that to 

 be expected for two prominent X-ray lines of the L series of the daughter element 

 (atomic number 83) and disagreed unmistakably with that to be expected for the 

 parent element. This invalidated a contrary result obtained in 1914, which long had 

 stood as an obstacle in the way of the conclusion that gamma-rays are emitted after 

 the transmutation. 



