CONTEMPORARY ADVANCES IN PHYSICS, XXVIII 591 



itself (either the very ones which are emitting the gamma-rays, in 

 which case the rays are said to undergo "internal conversion," or 

 their neighbours) or they may be those of other elements mixed with 

 the radioactive substances, or those of nearby solids or gases on which 

 the gamma-rays fall. Each gamma-ray line is responsible for several 

 different beta-ray lines, a circumstance which makes the analysis 

 more difficult at the beginning though it makes the inference more 

 reliable in the end. There may be gamma-rays having nothing to do 

 with alpha-particle emission, and there may be gamma-rays from 

 several different radioactive substances inextricably mixed up together, 

 so that the problem of analyzing the spectrum of one transformation is 

 preceded (or, more truly, accompanied) by that of distinguishing it 

 from the intermingled spectra of others. ^^ The experimental errors in 

 the estimates of L'^- values and /zi'-values may be so large that apparent 

 agreements are actually unreliable. Altogether, the comparison of a 

 rich alpha-ray spectrum with a rich gamma-ray spectrum is an 

 exceedingly intricate business, the outcome of which is not to be 

 summarized in a few sentences. To give a mere notion of the sort of 

 conclusion which is reached, I quote some lines from Rutherford, 

 Lewis and Bowden, in their comparison of the thirteen-line alpha-ray 

 spectrum and the very rich gamma-ray spectrum of RaC: 



"When we consider in broad terms the data which have been 

 presented, there can be no doubt that there is a high correlation 

 between the alpha-particle levels which have been observed and the 

 emission of gamma-rays. In more important cases the numerical 

 agreement is well within the experimental error of measurement, 

 while the relation between the intensity of the alpha-ray groups and 

 the gamma-rays associated with them is of the right order of magnitude 

 to be expected on general theoretical grounds. In other cases the 

 agreement is very uncertain, and more definite information on the 

 gamma-rays is required to make the deductions trustworthy. It is 

 unfortunate that we have been unable to detect the alpha-particle 

 groups corresponding to certain postulated levels [i.e. postulated from 

 the classification of the gamma-ray lines] . . . ." 



Thus it appears that there are excellent agreements between hv- 

 values and {Ui — Uj) values, and yet nothing approaching a perfect 

 one-to-one-correspondence. Nevertheless, the general programme is 

 fixed: to assume that each nucleus possesses a system of stationary 



" It is interesting to notice that after the /zj/- values of certain gamma-rays emitted 

 from mixtures of ThC and ThC" had been found to agree with values of {Ui — Uj) 

 taken from the alpha-ray spectrum of ThC, these gamma-rays were proved to proceed 

 from ThC in its transformation into ThC", whereas till then they had been supposed 

 to proceed from ThC" in its transformation into ThD (Meilner & Philipp, Ellis). 



