CONTEMPORARY ADVANCES IN PHYSICS, XXVIII 589 



at least five cases in which the distribution-in-range curve obtained 

 with the differential chamber shows a single symmetrical peak sug- 

 gesting only one group: from the radium series, Rn and RaA; from 

 the thorium series, Tn and ThA; from the actinium series, AcA. 

 Altogether there are twenty-three ^^ known alpha-emitters, so that 

 nearly half of the total remain to be investigated to this end. It may 

 be significant that out of the four known alpha-emitters having odd 

 atomic number, the high proportion of three at least is known to 

 display fine-structure (the fourth, Pa, being as yet uninvestigated). 



Interrelations of Alpha-Ray Spectra and Gamma-Ray Spectra 



Evidently, if two atoms of the same radioactive substance were to 

 emit alpha-particles of different speeds, there would be three obvious 

 possibilities. The resultant nuclei might be different: in this case 

 we should expect (though not with certainty) that they would be the 

 starting-points of different radioactive series, and we should speak of 

 "branching." The initial nuclei might have been different, in which 

 case it would have been improper to speak of them as belonging to the 

 same substance. Finally one at least of the two atoms might also 

 emit gamma-ray photons, of energies complementary to those of the 

 alpha-particles, in such a way that the total amount of energy released 

 by the one atom would be the same as the total amount released by 

 the other. 



The first of these possibilities is not to be excluded a priori (since 

 there are known cases of branching, though in them the alternative 

 is between emission of an alpha-particle and emission of an electron) 

 and neither is the second. The third, however, is the most agreeable, 

 since if realized it allows us to believe that in the transformation of 

 radium (to take one example) every radium nucleus is like every other 

 before its change begins and every resulting (radon) nucleus is like 

 every other after its change is over. Now alpha-ray emission and 

 gamma-ray emission often occur together, which suggests that often 

 the third possibility is the one which is realized; but this cannot be 

 proved without measuring the energies or the wave-lengths of the 

 gamma-rays. 



The simplest cases are those in which the alpha-ray spectrum con- 

 sists of two lines only. Here and always, there is an inconvenient 

 complication- at the start: when an alpha-particle is emitted, the 

 residual nucleus recoils, and it is the sum of the kinetic energies of 



'^ Not including Sm and other elements of atomic number lower than 81. The 

 rest are depicted (together with the beta-ray emitters of atomic numbers 81 and 

 greater) in Fig. 21. 



