148 BELL SYSTEM TECHNICAL JOURNAL 



Disintegration-with-capture is very advantageous for the theorist, 

 since when there are only two fragments after the interaction the 

 principle of conservation of momentum suffices to determine the 

 kinetic energy of either in terms of that of the other and that of the 

 alpha-particle. In equation (9), Tq stands for the kinetic energy of 

 the alpha-particle, Ti for the sum of the kinetic energies of the proton 

 and the residual fragment, which call Tp and Tr respectively. Now 

 excepting in the cloud-chamber experiments, it is only the proton 

 which is detected, and therefore only Tp can be estimated from the 

 data; but if the disintegration is by capture, then Tr and consequently 

 Ti can be deduced from Tq and Tp. If however there are three or 

 more final fragments, measurement of Tp is not sufficient to determine 

 T\. Also even in the case of disintegration-by-capture there will be 

 uncertainty if the transmuted element is a mixture of two or more iso- 

 topes, since the value of Tr corresponding to an observed Tp will de- 

 pend on the mass of the atom which is transmuted. 



In a case of disintegration-by-capture, the simplest possible assump- 

 tion is that {T\ — Tq) has a perfectly definite value, independent of Tq\ 

 there is conversion of a definite amount of kinetic energy into rest-mass 

 (or vice versa), whatever the velocity of the alpha-particle may be. 

 This may be tested by varying To; it may also be tested to some extent 

 by observing protons ejected in various directions (relatively to the 

 initial direction of the alpha-particles) since although the sum of Tp 

 and Tr (which is T\) should be the same for all of these protons those 

 two quantities individually should vary, and Tp in particular should 

 depend in a definite manner on the direction of the protons. Yet in 

 nearly all such tests, the target is so thick that the alpha-particles im- 

 pinging on various nuclei have very various speeds. How then shall 

 we know which speed of proton to associate with which speed of alpha- 

 particle, which value of Tp belongs with which of TqI One naturally 

 begins by assuming that the fastest of the primary particles produce 

 the fastest of the protons. But plausible as this assumption seems at 

 first, there are several cases known in which it is not true: cases in 

 which a definite group of protons is evoked by alpha-particles of a 

 definite interval of speeds, and neither faster nor slower particles are 

 capable of producing them. 



This phenomenon of " resonance, " as it is called,^" was first observed 

 by Pose in the experiments on aluminium to which many pages were 

 devoted in "Transmutation." It is evidently an important quality 

 of nuclei, destined to be prominent in experiment and theory both. 



5° There is a tendency to use the term "resonance" to express the mere existence 

 of groups, irrespective of whether they are evoked by alpha-particles of narrowly 

 limited speeds. This is to be deprecated. 



