CONTEMPORARY ADVANCES IN PHYSICS 145 



merits than the protons, excepting fragments of such great mass that 

 they could take up the necessary momentum without taking an 

 appreciable amount of kinetic energy. The heavy nucleus by itself 

 is able to do this. However there are also neutrons, of which the 

 energy is sufficient to let them be detected, and therefore by no means 

 negligible. This is gratifying for the theory, inasmuch as if a proton 

 is separated from a deuton, the residue should be a neutron (or else 

 another proton and a free electron) ; but one is then obliged to assume 

 that the neutron always takes the same kinetic energy, whatever 

 that of the impinging deuton may have been. This seems rather odd, 

 but nothing prohibits it. Streams of alpha-particles have been sent 

 against compounds ("heavy water') containing deuterium in abun- 

 dance, but as yet no neutrons have been detected coming off. 



Transmutation by Impacts of Alpha-Particles ^^ 



Impact of an alpha-particle against a nucleus may result in the 

 springing-off of one or more (or none) of four kinds of corpuscles: 

 protons, photons, neutrons, positive electrons. 



Trans77iutation ivith production of protons 



This is the earliest-discovered type, of which I told at length in 

 "Transmutation." The discovery was made by Rutherford in 1919 

 in experiments on nitrogen. At present the Cavendish school considers 

 that this mode of transmutation has been proved for thirteen elements, 

 none of atomic number greater than 19: the list comprises B, N, F, 

 Ne, Na, Mg, Al, Si, P, S, CI, A, K. The most frequently and fully 

 studied cases are those of boron, nitrogen and aluminium. 



The evidence that the fragments are protons is rather variegated. 

 In some cases this has been proved by deflection-experiments;^^ 

 recently it has been proved in some other cases by measuring both 

 the range of the fragments and the ionization which they individually 

 produce in a shallow chamber or a deep one (page 125) ; some observers 

 are able to tell the scintillations due to protons from those which are 

 due to alpha-particles. 



Integral distribution-in-range curves of the fragments have been 

 obtained for boron, nitrogen, fluorine, sodium, magnesium, aluminium 

 and phosphorus. Most of them show more or less conspicuous 

 plateaux, of which the most magnificent appear in the celebrated 

 curves of Pose for aluminium, reproduced in "Transmutation" 



^^ An expanded version of this section, with citations of additional data and 

 reproductions of some curves, appears in the Physics Forum of the Review of Scientific 

 Instruments for February 1934. 



^^ "Transmutation," pp. 636-640, B. S. T. J., Oct. 1931. 



