CONTEMPORARY ADVANCES IN PHYSICS 401 



of deuterium with any other substance must be very carefully con- 

 sidered and assessed, whenever that other substance is bombarded 

 with a beam containing deutons and it is observed that protons are 

 produced. 



The range of the protons due to the foregoing reaction is about 

 14 cm when To is low — 0.1 MEV or thereabouts — and rises with Tq. 

 Translate its minimum value into the corresponding kinetic energy 

 (obtaining about 3 MEV); compute the momentum of the proton — 

 this, save for a minor correction due to the relatively small momentum 

 of the impinging deuton, should be opposite in direction and equal in 

 magnitude to the momentum of the other fragment of the transmu- 

 tation, the nucleus H^. Thence compute the kinetic energy of this 

 other fragment, and estimate thence its presumable range; owing to 

 our lack of experience with such particles the estimate may not be 

 very exact; Oliphant, Harteck and Rutherford arrive at the figure 

 1.74 cm. Now, the protons of 14-cm range of which I have been 

 speaking are not the only fragments to be observed when deutons 

 impinge on deuterium. There are also particles of a much less range; 

 these are equally numerous with the 14-cm protons, and expansion- 

 chamber photographs by Dee have shown that a track of the one 

 variety is likely to be paired with a track of the other, after the fashion 

 of the paired tracks due to the transmutations H + Li = 2He (Figs. 

 14 and 15, Second Part) ; and their range of about 1.6 cm. is taken by 

 the Cavendish people as being in substantial agreement with the 

 estimate aforesaid. It is this interlocking of concordant observations 

 which speaks so strongly for the Tightness of this description of the 

 reaction, and therefore for the existence of the hitherto-unknown 

 isotope H'' of hydrogen. 



Meanwhile it has been discovered at Princeton that the new isotope 

 can be generated by maintaining a self-sustaining discharge in gaseous 

 deuterium: a way of achieving transmutation several times attempted 

 in past years, but never (so far as I know) with proved success. Out 

 from the discharge tube (where the voltage is 50,000 to 80,000) some 

 of the ionized atoms and molecules shoot through a hole in the cathode 

 into another and very large chamber filled with deuterium in which 

 they disperse themselves, thus having opportunities for transmutation 

 in both this chamber and the tube. A sample of the gas is afterwards 



of number-of-fragments vs. To shows the pecuHar shape common to such curves 

 when obtained with thick layers, which suggests that as 7"o is raised the increase 

 in the number of transmutations is at first i)artly due to an increase in the [irobabiiity 

 of transmutation at an impact, but hiter entirely due to the fact that the faster 

 particles enter farther into the layer and have more ojiportiuiities of striking nuclei 

 before their energy is gone than do the slower (The Nucleus, Second Part, p. 141). 

 The theory of such curves has, however, never been worked out. 



