Contemporary Advances in Physics, XXVII The Nucleus, Second Part

T R A N G E as it seems to speak of "bringing up to d a t e " something that was published only six months ago, one is sometimes obliged to do so by the rapid march of science; and three of the "elementary particles" of which I spoke in the First Part were and still are so young--or to speak more carefully, our acquaintance with them is still so young--that their role and situation in the body of physical knowledge is changing from month to month. The Positive Electron Of the positive electron the most striking new thing to be said is, that there is now a new way of generating it: by impacts of alphaparticles against metals. This so far has been applied only by its discoverers, M. and Mme. Joliot; only with alpha-particles from polonium, therefore of energy 5.3 millions of electron-volts; only to five metals, of which beryllium and boron and aluminium yielded positive electrons, while silver and lithium did not. It is as yet the most efficacious way of producing positive electrons, Joliot having evoked last summer as many as 30,000 of these corpuscles per second from aluminium. This of course looks small when compared with the torrents of negative electrons which incandescent metals will pour out, * " T h e Nucleus, First P a r t " was published in the July 1933 issue of the Bell Sys. Tech. Jour., Vol. X I I . 102 CONTEMPORARY ADVANCES IN PHYSICS « 103 but these are not a proper standard of comparison. Rather should one say that in the autumn of 1932 positive electrons were being observed at the rate of three or four a year, and already by the summer of 1933 this rate had been enhanced to thirty thousand in the second! The other voluntary way of generating positive electrons--by applying hard gamma-rays to heavy elements--has already been studied enough to yield the data of the following table.