Contemporary advances in physics, XXVIII the nucleus, third part

This article deals first with the newer knowledge of alpha-particle emission: that common and striking form of radioactivity, in which massive atom-nuclei disintegrate of themselves, emitting helium nuclei (alpha-particles) and also corpuscles of energy in the form of gamma-rays or high-frequency light. There follows a description of the contemporary picture of the atom-nucleus, in which this appears as a very small region of space containing various charged particles, surrounded by a potential-barrier; and the charged particles within, or those approaching from without, are by the doctrine of quantum mechanics sometimes capable of traversing the barrier even when they do not have sufficient energy to surmount it. The exponential law of radioactivity — to wit, the fact that the choice between disintegration and survival, for any nucleus at any moment, seems to be altogether a matter of pure chance — then appears not as a singularity of nuclei, but as a manifestation of the general principle of quantum mechanics: the principle that the underlying laws of nature are laws of probability. Moreover it is evident that transmutation of nuclei by impinging charged particles, instead of beginning suddenly at a high critical value of the energy of these particles, should increase very gradually and smoothly with increasing energy, and might be observed with energy-values so low as to be incomprehensible otherwise; and this agrees with experience.