Artificial disintegration by ∝-particles. Part II. ─ fluorine and aluminium

1. In a previous paper an account has been given of some experiments on the artificial disintegration of certain elements by bombardment with ∝-particles, in which an electrical method—the valve amplifier—was used to detect the protons emitted in the disintegrations. It was found that the disintegration protons emitted from all the elements investigated except fluorine and sodium could be resolved into distinct groups. A general description of the origin of these groups was given on the assumption that the protons and ∝-particles contained in an atomic nucleus are in definite energy levels. It will be shown in this paper that the protons from fluorine also consist of well-defined groups and that the failure to resolve them in our earlier experiments was due partly to their complexity and partly to the poor geometry of the experimental arrangement. An important question discussed in the previous paper concerned the penetration of a nucleus by an ∝-particle of insufficient energy to surmount the potential barrier of the nucleus. It was first pointed out by Gurney that there may be a resonance effect between the incident ∝-particle and the atomic nucleus. If the ∝-particle has exactly the energy corresponding to a resonance level of the nucleus its chance of penetrating the potential barrier and entering the nucleus will be very much greater than if its energy is slightly more or less than this. The first evidence for such a resonance effect was obtained by Pose in an investigation of the disintegration of aluminium. His results seemed very definite, and suggested that the disintegration of aluminium by ∝-particles from polonium was due entirely to penetration by means of two resonance levels. Later observations by Meitner and by de Broglie and Leprince Ringuet have not confirmed Pose’s results and have thrown doubt upon the real existence of this resonance phenomenon.