Summary A homogeneous blood-thorium sulphate mixture has been used as a source of polonium-216 alpha-particle activity in two determinations of the half-life of this nuclide. The first method consisted of scintillation counting, by means of a scaler and six electro-mechanical registers, of the alpha-particle pairs produced by the decay of polonium-216 subsequent to that of its precursor, thoron-220. A more accurate result was provided by pulse-counting with a multi-channel analyser. Results are compared with those of three earlier experiments. The weighted mean of our two values for the half-life of polonium-216 (0,155 plusmn 0,004 seconds) is in better agreement with the result of Ward (1942) than with the determination by Diamond & Gindler (1963).

The nature of the self-energy term in the mirror nucleus energy-difference formula is investigated. Two approaches are used. In the first this self-energy term is assumed to be a constant equal to the Coulomb self-energy of a single proton, and in the second a more refined quantum mechanical approach based on the Swamy and Green Coulomb exchange energy calculations is used. Both approaches yield ${r}_{0}$ values which possess the correct general trend with increasing $A$, but which disagree with theoretical values for very low $A$. The effect of nonuniform charge distributions on the values of nuclear radii obtained from mirror nuclei is investigated, and expressions for the Coulomb energy for various charge distributions are given. A direct comparison between the mirror nucleus radii and those obtained from electron scattering is made in the few cases where this is possible. Finally, the possible validity of a suggested value of 0.58 Mev for the Coulomb self-energy of the proton is discussed briefly.