The Actinium Series and the Order of Stability of Radioactive Isotopes

Abstract

ONE of the outstanding problems of radioactivity is the exact relation of the actinium to the thorium and uranium-radium disintegration series. That the actinium series starts at uranium I or an isotope of uranium, includes uranium-Y and ends at an isotope of lead after five α-particles have been expelled between protoactinium and the end product is regarded as proved by the experimental evidence. But the atomic weights of members of the series are not known; protoactinium is the only member of the series whose atomic weight might be determined by the usual methods, but this constant is still lacking owing to the very great difficulty of making a complete separation of protoactinium from its homologue tantalum. An investigation with the mass-spectrograph of uranium or of uranium-load, loading to the discovery of isotopes not ascribable to the uranium-radium series, might also throw light on the atomic weights of members of the actinium series. This has yet to be made.