VII. The natural radioactivity of rubidium

Abstract

The mode of decay of ⁸⁷Rb is investigated chiefly by means of a shielded proportional tube operating as a spectrometer. It is shown to consist simply of a β- transition between the ground states of ⁸⁷Rb and ⁸⁷Sr the limiting energy of the particles being 275 kev and the average energy 44 kev. The spectrum, which was investigated using sources of thickness 1 mg/cm² and 0•128 mg/cm², shows no maximum, differs very considerably from that for an allowed transition and is in agreement qualitatively with that for a third forbidden transition with parity change, Δ I=3, yes. The period is found to be τ=6·15 × 10¹⁰ yr. No ψ-rays, x-rays or internal conversion electrons ascribable to the decay process are emitted in appreciable intensity. It is suggested that the established natural radio-elements with Z <80 form a well-defined and isolated group with log ft.= 18 approximately.