VII. The natural radioactivity of rubidium

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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.