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A new kind of natural radioactivity

Abstract

From a systematic study of the properties of nuclei heavier than lead, we have concluded that, in one or two cases, radioactive decay by emission of a particle heavier than the α-particle might be observable in competition with the latter. We have observed such decay in 223Ra which occurs in the natural radioactive series emanating from 235U. Two ‘by-pass‘ modes seem likely: emission of 14C, to 209Pb, with a Q-value of 32 MeV; and of 12C, to 211Pb, with a Q-value of 28 MeV. (Emissions of 13C and 15C from the same nucleus have a priori emission rates similar to that of 12C, but the emission of α-like nuclei seems more likely.) In this study we have used a solid-state counter telescope to identify the charge of the particle emitted, and, for greater convenience, a source of 227Ac (half life 21 yr), with which the lower members of the series, including 223Ra, are in secular equilibrium. We have observed particles identifiable as carbon ions and, from their energy and emission rate, they are 14C rather than other carbon isotopes. The branching ratio for emission of 14C nuclei relative to α-particles from 223Ra is 8.5±2.5×l0−10, corresponding to a reduced width (preformation probability) smaller by a factor of 105 to 106.

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Rose, H., Jones, G. A new kind of natural radioactivity. Nature 307, 245–247 (1984). https://doi.org/10.1038/307245a0

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