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.