218Ra alpha reduced width and its consequences for alpha clustering in the heavy elements.

Abstract

Nuclei of $^{218}\mathrm{Ra}$ were produced in the reaction $^{208}\mathrm{Pb}(^{13}\mathrm{C},3n)$. After being separated from the beam by a velocity filter, they were implanted in a Si(Au) surface-barrier detector and their $\ensuremath{\alpha}$-particle decays were observed in the same detector. From these $\ensuremath{\alpha}$-decay events the half-life of $^{218}\mathrm{Ra}$ was measured to be 25.6 \ifmmode\pm\else\textpm\fi{} 1.1 \ensuremath{\mu}s, almost a factor of 2 greater than the previously reported value of 14 \ifmmode\pm\else\textpm\fi{} 2 \ensuremath{\mu}s. The resultant $\ensuremath{\alpha}$ width fits well into the overall picture of $\ensuremath{\alpha}$-decay-rate systematics and weakens one piece of evidence quoted for the existence of $\ensuremath{\alpha}$ clusters in heavy elements.