Emission of 23F and 24Ne in cluster radioactivity of 231Pa.

Abstract

Using a track-recording phosphate glass detector with a standard deviation ${\mathrm{\ensuremath{\sigma}}}_{\mathit{Z}}$=0.23 charge unit, we collected \ensuremath{\sim}2100 tracks of energetic clusters emitted from $^{231}\mathrm{Pa}$. Of these, 1348 survived cuts on energy, angle of incidence to the detectors, and ${\mathrm{\ensuremath{\chi}}}^{2}$. All but one of them are consistent with $^{24}\mathrm{Ne}$, and our measured branching ratio, B(Ne/\ensuremath{\alpha})=(13.4\ifmmode\pm\else\textpm\fi{}1.7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$, for an alpha-decay half-life for $^{231}\mathrm{Pa}$ of 3.28\ifmmode\times\else\texttimes\fi{}${10}^{4}$ yr. This value of B is a factor of 2.2 greater than was determined by Tretyakova et al., who observed 252 Ne decays. We identify one event as $^{23}\mathrm{F}$, the nucleus predicted by cluster emission models to have the second highest branching ratio for emission from $^{231}\mathrm{Pa}$. This corresponds to a branching ratio B(Ne/F)=${1347}_{\mathrm{\ensuremath{-}}940}^{+6440}$, the 1\ensuremath{\sigma} limits being governed by inverse Poisson statistics. Published models predict lower values of B(Ne/F), ranging from 3 to 400. Our event, if its 4\ensuremath{\sigma} deviation from Ne is regarded as sufficient evidence for F, would be the first example of emission of an odd-Z cluster in spontaneous cluster radioactivity.