Using a track-recording phosphate glass detector with a standard deviation ${\mathrm{\sigma }}_{\mathit{Z}}$=0.23 charge unit, we collected ∼2100 tracks of energetic clusters emitted from ${}_{}{}^{231}{}_{}{}^{}\mathrm{Pa}$. Of these, 1348 survived cuts on energy, angle of incidence to the detectors, and ${\mathrm{\chi }}^2$. All but one of them are consistent with ${}_{}{}^{24}{}_{}{}^{}\mathrm{Ne}$, and our measured branching ratio, B(Ne/α)=(13.4±1.7)×${10}^{\mathrm{-}12}$, for an alpha-decay half-life for ${}_{}{}^{231}{}_{}{}^{}\mathrm{Pa}$ of 3.28×${10}^4$ 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)=${1347}_{\mathrm{-}940}^{+6440\mathrm{}}$, the 1σ 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 4σ 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.

• Received 29 June 1992

DOI:https://doi.org/10.1103/PhysRevC.46.1939