EMISSION OF F-23 AND NE-24 IN CLUSTER RADIOACTIVITY OF PA-231

Using a track-recording phosphate glass detector with a standard deviation sigma(Z)=0.23 charge unit, we collected approximately 2100 tracks of energetic clusters emitted from Pa-231. Of these, 1348 survived cuts on energy, angle of incidence to the detectors, and chi2. All but one of them are consistent with Ne-24, and our measured branching ratio, B(Ne/alpha)=(13.4+/-1.7) X 10(-12), for an alpha-decay half-life for Pa-231 of 3.28 X 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 F-23, the nucleus predicted by cluster emission models to have the second highest branching ratio for emission from Pa-231. This corresponds to a branching ratio B (Ne/F)=1347(-940)+6440, the 1sigmalimits being governed by inverse Poisson statistics. Published models predict lower values of B (Ne/F), ranging from 3 to 400. Our event, if its 4sigma 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.