High-Spin States in the Continuum. I.Ne20

We describe a new angular-correlation method for assigning spins to states that are selectively populated in reactions of the type ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^{12}{}_{}{}^{}\mathrm{C},\alpha ){}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ and which decay by the emission of an $\alpha$ particle to a state of nonzero spin, which decays in turn to a spin-zero state by the emission of a $\gamma$ ray. Such states are not accessible to conventional particle-particle angular-correlation techniques. The new method consists of measuring the triple-angular correlation of the two $\alpha$ particles and the $\gamma$ ray from the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^{12}{}_{}{}^{}\mathrm{C},{\alpha }_1{\alpha }_2\gamma ){}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ with the first emitted $\alpha$ particle observed at zero degrees, the $\gamma$ ray observed at a fixed angle, and the second emitted $\alpha$ particle observed over a range of angles. Applications of this method to the determination of the spins of two states in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$, at 15.18 ± 0.02 MeV ($6^{+}$) and 17.40 ± 0.02 MeV ($9^{-}$), are discussed. The 17.40-MeV state has been identified as the $9^{-}$ member of the lowest negative-parity band in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$; the $6^{+}$ assignment to the level at 15.18 MeV is in disagreement with an earlier assignment of spin and parity $9^{-}$ to this level.