States of Be11, B11, and C11

Abstract

The gamma-decay of the 4.46- and 5.03-Mev levels of ${\mathrm{B}}^{11}$ has been determined by ($p, \gamma$) coincidences using the reaction ${\mathrm{Be}}^9({\mathrm{He}}^3, p){\mathrm{B}}^{11}$. The 5.03-Mev level is found to decay with a probability of 0.14±0.03 via the state at 2.13 Mev; the intensity of the branch to the state at 4.46 Mev is less than 3×${10}^{-3\mathrm{}}$. The intensity of the branch from the 4.46-Mev level via that at 2.13 Mev is less than 5×${10}^{-3\mathrm{}}$. The data on these states are reviewed and it is concluded that the present results strongly favor the choices $J=\frac{1}{2}-, \frac{5}{2}-, \frac{3}{2}-$ for the states at 2.13, 4.46, and 5.03 Mev, respectively, without appeal to a model. It is, however, emphasized that the body of data as it stands at the moment might admit $J=\frac{1}{2}+$ for the 2.13-Mev state and $J=\frac{1}{2}-$ for the 5.03-Mev state. The previsions of the independent-particle model (IPM) are then examined and it is concluded that best general agreement between theory and experiment is achieved for $\frac{a}{K}\approx 4-4.5$. The likely relevance of the collective model is remarked upon. It is demonstrated with the aid of the reaction ${\mathrm{B}}^{10}(p, \gamma ){\mathrm{C}}^{11}$ that the ordering of the first three excited states of ${\mathrm{C}}^{11}$ is the same as in ${\mathrm{B}}^{11}$. The beta-decay of ${\mathrm{Be}}^{11}$ is reconsidered in the light of the increased firmness of the assignments in ${\mathrm{B}}^{11}$ and of the previsions of the IPM; it is concluded that ${\mathrm{Be}}^{11}$ is most probably of even parity.