States inB12Observed in the Scattering of Neutrons byB11

The polarization $P\left(\theta \right)$ and differential cross section $\sigma \left(\theta \right)$ for the scattering of neutrons by ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ have been measured for $0.075<~E_n<~2.2$ MeV. Both $\sigma \left(\theta \right)$ and $P\left(\theta \right)$ are simultaneously fitted reasonably well by $R$-matrix parameters for broad states in ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ with assignments $2^{-}$ ($l=0\mathrm{}$) and $4^{-}$ ($l=2\mathrm{}$) at excitation energies $E_x=4.37\mathrm{} \mathrm{and} 4.54$ MeV, respectively. The $2^{-}$ level has not previously been observed, while the $4^{-}$ level has previously been assigned to be $3^{-}$. These results, together with a previous $3^{-}$ assignment for a state at $E_x=3.39\mathrm{}$ MeV, give experimental evidence for proton-hole-neutron-particle shell-model configurations ${\left(1\mathrm{}{p}_{\frac{3}{2}}\right)}^{-1\mathrm{}}\left(1\mathrm{}{d}_{\frac{5}{2}}\right)$ and ${\left(1\mathrm{}{p}_{\frac{3}{2}}\right)}^{-1\mathrm{}}\left(2\mathrm{}{s}_{\frac{1}{2}}\right)$ in ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$. Shell-model calculations were made with $\delta$-function residual interactions and include configuration mixing of the resulting $2^{-}$ states. Work on $T=1\mathrm{}$ states in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}_{}^{*}{}_{}{}^{}$ has been compared with the present work.