Excited states in ${\mathrm{B}}^{11}$ between 9.5 and 11 MeV were studied by use of the ${\mathrm{Li}}^7(\alpha , \gamma ){\mathrm{B}}^{11}$ and ${\mathrm{Li}}^7(\alpha , {\alpha }^{\prime }){\mathrm{Li}}^{7*}$ (478 keV) reactions. The thin-target excitation function of the capture reaction, which was obtained at 90° for the ground-state transition from 1.3- to 3.2-MeV bombarding energy, displays a structured peak around 2.5 MeV with a peak differential cross section of 2 μb/sr. A transition to the first excited state was not observed, setting an upper limit for $\frac{d\sigma }{d\Omega }<0.2\mathrm{}$ μb/sr. By comparing the yield with that of the ${\mathrm{C}}^{13}(p, {\gamma }_0){\mathrm{N}}^{14}$ reaction, an absolute peak cross section of 22.5 μb/sr (±20%) was obtained for the 951-keV resonance. Reinvestigation of the inelastic scattering cross section over the same bombarding energy range, coupled with a Breit-Wigner analysis, yielded for the states at 9.87, 10.26, and 10.62 MeV the assignments for spin and parity of ${\frac{3}{2}}^{+}$, ${\mathrm{½}}^{\pm }$ or ${\frac{3}{2}}^{\pm }$, and ${\frac{7}{2}}^{+}$, respectively. This is essentially in agreement with the recent work by Cusson on elastic and inelastic $\alpha$ scattering. Analysis of the radiative capture cross sections (assuming isotropy) with resonance parameters from the particle reactions gives the following ground-state $\gamma$ widths for the established states: <0.5 eV (at 9.88 MeV); 17 eV (10.26 MeV); 1 eV (10.32 MeV); ≤0.2 eV (10.61 MeV). There is evidence for a new state at 10.45±0.05 MeV with $\Gamma (\mathrm{c}.\mathrm{m}.\mathrm{})\simeq 140$ keV and $\mathrm{}(2J+1)\mathrm{}{\Gamma }_{\gamma }=10\mathrm{}$ eV. Possible analog states in ${\mathrm{C}}^{11}$ have been reinvestigated with the ${\mathrm{B}}^{10}(p, {\gamma }_0){\mathrm{C}}^{11}$ reaction. The $\gamma$ transition strengths observed in ${\mathrm{B}}^{11}$ do not agree with present shell-model calculations.

• Received 4 August 1967

DOI:https://doi.org/10.1103/PhysRev.164.1332