Excited States inB11Observed in theLi7(αγ)Reaction

Abstract

Excited states in ${\mathrm{B}}^{11}$ between 9.5 and 11 MeV were studied by use of the ${\mathrm{Li}}^{7}(\ensuremath{\alpha}, \ensuremath{\gamma}){\mathrm{B}}^{11}$ and ${\mathrm{Li}}^{7}(\ensuremath{\alpha}, {\ensuremath{\alpha}}^{\ensuremath{'}}){\mathrm{Li}}^{7*}$ (478 keV) reactions. The thin-target excitation function of the capture reaction, which was obtained at 90\ifmmode^\circ\else\textdegree\fi{} 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 \ensuremath{\mu}b/sr. A transition to the first excited state was not observed, setting an upper limit for $\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}<0.2$ \ensuremath{\mu}b/sr. By comparing the yield with that of the ${\mathrm{C}}^{13}(p, {\ensuremath{\gamma}}_{0}){\mathrm{N}}^{14}$ reaction, an absolute peak cross section of 22.5 \ensuremath{\mu}b/sr (\ifmmode\pm\else\textpm\fi{}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 ${\frac{3}{2}}^{+}$, ${\mathrm{\textonehalf{}}}^{\ifmmode\pm\else\textpm\fi{}}$ or ${\frac{3}{2}}^{\ifmmode\pm\else\textpm\fi{}}$, and ${\frac{7}{2}}^{+}$, respectively. This is essentially in agreement with the recent work by Cusson on elastic and inelastic $\ensuremath{\alpha}$ scattering. Analysis of the radiative capture cross sections (assuming isotropy) with resonance parameters from the particle reactions gives the following ground-state $\ensuremath{\gamma}$ widths for the established states: 0.5 eV (at 9.88 MeV); 17 eV (10.26 MeV); 1 eV (10.32 MeV); \ensuremath{\le}0.2 eV (10.61 MeV). There is evidence for a new state at 10.45\ifmmode\pm\else\textpm\fi{}0.05 MeV with $\ensuremath{\Gamma}(\mathrm{c}.\mathrm{m}.)\ensuremath{\simeq}140$ keV and $(2J+1){\ensuremath{\Gamma}}_{\ensuremath{\gamma}}=10$ eV. Possible analog states in ${\mathrm{C}}^{11}$ have been reinvestigated with the ${\mathrm{B}}^{10}(p, {\ensuremath{\gamma}}_{0}){\mathrm{C}}^{11}$ reaction. The $\ensuremath{\gamma}$ transition strengths observed in ${\mathrm{B}}^{11}$ do not agree with present shell-model calculations.