Studies of Electromagnetic Transitions in B 11 and C 11

Electromagnetic transitions in ${\mathrm{B}}^{11}$ and ${\mathrm{C}}^{11}$ have been investigated through the reactions induced (a) by ${\mathrm{He}}^{3}$ bombardment of ${\mathrm{Be}}^{9}$ targets and (b) by deuteron bombardment of ${\mathrm{B}}^{10}$ targets employing bombarding energies ranging from 2.0 to 3.5 MeV. Branching ratios for the decay of the bound levels of ${\mathrm{B}}^{11}$ populated in (a) and (b) by the ${\mathrm{Be}}^{9}({\mathrm{He}}^{3}, p){\mathrm{B}}^{11*}$ and ${\mathrm{B}}^{10}(d, p){\mathrm{B}}^{11*}$ reactions were determined through measurements of proton-gamma coincidence spectra. The ratios of radiative width to total width for the unbound 9.19- and 8.92-MeV levels of ${\mathrm{B}}^{11}$ were obtained as $\frac{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}}{\ensuremath{\Gamma}(9.19)}={{0.1}_{\ensuremath{-}0.05}}^{+0.2}$ and $\frac{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}}{\ensuremath{\Gamma}(8.92)}=1.08\ifmmode\pm\else\textpm\fi{}0.12$. Additional measurements of the direct gamma spectra employing a three-crystal Nal(Tl) pair spectrometer, and of the internal pair transitions using an intermediate-image magnetic spectrometer, complement the above results and provide information on the decay of those bound levels of ${\mathrm{C}}^{11}$ populated by the ${\mathrm{Be}}^{9}({\mathrm{He}}^{3}, n){\mathrm{C}}^{11*}$ and ${\mathrm{B}}^{10}(d, n){\mathrm{C}}^{11*}$ reactions. The angular distributions of gamma rays from the four listed reactions were measured with the three-crystal pair spectrometer, and indicate significant anisotropies for those gamma rays arising from decay of the 8.92- and 7.99-MeV levels of ${\mathrm{B}}^{11}$ and the 7.50-MeV level of ${\mathrm{C}}^{11}$ formed in the ${\mathrm{Be}}^{9}$+${\mathrm{He}}^{3}$ bombardment. From the Doppler shifts apparent in the angular-distribution data an upper limit of $\ensuremath{\tau}l5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ sec was extracted for the mean lifetime of all bound levels of ${\mathrm{B}}^{11}$ and ${\mathrm{C}}^{11}$ except the 6.81-MeV level of ${\mathrm{B}}^{11}$, which could not be studied in this manner. The intermediate-image spectrometer was used to determine the multipolarity of those internal pair transitions in ${\mathrm{B}}^{11}$ and ${\mathrm{C}}^{11}$ having transition energies greater than 4 MeV. These results show that the 7.99- and 7.30-MeV levels of ${\mathrm{B}}^{11}$ and the 7.50-, 6.90-, and 6.35-MeV levels of ${\mathrm{C}}^{11}$ decay by $E1$ ground-state transitions and hence have even parity, while the 8.92- and 6.76-MeV levels of ${\mathrm{B}}^{11}$ and 6.49-MeV level of ${\mathrm{C}}^{11}$ decay by $M1$ and/or $E2$ ground-state transitions, and hence have odd parity. Combining these results with previously available information leads to spin-parity assignments of ${\frac{5}{2}}^{\ensuremath{-}}$ and ${\frac{3}{2}}^{+}$ for the 8.92- and 7.99-MeV levels of ${\mathrm{B}}^{11}$, respectively, and strongly suggests the assignment ${\frac{3}{2}}^{+}$ for the 7.50-MeV level of ${\mathrm{C}}^{11}$. The odd-parity assignment to the 2.14-MeV level of ${\mathrm{B}}^{11}$ is confirmed while the parity of the ${\mathrm{C}}^{11}$ 2.00-MeV level is fixed as odd. For the remaining levels the measurements serve to further restrict the range of possible spin-parity assignments. In addition the parities of the ${\mathrm{Be}}^{11}$ ground state and ${\mathrm{B}}^{11}$ 6.81-MeV level are determined to be even since the beta decay of ${\mathrm{Be}}^{11}$ to the ${\mathrm{B}}^{11}$ ${\frac{3}{2}}^{+}$ 7.99-MeV level and the 6.81-MeV level are known to be allowed. The level schemes of ${\mathrm{B}}^{11}$ and ${\mathrm{C}}^{11}$ as deduced from the present results are discussed in terms of the predictions of the intermediate-coupling shell model and the weak-coupling scheme.