$\gamma$ transitions to the pair of $J^{\pi }=2^{+}$ states at 16.64 and 16.90 MeV in ${\mathrm{Be}}^8$ have been observed in the reaction ${\mathrm{Li}}^7(p,\gamma ){\mathrm{Be}}^{8^{*}}\to 2\alpha$. The transitions to both states are resonant at the 17.64- and 18.15-MeV states, which have $J^{\pi }=1^{+}$. The following resonant cross sections and branching ratios have been obtained: $\sigma (17.6\mathrm{}\to 16.6)=11\pm 2$ μb; $R\left(\frac{17.6\to 16.90}{17.6\to 16.6}\right)=(7\mathrm{}\pm 2)\%$; $\sigma (18.2\mathrm{}\to 16.6)=1.30\pm 0.32$ μb; $R\left(\frac{18.2\to 16.90}{18.2\to 16.6}\right)=(50\mathrm{}\pm 10)\%$. From these transitions and the reported strengths for the transitions from the $J^{\pi }=1^{+}$ states to the ground and first excited states, the amount of isospin mixing in the four highly excited states has been determined using intermediate-coupling shell-model wave functions. The squared $T=1\mathrm{}$ components amount to 40% and 60% in the 16.6- and 16.9-MeV states, and 95% and 5% in the 17.6- and 18.2-MeV states, respectively. Using these mixing coefficients and the assumption of pure $T=0\mathrm{}$ character for the ground and first excited states, a shell-model calculation accounts approximately for the strengths of most of the observed or reported magnetic dipole transitions from the 17.64- and 18.15-MeV states. Inclusion of a possible $J^{+}=1^{+}$, $T=1\mathrm{}$ level at 19.4 MeV improves the agreement. A large nonresonant transition to the 16.6-MeV state with $\sigma \simeq 2$ μb can be ascribed to direct radiative proton capture which yields a reduced proton width of ${{\theta }_p}^2=0.7\mathrm{}$ for the final state. This transition, and the fact that the equivalent transition to the 16.9-MeV state is not observed, substantiate the predominance of a (${\mathrm{Li}}^7+p$) configuration for the 16.6-MeV state and agree with an assumed (${\mathrm{Be}}^7+n$) configuration for the 16.9-MeV level.

• Received 27 December 1967

DOI:https://doi.org/10.1103/PhysRev.173.919