Gamma-Ray Transitions Involving Isobaric-Spin Mixed States inBe8

The cross sections for the reactions ${\mathrm{Li}}^7(p,\gamma ){\mathrm{Be}}^{8*} \left(16.63\mathrm{} \mathrm{and} 16.90 \mathrm{MeV}\right)\to 2\alpha$ have been measured over the range of bombarding energies 0.441-2.45 MeV. The nonresonant portion of the cross section for the population of the 16.63-MeV level has been compared successfully with cross sections calculated on the assumption of an extranuclear direct-capture process. Resonant cross sections populating the 16.63- and 16.90-MeV levels of ${\mathrm{Be}}^8$ were measured at incident energies of 0.441, 1.03, and 1.89 MeV, corresponding to levels at 17.64, 18.15, and 18.9 MeV in ${\mathrm{Be}}^8$. Partial widths and reduced matrix elements were calculated for the resonant contributions to the cross sections. The experimental results are consistent with the hypothesis of nearly maximal isobaric-spin mixing in the $J^{\pi }=2^{+}$ levels of ${\mathrm{Be}}^8$ (16.63 and 16.90 MeV). There is no evidence for transitions from the $J^{\pi }=3^{+}$ levels near 19 MeV to either of the 16-MeV states, but decay to these states was observed from a level (probably $J^{\pi }=2^{-}$) at 18.9 MeV.