We measured the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(α,γ) cross section at ${\mathrm{\theta }}_{\gamma }$=90° across the 16.6 and 16.9 MeV resonances in ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ and obtained angular distributions on the peaks of both resonances. The decays of the 16 MeV doublet to the 3.0 MeV $2^{+}$ final state have an (isovector E2)/(isovector M1) mixing ratio of (0.01±0.03), an (isoscalar E2)/(isovector M1) mixing ratio of (0.21±0.04), and an isovector M1 width of (2.80±0.18) eV. Our result for the isovector E2 strength disagrees markedly with previous work. We observe ground-state E2 decays of both members of the doublet. We use our results, plus existing ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ β-decay data, to test the conservation of the vector current and to constrain the second-class induced-tensor form factor. Finally, we compare our results to shell-model calculations.

• Received 26 October 1994

DOI:https://doi.org/10.1103/PhysRevC.51.2778