Evidence for GiantE2Transitions in High-Energyα-Particle Capture

High-energy $\gamma$ rays from the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($\alpha$,$\gamma$) have been observed decaying from states between 32 and 39 MeV in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. A narrow unexpectedly strong line, corresponding to decay to a state at 14.8±0.1 MeV in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, was observed with an angular distribution suggesting a strong $E2\mathrm{}$ component. Structure in the excitation function of this transition favors a resonant rather than direct-capture mechanism. Comparisons with SU(3)- and shell-model calculations suggest a giant quadrupole excitation built on the $6_1^{+}$ state in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$.