Direct observation of 4+ to 2+ gamma transition in 8Be

The low lying states in ${}^8$Be are believed to have a two-alpha cluster structure and hence a large intrinsic quadrupole deformation. An earlier calculation showed a large collective enhancement in gamma transition probability between the low lying states leading to a 4${}^{+}$ to 2${}^{+}$ gamma branch of $\sim {10}^{-7}$ and a resonant radiative cross section of 134 nb for the $\alpha +\alpha$ entrance channel. We report here the first experimental evidence for this transition through a $\gamma -\alpha -\alpha$ coincidence measurement in the reaction ${}^4$He($\alpha ,\alpha \gamma$)${}^4$He using a gas target. The measured cross sections on and off the 4${}^{+}$ resonance are 165 $\pm$ 41 (stat) $\pm$35 (sys) nb and 39 $\pm$ 25 (stat) $\pm$7 (sys) nb, respectively.