Alpha structure of O at high excitation energies by He+C nuclear reactions

This work investigates high energy states in the self-conjugate O nucleus, through the analysis of C(He,α)C reactions in the 1.4-2.2 MeV bombarding energy range. The cross-section of the decay channel in which an α particle is emitted in coincidence with a C residual nucleus in the Hoyle state, leading to four α particles in the final channel, has been measured thanks to a low-threshold and high-resolution array. The occurrence of two low-spin resonant states is highlighted by the analysis of angular distributions, with J = 2 and 3, at respective energies of ≃ 24.1 and 24.5 MeV. In this energy window, the observed branching ratios for the transition involving the emission of the Hoyle state appear to be way larger than calculations based on the barrier penetration alone, pointing out to the possible occurrence of largely clustered high-energy states in O.