Alpha structure of 16O at high excitation energies by 3He+13C nuclear reactions

This work investigates high energy states in the self-conjugate 16O nucleus, through the analysis of 13C(3He,α)12C 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 12C 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 Jπ = 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 16O.