Clarifying the radiative decay of the Hoyle state with charged-particle spectroscopy

A detailed knowledge of the decay properties of the so called Hoyle state in the 12C nucleus (Ex=7.654 MeV, 0+) is required to calculate the rate at which carbon is forged in typical red-giant stars. This paper reports on a new almost background-free measurement of the radiative decay branching ratio of the Hoyle state using advanced charged particle coincidence techniques. The exploitation, for the first time in a similar experiment, of a bidimensional map of the coincidence efficiency allows to reach an unitary value and, consequently, to strongly reduce sources of systematic uncertainties. The present results suggest a value of the radiative branching ratio of Γrad/Γtot=4.4(6)·10-4. This finding helps to resolve the tension between recent data published in the literature.