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 12 C nucleus ( E x = 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 Γ rad / Γ tot = 4.2 ( 6 ) · 10 - 4 . This finding helps to resolve the tension between recent data published in the literature.