Radiative Width of the Hoyle State from γ -Ray Spectroscopy

The cascading 3.21 and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in C12, excited by the C12(p,p′) reaction at 10.7 MeV proton energy. From the proton-γ−γ triple coincidence data, a value of Γrad/Γ=6.2(6)×10−4 was obtained for the radiative branching ratio. Using our results, together with ΓπE0/Γ from Eriksen et al. [Phys. Rev. C 102, 024320 (2020)] and the currently adopted Γπ(E0) values, the radiative width of the Hoyle state is determined as Γrad=5.1(6)×10−3 eV. This value is about 34% higher than the currently adopted value and will impact models of stellar evolution and nucleosynthesis.