Abstract: Stellar formation of carbon occurs when three alpha particles fuse and form the excited 7.654-MeV 0+ Hoyle state in ¹²C. Stable carbon is created only if the excited nucleus decays to the ground state. The Hoyle state is located above the 3α threshold, which makes the triple alpha process very unlikely as the excited carbon nucleus decays back to three alpha particles ~99.96% of the time. The remaining 0.04% will lead the formation of stable carbon. The process is therefore a bottleneck in nuclear astrophysics, and good knowledge about the production rate is imperative for accurate modelling of carbon formation in the universe. The internal decay of the Hoyle state occurs either by a 7.654-MeV E0 transition to the 0⁺ ground state, or by a 3.215-MeV E2 transition to the first-excited 2⁺ state. The current value of the radiative width, Γ_rad, has been determined in an indirect way, resulting in a ~12.5% uncertainty on the 3α rate.

Here we report on two experiments to improve our knowledge on Γrad. The Hoyle state was excited with proton bombardment of natural carbon. In the first experiment, carried out at the Oslo Cyclotron Laboratory, using the CACTUS and SiRi arrays, the cascading gamma-rays of E2 multipolarity and 3.215-MeV and 4.439-MeV energy were observed in coincidence with protons. The Γ_rad / Γ ratio was determined from the ratio of singles proton events to number of proton-gamma-gamma triple coincidences. The new value of Γ_rad/Γ is about 50% larger than the currently adopted value. The second experiment, using the ANU Super-e spectrometer, the Γ_E0/Γ ratio was determined from the 7.654-MeV E0 and 4.439-MeV E2 pair conversion ratios. From our measurements the recommended value of Γ_E0/Γ has increased by 11% and the uncertainty has been reduced to ~5%. The combined effect of the two measurements is a 34% increase in the triple-α reaction rate.

In this talk, details of the experiments and the implications of the new rates will be discussed.