New results on the Radiative width of the Hoyle State from Pair Conversion and G-Ray Measurements

Tibor Kibedi, Australian National University
Wednesday, Feb 24, 4:10 PM - Virtual Nuclear Science Seminar
Online via Zoom

Abstract:  Stellar formation of carbon occurs when three alpha particles fuse and form the 7.654 MeV 0+ state in 12C, the so-called Hoyle state. The Hoyle state is located above the 3a 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 the knowledge of 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 directly 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, Grad, has been determined in an indirect way, resulting in a ~12.5% uncertainty on the 3a rate. Here we report on two experiments to improve our knowledge on Grad. The Hoyle state was excited with proton bombardment of natural carbon. In the first experiment [1], carried out at the Oslo Cyclotron Laboratory. Using the CACTUS and SiRi arrays, the 3.215 and 4.439 MeV gamma-rays were observed in coincidence with protons. The Grad / G ratio was determined from the ratio of singles proton events to number of proton-g-g triple coincidences. The new value of Grad/G is about 50% larger than the currently adopted value [3]. The second experiment [2], using the ANU Super-e spectrometer [3], the GE0/G ratio was determined from the 7.654 MeV E0 and 4.439 MeV E2 pair conversion ratios. From our measurements the recommended value of GE0/G has increased by 11% and the uncertainty has been reduced to ~5% [3]. The combined effect of the two measurements is a 34% increase in the triple-a reaction rate [1,4]. In this talk details of the experiments and the implications of the new rates will be discussed. [1] T. Kibèdi, et al., Phys. Rev. Lett. 125, 1(2020) 82701 [2] T.K. Eriksen, et al., Phys. Rec. C 102 (2020) 024320 [3] M. Freer, H.O.U. Fynbo, Prog. Part. Nucl. Phys. 78 (2014) 1 [4] * Project supported by the Australian Research Council - DP140102986 and DP170101673