The triple-alpha process is a fundamental reaction in stellar nucleosynthesis, playing a crucial role in the production of 12C. This process involves the fusion of three helium nuclei to form a short-lived intermediate state, which can either decay back into its constituent alpha particles or undergo radiative decay to form stable 12C. At temperatures between 0.1 and 2 GK, the triple-alpha reaction is predominantly mediated by the Hoyle state of 12C, making an understanding of its properties essential for modelling subsequent stellar evolution.
The synthesis of stable carbon occurs primarily through two decay pathways, resulting in 12C in its ground state. The radiative decay of the Hoyle state to stable 12C is mainly facilitated by gamma decay and pair production. The radiative width of the gamma decay branch has been measured multiple times between 1961 and 1976, with most of those measurements showing consistent results. However, a recent study published in 2020 by Kibédi et al. [Phys. Rev. Lett. \textbf{125}, 182701 (2020)] reported a significantly larger radiative branching ratio, raising questions and prompting further investigation from the scientific community.
Given the astrophysical significance of the Hoyle state, resolving this discrepancy is imperative. New measurements have been conducted to reassess the gamma-decay branching ratio of the Hoyle state, including a comprehensive reanalysis of the data from Kibédi et al. These experiments were carried out at the Oslo Cyclotron Laboratory using the 12C(p,p')-reaction. In this presentation, I will share the results from the new measurements and the reanalysis of Kibédi et al.'s data, along with an overview of the research conducted at the Oslo Cyclotron Laboratory and the nuclear physics experiments typically performed there.
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Wanja Paulsen
PhD Research Fellow
Department of Physics
University of Oslo
P.O.Box 1048 Blindern
N-0316 Oslo, Norway
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