Speaker
Description
The triple-alpha process is one of the most fundamental processes in stellar nucleosynthesis, entailing the fusion of three helium nuclei to form $^{12}$C. At temperatures between 0.1 - 2 GK, the triple-alpha reaction is almost exclusively mediated by the Hoyle state in $^{12}$C [1]. Accurate measurements of the radiative width of the Hoyle state is important for understanding the production of $^{12}$C, but also for understanding subsequent stellar reactions such as the $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction [2].
The radiative branching ratio of the Hoyle state was measured several times between the period 1961 to 1976, yielding the presently accepted value of $\Gamma_{rad}/\Gamma=4.16(11)\times 10^{-4}$ [3]. However, a measurement published in 2020 by Kib{\'e}di \textit{et al.} [4] resulted in a significantly larger radiative branching ratio of $\Gamma_{rad}/\Gamma=6.2(6)\times 10^{-4}$. Several measurements of the radiative width of the Hoyle state have been published as a direct result of this discrepancy recently [5-8].
Given the astrophysical significance of the Hoyle state, resolving this conflict is crucial. Therefore, new measurements have been performed to reinvestigate the $\gamma$-decay branching ratio of the Hoyle state, with a complete reanalysis of the data published by Kib{\'e}di \textit{et al.} [4]. The experiments have been performed at the Oslo Cyclotron Laboratory through the $^{12}$C(p, p'$\gamma \gamma$)-reaction. In these experiments, the SiRi particle telescope [9] was employed to detect proton ejectiles and the OSCAR [10] LaBr3 array was used to detect the coincident $\gamma$-ray decays. Results from the new measurement and the reanalysis of the data published by Kib{\'e}di \textit{et al.} [4] will be presented.
In addition, I will outline plans for our new FRIPRO project AMiCARE, which will perform precision measurements of the $^{12}$C+$^{12}$C and $^{12}$C+$^{16}$O fusion reactions at the CIRCE laboratory in Italy, in collaboration with the University of Naples Federico II. The $^{12}$C+$^{12}$C campaign will extend the work of Morales-Gallegos \textit{et al.} [11] through several upgrades to the GASTLY detector system [12].
References
[1] M. Freer and H. Fynbo, The Hoyle state in 12 C, Prog. Part. Nucl. Phys. 78, 1 (2014).
[2] R. J. de Boer, A. Best, C. R. Brune, A. Chieffi, C. Hebborn, G. Imbriani, W. P. Liu, Y. P. Shen, F. X. Timmes, M. Wiescher, The 12C(α, γ )16O reaction, in the laboratory and in the stars, Eur. Phys. J. A 61, 70 (2025).
[3] J. Kelley, J. Purcell, and C. Sheu, Energy levels of light nuclei A=12, Nucl. Phys. A 968, 71 (2017).
[4] T. Kibédi, B. Alshahrani et al., Radiative Width of the Hoyle State from $\gamma$-ray spectroscopy, Phys. Rev. Lett, 125, (Oct 2020) 182701-182707.
[5] M. Tsumura et al., First experimental determination of the radiative-decay probability of the $3_{1}^{-}$ state in C-12 for estimating the triple alpha reaction rate in high temperature environments, Phys. Rev. B, 817, (2021) 136283.
[6] Z. Luo et al., Radiative branching ratio of the Hoyle state, Phys. Rev. C, 109, (2024) 025801.
[7] D. Dell’Aquila et al., Clarifying the radiative decay of the Hoyle state with charged-particle spectroscopy, Scientific Reports, 14, (2024) 18958.
[8] T. Rana et al., New measurement of the Hoyle state radiative transition width, Phys. Lett. B., 859, (2024) 139083.
[9] M. Guttormsen et al., The SiRi particle-telescope system, Nucl.Instrum.Meth., 648(1), (2011) 168-173.
[10] F. Zeiser et al., The $\gamma$-ray response of the Oslo scintillator array OSCAR, Nucl.Instrum.Meth., 985, (2021) 164678.
[11] L. Morales-Gallegos et al., Direct measurements of the $^{12}$C-$^{12}$C reactions cross-sections towards astrophysical energies, Eur. Phys. J. A., (2024) 60:11
[12] M. Romoli et al.*, Development of a two-stage detection array for low-energy light charged particles in nuclear astrophysics applications, Eur. Phys. J. A. (2018) 54:112