Nippert, J; Courtin, S; Heine, M; Jenkins, D G; Adsley, P; Bonhomme, A; Canavan, R; Curien, D; Dumont, T; Gregor, E; Harmant, G; Monpribat, E; Morrison, L; Moukaddam, M; Richer, M; Rudigier, M; Romero, J G V; Catford, W N; Cotte, P; Della Negra, S; Haefner, G; Hammache, F; Lesrel, J; Pascu, S; Podolyák, Zs; Regan, P H; Ribaud, I; de Séréville, N; Stodel, C; Vesić, J (2025) Refining the deep sub-barrier 12C + 12C fusion excitation function with the STELLA apparatus. Physical Review C, 111 (6). 065804 ISSN 2469-9985

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Abstract

Background: The fusion excitation function of 12C+12C contains many resonances from above the Coulomb barrier towards sub-barrier energies. These oscillations might be linked to molecular states in the compound nucleus 24Mg with drastic consequences for stellar carbon burning. Moreover, these structures render the common extrapolations from measurements into the astrophysics region of interest at deep sub-barrier energy extremely uncertain.

Purpose: We have investigated the cross section around the lowest direct coincident gamma-particle measurements, where previously only limits could be established with the aim of obtaining a detailed description of the excitation function. We have furthermore analysed the ratio of extreme decay branching into the first excited state of daughter nuclei with alpha or proton emission at relative energies where previous measurements are in disagreement with each other.

Methods: The experiments were carried out at the Androm`ede accelerator, Orsay (France), with carbon beam of up to 5 pμA on thin rotating carbon target foils. Light charged particles and gamma decays were detected using silicon strip detectors and LaBr3(Ce) crystals, respectively, in coincidence measurements with nanoseconds precision.

Results: We establish data points with highly improved accuracy at the high energy tail of the lowest resonance detected so far in direct measurements, right in the astrophysics region of interest (RoI). The findings are in agreement with an earlier interpretation of the fusion excitation function composed of resonances on top of a global trend of empirically determined fusion hindrance behaviour. At a relative energy between 3.1 MeV and 3.3 MeV, an oscillating behaviour of the branching ratio into first excited final state with alpha or proton emission is observed, with extreme values significantly deviating from data at higher energy.

Conclusions: Our findings in the astrophysics RoI support reaction-rate models with a lower average S-factor trend, that deviate significantly from standard extrapolations between 2.2 MeV and 2.6 MeV, for stellar carbon burning simulations of up to 25 M⊙ stars. Based on our data, an overall increase of the S-factor at deep subbarrier energy cannot be confirmed. The extremely low branching into the first excited state wit proton emission of 3% at 3.3 MeV might indicate the presence of alpha cluster compound states in 24Mg. This highly favours α emission with fundamental consequences in possible stellar carbon burning sites.

Item Type:	Article
Keywords:	nuclear astrophysics ; nucleosynthesis ; LaBr3 detectors.
Subjects:	Ionising Radiation > Radioactivity
Divisions:	Medical, Marine & Nuclear
Identification number/DOI:	10.1103/PhysRevC.111.065804
Last Modified:	16 Jun 2026 13:38
URI:	https://eprintspublications.npl.co.uk/id/eprint/10457