An experiment with magnesium 24 challenges nuclear astrophysics models

Nuclear physics Scientific results

A collaboration of iThemba LABS - Cape Town, STELLA- IPHC Strasbourg and the University of York has experimentally identified « cluster states » in magnesium nuclei at energies that correspond to thermal conditions in the cores of stars. Their results, that have been published in Physical Review Letters, reveal that the probability of fusion of 12C-12C to 24Mg at stellar temperatures can be much higher than previously thought.

The work of the collaboration is based on theoretical (AMD) models which predict that the 24 nucleons of a magnesium core can form cluster states at excitation energies compatible with stellar conditions, that might play a key role in the synthesis of nuclei in stars. In a first identified state, the magnesium 24 is formed from two symmetric distributions very similar to two carbon 12 nuclei (panel (e) and (f) in the figure below), which represents an intuitive path of formation for carbon 12 fusion into magnesium 24. The second identified state is composed of a neon 20 core and a distribution of 4 valence nucleons (panel (c) and (d) in the figure), and this configuration can have a largely enhanced probability of decaying by emitting an alpha particle.

Simulation des états cluster de magnsium 24
Modeling of different cluster states that magnesium can form at stellar temperatures. The cluster state (e) and (f) with two identical wings represents an intuitive and resonant path of formation of magnesium 24 from two carbon 12 nuclei. The cluster states (c) and (d) can be associated to enhanced disintegration with alpha particle emission. Courtesy: Y. Chiba


In remarkable agreement with theoretical predictions

These models were tested by the collaboration with an experiment carried out at the iThemba LABS, close to Cape Town in South Africa, where an alpha particle beam of 200 MeV was directed onto the nucleons of a magnesium 24 target reaching excitation energies relevant at the temperature conditions at astrophysics sites (massive stars, type 1a supernovae, superbursts in x ray binary systems). The spin, the excitation energy and the decay probability of the cluster states in 24Mg where measured by characterizing in coincidence light charged particles and the beam in 24Mg(α, α')24Mg reactions using the K600 spectrometer and the CAKE setup. The cluster states, that were identified during the experiment, are close to the 12C-12C formation threshold and in remarkable agreement with theoretical predictions of such C-C and α-cluster states in 24Mg. Based on these results, the collaboration could prove that the existence of these cluster configurations can change the reaction rate by up to an order of magnitude, compared to previous extrapolations, with enormous consequences for stellar modelling.

Link to the publication :


Marcella Grasso
IN2P3 deputy scientific director for nuclear physics and applications
Marcel Heine
Chercheur CNRS à l'Institut Pluridisciplinaire Hubert Curien (IPHC)
Sandrine Courtin
Professeure à l'Université de Strasbourg et directrice de l'Institut Pluridisciplinaire Hubert Curien (IPHC)
Emmanuel Jullien
Responsable du service communication de l'IN2P3