12C+12C
PI: Federico Ferraro
Astrophysical Motivation
The 12C+12C reaction is fundamental to several critical astrophysical processes. In stars with masses greater than 8 solar masses, carbon burning occurs after the exhaustion of hydrogen and helium. This reaction plays a key role in shaping the star’s core structure and energy production, determining its evolutionary path toward later stages like supernovae. The 12C+12C reaction is critical in the ignition of Type Ia supernovae, particularly in carbon-oxygen white dwarfs, where it triggers a thermonuclear runaway that results in the explosion of the white dwarf. In Type II supernovae, carbon fusion also plays a role in the advanced stages of carbon burning in massive stars, contributing to the nucleosynthesis that leads to the star's collapse and explosive death. In neutron star accretion disks, superbursts (intense, rapid outbursts) are thought to be ignited by carbon fusion. Understanding this reaction is key to understanding the physics behind these extreme events, including the ignition depth. Moreover, at very high temperatures the products of 12C+12C reaction can be responsible of a high-temperature revival of hydrogen and helium burning. This process is important in the advanced stages of stellar evolution and can influence the production of heavier elements.
However, the cross section of the 12C+12C is very difficult to measure and the thermonuclear reaction rate is still very uncertain.
Accurate, direct measurements of the 12C+12C cross-section, especially at low energies, are crucial for resolving the long-standing uncertainties surrounding this reaction. These measurements are key to refining our models of stellar evolution, supernovae, and element formation in the universe, helping us better understand how stars live, die, and produce the elements that make up everything around us.
Experiment
LUNA is conducting a direct measurement of this reaction deep underground at the Bellotti Ion Beam Facility. This study focuses on the detection of photons emitted in the de-excitation of Ne and Na populated via the two key reaction channels: 12C(12C,α)20Ne and 12C(12C,p)23Na. The experimental setup includes a massive lead and copper shielding (about 13 tons in weight) provided with a sliding door, a very sensitive High Purity Germanium (HPGe) detector, a retractable beamline and a syntherized graphite target placed on a water-cooled aluminum target holder produced via additive manufacturing. The rock of the Gran Sasso massif significantly reduces cosmic-ray-induced muons, which results in a clean high-energy region in the gamma spectra. At the same time, the lead and copper shielding effectively suppress the low-energy background caused by natural radioactivity from the rock and the materials surrounding the detector, ensuring a clearer signal for the experiment.
Measurements are already underway, but in a few months, a new component will be installed to achieve even higher sensitivity. A segmented 16-fold NaI detector with large angular coverage will be placed around the target and the HPGe detector. It will be used both as an active anti-Compton shield and as a total absorption calorimeter.
The very special setup and the unique features of the Bellotti IBF position the LUNA experiment to make the first-ever direct measurement of the C + C reaction in the crucial low-energy regime below 2.2 MeV, with a potential to provide key insights into the carbon fusion process at stellar energy.