A research team from the Institute of Modern Physics and Sichuan University has performed a direct measurement of the 12C+12C fusion reaction at a center-of-mass energy of 2.22 MeV using the LEAF accelerator facility. The experiment employed a highly intense 12C2+ beam, a highly oriented pyrolytic graphite (HOPG) target known for its low background, and a ΔE–E telescope combining a Time Projection Chamber and silicon detectors. This setup enabled detection of extremely rare fusion events, with a thick-target yield on the order of 10−17 per incident carbon ion in the 12C(12C,α0)20Ne channel. Under continuous irradiation with a total beam dose of 5 coulombs, the HOPG target suffered approximately 51% reduction in alpha particle yield due to radiation damage. These results represent the most sensitive direct measurement within the Gamow window relevant for stellar carbon burning.
A key step in stellar evolution
The fusion of carbon nuclei (12C+12C) is a fundamental reaction that occurs in the late stages of stellar evolution and in explosive phenomena like Type Ia supernovae and X-ray bursts. However, in stars this reaction proceeds at center-of-mass energies well below 3 MeV, far under the 5.8 MeV Coulomb barrier, where the fusion cross section is extremely small. Direct laboratory measurements at these low energies are therefore very challenging, requiring very intense beams and ultra-pure targets.
Fusion products identified by TPC+silicon telescope system
In the detector, outgoing charged particles from the fusion reaction were tracked by a Time Projection Chamber (TPC) and identified in a silicon-strip array. This ΔE–E telescope design allowed the team to distinguish fusion alphas and protons from any remaining background. With this setup, the experiment directly measured the thick-target yield of the 12C(12C, α0)20Ne (ground-state alpha) channel at Ec.m.=2.22 MeV. Given the extremely small reaction yield, the detected count of alpha particles corresponded to a yield on the order of 10−17 per incident 12C (after correcting for losses) – far smaller than any previous direct measurement. In fact, the authors note that this result "represents the highest sensitivity achieved to date" for the 12C(12C,α0)20Ne channel. The inferred cross section at this energy is on the order of picobarns or less, consistent with theoretical expectations in this deeply sub-barrier regime.
Radiation damage reduces fusion yields
A key finding of the study is the observation of radiation damage in the HOPG target caused by the intense carbon beam. As more beam was delivered, the target surface was progressively altered, reducing its hydrogen content but also degrading fusion yields. The team measured that after accumulating about 5 coulombs of charge, the detected alpha yield fell by roughly 51% and the proton yield by about 25%, compared to initial values. In other words, prolonged irradiation significantly damaged the target surface and suppressed the reaction yield. The researchers applied corrections for this effect when reporting their final yields. In the words of the experimenters, "we find that the yield of α and proton are reduced significantly under intense beam bombardment due to radiation damage of the HOPG." This underlines the practical difficulty of sustaining long runs at high current in such measurements.
The complete study is accessible via DOI: 10.1007/s41365-025-01714-3
The research is published by Nuclear Science and Techniques. Nuclear Science and Techniques (NST) is a peer-reviewed international journal sponsored by the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. The journal publishes high-quality research across a broad range of nuclear science disciplines, including nuclear physics, nuclear energy, accelerator physics, and nuclear electronics. Its Editor-in-Chief is the renowned physicist, Professor Yu-Gang Ma.
