Imagine trying to hear a single voice humming in a stadium of screaming fans. That’s how difficult it is to detect the fusion of two carbon atoms at the same low energies, where it happens inside stars, and yet, a team of Chinese researchers has done just that.
They measured a rare reaction, called 12C+12C fusion, which plays a crucial role in the late stages of stellar evolution and is responsible for triggering spectacular cosmic events, such as supernova explosions and X-ray bursts.
“These results represent the most sensitive direct measurement within the Gamow window (the energy range at which nuclear reactions occur in stars) relevant for stellar carbon burning,” the study authors note.
They suggest that their achievement opens a new window into how stars burn and die, and may help scientists understand the origins of heavy elements in the universe.
Observing the rare carbon nuclei fusion
For a long time, physicists have struggled to measure the 12C+12C fusion reaction at the same energies where it actually occurs inside stars, i.e., below three million electron volts (MeV). At these low energies, the repulsive electric force between the two carbon nuclei (known as the Coulomb barrier, which is at 5.8 MeV) makes it extremely unlikely for them to fuse.
Plus, the fusion cross section, which is a measure of the reaction’s probability, becomes extremely small, down to a few trillionths of a trillionth. These factors make direct laboratory detection very hard. Scientists need extremely intense beams of carbon ions and ultra-clean targets, along with detectors sensitive enough to catch such rare fusion events among billions of background signals.
The researchers overcame this hurdle using a powerful machine called the LEAF accelerator. This facility generates high-energy beams of ions, in this case, carbon ions, and directs them with great precision toward a target. The aim was to force two carbon nuclei (12C) that strongly repel each other to fuse.
To improve the odds of detecting this rare fusion, the team used a special type of target material known as highly oriented pyrolytic graphite (HOPG). This is a very pure, crystal-like form of carbon, commonly used in high-tech applications like X-ray instruments or neutron scattering, because it produces little background interference. Its clean structure made it ideal for detecting only the reactions the researchers were interested in, without excess noise.
Once the carbon beam hit the HOPG target, some carbon nuclei managed to overcome their repulsion and fuse. These fusion events released alpha particles, which the study authors detected using a Time Projection Chamber (TPC) and silicon-strip detectors arranged as a special telescope.
While the TPC acted like a 3D tracking camera, capturing the paths of particles created in the reaction, the silicon detectors identified the type of each particle based on how much energy it lost while passing through. This entire setup allowed the team to directly measure the number of alpha particles produced by the 12C-12C fusion reaction at 2.22 MeV, an energy level well within the so-called Gamow window.
A super-sensitive measurement
Apart from the detection, what’s even more interesting is that this fusion reaction occurred for every 100 quadrillion carbon ions fired. That’s a yield of about 10⁻¹⁷ per incident ion, making it the most sensitive direct measurement of this reaction ever achieved.
“This result represents the highest sensitivity achieved to date for the 12C(12C,α0)20Ne channel,” the study authors said.
This level of precision is a big deal. Understanding how and when carbon atoms fuse is key to explaining how stars evolve after their helium runs out, and what triggers massive cosmic events like supernovae. The data can also help refine models of element formation in the universe.
However, the experiment wasn’t without setbacks. After continuous bombardment, the carbon beam damaged the HOPG target, reducing the number of detected alpha particles by about 51 percent and protons by around 25 percent.
This radiation damage altered the surface and reduced the hydrogen content of the target, limiting the duration and accuracy of long experiments. Although the team corrected for this damage in their final results, this problem highlights that the setup needs further improvement to sustain measurements that take up a lot of time.
The study is published in the journal Nuclear Science and Techniques.
