Nuclear astrophysics delves into the universe’s element evolution since its inception, relying on parameters derived from lab experiments. Within stars, nuclear reactions are pivotal. Researchers from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), along with colleagues from Italy, Hungary, and Scotland, have reexamined a crucial reaction at the Dresden Felsenkeller accelerator, yielding a surprising outcome, detailed in the journal Physical Review C.
They focused on a well-known nuclear reaction, fundamental in crafting elements within massive stars: the collision of a carbon-12 nucleus and a hydrogen nucleus, yielding nitrogen-13 and gamma radiation, initiating the CNO cycle. Their interest lay in the reaction cross section, revealing its likelihood. Professor Daniel Bemmerer of HZDR-Institute of Radiation Physics explains that the previously accepted value needed a correction, lowering it by approximately 25%. This result implies a lengthier burn-in phase for the CNO cycle and a shift in the emission of 13N neutrinos closer to the sun’s center. Furthermore, this data enhances theoretical predictions for the carbon isotope ratio, aiding in refining models of stellar interior processes.
A miniature sun in the laboratory
Stars derive their energy from the fusion of hydrogen into helium, with the specific process varying based on a star’s mass. In low-mass stars like our sun, the dominant process is the proton-proton chain. In more massive stars, the higher temperatures created by gravitational pressure allow for the fusion of hydrogen and carbon nuclei.
Interestingly, even though interstellar matter contains less than 2% carbon, this amount is adequate to initiate and sustain the CNO cycle. Carbon serves as a catalyst, accelerating the fusion process without being consumed. Ultimately, the net result is still the conversion of hydrogen into helium, much like the proton-proton chain, but in massive stars, the CNO cycle operates at a significantly faster pace.
To conduct these experiments, tantalum disks coated with a layer of carbon are used as targets. Protons from a 5 MV Pelletron accelerator strike these targets, generating gamma-rays that are detected by 20 high-purity germanium detectors, as explained by Bemmerer.
The Felsenkeller Underground Laboratory, jointly operated by HZDR and TU Dresden, is an ideal location for such measurements. Situated in a tunnel previously utilized for storing ice for the Dresden Felsenkeller brewery, it offers a protective shield of 45 meters of rock, safeguarding the detectors from cosmic radiation, which could disrupt the sensitive measurements. This work exemplifies European collaboration within the astrophysics community, with a University of Padua Ph.D. student conducting research at the Felsenkeller for six months during the experiment.