A new process for the synthesis of rare nuclei in the universe

A new process for the synthesis of rare nuclei in the universe
by Robert Schreiber
Berlin, Germany (SPX) May 14, 2024

A new nucleosynthesis process, called the r-Process, has been proposed by scientists from GSI, and the Max Planck Institute for Astrophysics. This r-Process occurs when neutron-rich material is exposed to a high flux of neutrinos. The theoretical proposal, recently published in "Physical Review Letters," may solve the long-standing issue of the production of certain rare isotopes found in the solar system, known as p-nuclei.

Fusion processes in massive stars produce nuclei up to iron and nickel. Beyond these elements, most heavy stable nuclei, such as lead and gold, are produced via slow or rapid neutron capture processes. However, explaining the large abundances of 92,94Mo, 96,98Ru, and 92Nb in the early solar system has been challenging.

The r-Process allows for the simultaneous production of these nuclei through a series of capture reactions catalyzed by neutrinos. Initially, in astrophysical explosions, neutron-rich outflows consist of neutrons and nuclei around iron and nickel. As the temperature decreases, heavier nuclei form from lighter ones through neutron captures and weak interaction processes. Unlike the rapid neutron capture process, the r-Process involves neutrino absorption reactions instead of beta-decays.

When free neutrons are depleted, further neutrino absorption reactions convert bound neutrons into protons, pushing the produced nuclei beyond the beta-stability line. The high-energy neutrinos excite nuclei, causing the emission of neutrons, protons, and alpha particles, which are then captured by heavy nuclei, leading to a series of capture reactions that determine the final element abundances produced by the r-Process.

"Our finding opens a new possibility to explain the origin of p-nuclei via neutrino absorption reactions on nuclei," says Zewei Xiong, scientist at GSI/FAIR Nuclear Astrophysics and Structure Department and the corresponding author of the publication.

While the reactions driving the r-Process have been identified, the type of stellar explosion where it occurs is still unknown. The authors suggested that the r-Process operates in material ejected in environments with strong magnetic fields, such as magneto-rotational supernovae, collapsars, or magnetars. This suggestion has prompted astrophysicists to search for suitable conditions, with a first publication reporting that magnetically driven ejecta meet the necessary conditions.

The r-Process requires understanding neutrino reactions and neutron-capture reactions on nuclei on both sides of the beta-stability line. These reactions can be measured using the unique storage ring capabilities at the GSI/FAIR facility.

Research Report:Production of p Nuclei from r-Process Seeds: The r-Process