Researchers develop novel technique for constraining nuclear reactions in stellar explosions

07 June 2023

In a recent Physical Review Letters paper, researchers from Oak Ridge National Laboratory, the University of Tennessee, the University of Santiago de Compostela (Spain), the Johannes Gutenberg-University Mainz (Germany), Argonne National Laboratory, and FRIB explain their new technique for constraining nuclear reactions in stellar explosions.

Core-collapse supernovae are cataclysmic events, in which stars that are more than eight times heavier than the sun collapse at the end of their life cycle and explode. Elements are ejected into the universe and a black hole or very dense neutron star is created. During the collapse, atomic nuclei in the core of the star capture electrons. These reactions reduce the pressure in the star and produce neutrinos that carry away energy. Supernovae play an important role in the evolution of the universe. Therefore, researchers work to model and understand them, including these so-called electron-capture reactions.

Researchers can indirectly extract information about the electron-capture reaction rates by using so-called charge-exchange reactions. In these reactions, the initial nucleus and the final reaction product are the same as in the stellar electron-capture reactions. By obtaining information about the probability of the charge-exchange reaction, researchers can calculate the electron-capture reaction rate in a supernovae. Until now, the problem with this method was that only isotopes that are stable on Earth can be studied, whereas many of the isotopes in the collapsing star only live for a fraction of a second when produced in a laboratory.

In an experiment performed at the National Superconducting Cyclotron Laboratory (NSCL), the researchers developed a novel tool to overcome this hurdle. They injected a beam of unstable oxygen-14 into an Active-Target Time-Projection Chamber (AT-TPC) filled with deuterium gas. They identified events in which nitrogen-14 was produced, the same product that would appear after an electron-capture on oxygen-14. In this reaction, a deuterium nucleus in the gas, consisting of one proton and one neutron, is converted into two protons. The AT-TPC allows one to take a “picture” of the tracks of these two protons. This is sufficient to obtain the necessary information to constrain the electron-capture reactions. The nitrogen-14 reactions products were detected in the S800 magnetic spectrometer and used to simplify the identification of the relevant reactions.

The experiment on oxygen-14 was also important and successful in understanding differences between well-established theoretical calculations, which require a correction factor to match experimental data, with more recently developed theoretical models that are based on fundamental principles, which do not require a scaling factor. That result is not only important for astrophysical applications, but also for a better understanding of fundamental properties on atomic nuclei.

The experiment paves the way for future experiments with heavier and less stable nuclei that can be produced at FRIB.

The experimental effort was the result of a collaboration between the FRIB/MSU charge-exchange group led by Prof. Remco Zegers, with Research Associates Simon Giraud and Juan Zamora in key roles, and the international and multi-institutional Active-Target Time-Projection Chamber Collaboration led by Prof. Daniel Bazin (FRIB/MSU). The analysis relied on a simulation framework whose development is led by Ramon Y Cajal Fellow Dr. Yassid Ayyad from the University of Santiago de Compostela. The theoretical calculations were performed by Dr. Sam Novario and Prof. Gaute Hagen from Oak Ridge National Laboratory and the University of Tennessee, Prof. Sonia Bacca from the Johannes Gutenberg-University Mainz in Germany, and Prof. Alex Brown from FRIB/MSU.

This material is based upon work supported by the National Science Foundation; the U.S. Department of Energy Office of Science; the U.S. Department of Energy Office of Science Office of Nuclear Physics; and the German Research Foundation.

Research by the Charge-Exchange group of Remco Zegers is funded by the National Science Foundation, under grants PHY-1913554 (Windows on the Universe: Nuclear Astrophysics at the NSCL) and PHY-2209429 (Windows on the Universe: Nuclear Astrophysics at FRIB).

NSCL was a national user facility funded by the National Science Foundation (NSF), supporting the mission of the Nuclear Physics program in the NSF Physics Division.

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. User facility operation is supported by the DOE-SC Office of Nuclear Physics as one of 28 DOE-SC user facilities.

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