Pinning down the unstable magnesium burning reaction rate essential for X-ray bursts
Overview of the press release
Researchers at the Center for Nuclear Study (CNS) and their colleagues have obtained the most precise unstable magnesium burning reaction rate of 22Mg+α→25Al+p (or 22Mg(α, p) in the common notation) in the world, which is highly essential for the stellar explosion referred to as an X-ray burst. They also performed a new X-ray burst model calculation and demonstrated that the new rate reproduces the observational data of X-ray bursts most accurately.
Figure1: Scheme of the present interdisciplinary study, where the astrophysically observed X-ray burst lightcurve is compared with the simulation based on the nuclear reaction rate determined by the accelerator experiment. The upper-right boxes show the location of the 22Mg(α, p)25Al reaction in the “αp-process”, a nuclear burning path that is particularly important in high-temperature stellar environments such as X-ray bursts. The boxes in blue indicate the location of stable nuclides and show that the burning path runs through proton-rich unstable nuclides, which usually cannot be found on the Earth.
The 22Mg(α, p) reaction has been considered an important reaction in explosive stellar environments, but the reaction rate had a large discrepancy of several orders of magnitude among previous theoretical and experimental works. It is extremely difficult to experimentally investigate this reaction, since it is a reaction from a radioactive isotope (RI) to another RI, where the RI cannot be found naturally on the Earth and must be created artificially. In a previous study, Randhawa and colleagues (2020) measured the reaction directly using an RI beam technique; however, due to the limited intensity of the RI beam, the measurement range was far from the temperature of actual X-ray bursts. They evaluated the 22Mg(α, p) reaction rate by an extrapolation down to the stellar temperature that should accompany a large uncertainty that they did not account for.
“As stars are enormously large and contain a vast amount of atoms, it is often difficult to simulate them in a laboratory. Unstable nuclei (RI) in the burning process makes this even more difficult, since they do not survive on the Earth and have to be created with accelerators,” said Hidetoshi Yamaguchi, a lecturer at the University of Tokyo’s Center for Nuclear Study. “It’s a challenging yet amusing puzzle given by nature.”
Yamaguchi and his colleagues produced an intense RI beam of aluminum-25 (25Al) with a cryogenic deuterium (D2) target irradiated by an intense beam of 24Mg using the CNS Radioactive Ion Beam separator (CRIB), coupled with an efficient measurement method called “resonant scattering” to precisely determine the 22Mg(α, p) reaction rate. The RI beam bombarded a hydrogen target, and the scatterings of 25Al with 1H were identified with an array of semiconductor detectors. After analyzing the property of the resonances, the researchers then evaluated the stellar reaction rate. Lastly, they performed a model calculation to test if previous observations of X-ray bursts are reproduced better using their new reaction rate.
They found that the new 22Mg(α, p) reaction rate they obtained is the most accurate in the world. Their results further demonstrated that the “RI to RI” reactions, which are not easy to measure in most cases, can be studied with the advanced RI beam technique and the resonant scattering method. The research group believes that after further developing the RI beam technology, they should be able to measure all the relevant reactions in X-ray bursts.
“This experiment was carried out rather smoothly in spite of the difficulty of conducting an accelerator experiment with a large international collaboration and the complicated analysis, and we successfully simulated a star in a laboratory,” said Yamaguchi. “There was some uncertainty in the determination of the nuclear spin (the angular momentum attributed to each nuclear resonance) of the resonance states, but we were relieved to find that its effect to the final result was not too large.”
X-ray bursts are still mysterious stellar explosions but occur very frequently in the galaxy, providing a test ground of our knowledge of astrophysics and nuclear physics. They are also considered a typical stellar site of the “rp-process”, which is one of the major paths of nuclear synthesis of heavy elements. Studying X-ray bursts is important not only to understand a stellar explosion phenomenon, but also to find the origin of the elements that constitute all the material around us, from our bodies to the Earth.
Figure2: An image of the experimental setup. An intense radioactive isotope (RI) beam of aluminum-25 (25Al) was produced with the RI beam separator CRIB. The beam is projected from the right side, identified with monitoring detectors (PPACs and MCPs), and stopped in the polyethylene (CH2）target, which contains hydrogen. Recoiled protons (hydrogen ions) from the scattering with 25Al are detected with the silicon detector array on the left side.
Hidetoshi Yamaguchi (Lecturer, Center for Nuclear Study, the University of Tokyo)
Seiya Hayakawa (Project Assistant Professor, Center for Nuclear Study, the University of Tokyo)
Jun Hu (Researcher, Institute of Modern Physics, Chinese Academy of Science / former Project Researcher of the University of Tokyo.)
Physical Review Letters
TitleFirst measurement of 25Al+p resonant scattering for 22Mg（α,p）25Al reaction and implications on understanding type-I x-ray bursts Authors J. Hu*, H. Yamaguchi, Y. H. Lam, A. Heger, A. M. Jacobs, S. W. Xu, N. T. Zhang, S.B. Ma, L.H. Ru, E. Q. Liu, T. Liu, S. Hayakawa, L. Yang, H. Shimizu, D. Kahl, C.B. Hamill, A. St J. Murphy, J. Su, X. Fang, K. Y. Chae, M.S. Kwag, S. M. Cha, N.N. Duy, N.K. Uyen, D.H. Kim, R.G. Pizzone, M. La Cognata, S. Cherubini, S. Romano, A. Tumino, J. Liang, A. Psaltis, M. Sferrazza, D.H. Kim, Z. Johnston, Y.Y. Li, and S. Kubono DOI10.1103/PhysRevLett.127.172701 Paper link
― Office of Communication ―