A research team from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) has directly measured the masses of two highly unstable atomic nuclei, phosphorus-26 and sulfur-27. These precise measurements provide crucial information for determining the nuclear reaction rate during X-ray bursts, advancing our understanding of how elements are synthesized under such extreme conditions.
The findings were published in The Astrophysical Journal on December 1.
Type I X-ray bursts are frequent and violent thermonuclear explosions in the galaxy, primarily observed in low-mass X-ray binary systems consisting of a neutron star and its companion star. The energy for these bursts comes from the unstable thermonuclear combustion of hydrogen and helium on the surface of the neutron star, which involves rapid proton capture reactions, known as the rp-process. In this process, atomic nuclei rapidly capture protons to form heavier elements. The speed and path of these reactions critically depend on the precise masses of the relevant atomic nuclei.
However, the rp-process involves many nuclei close to the proton drip line, which typically have short lifetimes and unknown masses. As a result, accurately calculating the nuclear reaction pathways is very challenging.
According to Dr. YAN Xinliang of IMP, one of the corresponding authors of the study, the significance of a potential reaction branch involving phosphorus-26 and sulfur-27 in the rp-process has been debated for years due to the lack of precise mass data for these nuclei.
To acquire accurate mass data for phosphorus-26 and sulfur-27, the researchers made direct measurements using magnetic-rigidity-defined isochronous mass spectrometry at the Cooling Storage Ring of the Heavy Ion Research Facility in Lanzhou (HIRFL-CSR). They found that the proton separation energy of sulfur-27 is 129-267 keV higher than previously thought, with its precision showing an eightfold improvement over previous measurements.
With the new mass data, the researchers discovered that under X-ray burst conditions, the updated reaction rate of 26P(p,γ)27S is significantly enhanced within the temperature range of 0.4-2 Gigakelvin (GK), reaching up to five times the previously estimated rate at 1 GK. The uncertainty of the reverse reaction rate has been greatly reduced. The new reaction rate was found to increase the abundance ratio of sulfur-27 to phosphorus-26, indicating a more efficient reaction flow toward sulfur-27.
"Our high-precision mass results and the corresponding new reaction rate provide more reliable input for astrophysical reaction networks, resolving the uncertainties in the nucleosynthesis pathways within the phosphorus-sulfur region of X-ray bursts," said Dr. HOU Suqing from IMP, another corresponding author of the study.
The study was conducted in collaboration with researchers from Germany's GSI Helmholtz Centre for Heavy Ion Research and the Max Planck Institute for Nuclear Physics, as well as Japan's Saitama University.
This work was supported by the National Key Research and Development Program of China, the Youth Innovation Promotion Association of CAS, and the Regional Development Young Scholars Project of CAS.
Figure 1. The mass abundance distribution map at 100 seconds after the start of the rp-process. (Image from IMP)
Figure 2. The detector system of nuclear mass spectrometer based on the Cooling Storage Ring in Lanzhou. (Image from IMP)
