A team at the Institute of Modern Physics of the Chinese Academy of Sciences has directly measured the masses of the short lived nuclei phosphorus-26 and sulfur-27, providing data needed to determine a key nuclear reaction rate in Type I X ray bursts on neutron stars.
The study reports high precision mass measurements of these proton rich nuclei, which are important for modeling the rapid Tokyo, Japan (SPX) Dec 05, 2025
A team at the Institute of Modern Physics of the Chinese Academy of Sciences has directly measured the masses of the short lived nuclei phosphorus-26 and sulfur-27, providing data needed to determine a key nuclear reaction rate in Type I X ray bursts on neutron stars.
The study reports high precision mass measurements of these proton rich nuclei, which are important for modeling the rapid proton capture process, or rp process, that drives thermonuclear explosions in low mass X ray binaries.
Type I X ray bursts occur when hydrogen and helium accumulated on a neutron star surface undergo unstable thermonuclear burning, with reaction paths and timescales set by the exact masses of many short lived nuclei near the proton drip line. Large uncertainties in those masses have limited efforts to calculate reliable reaction pathways and energy generation in these events.
One debated branch in the rp process involves reactions that pass through phosphorus-26 and sulfur-27, but the lack of precise mass values for these nuclides has prevented clear evaluation of its role.
To address this, the researchers used magnetic rigidity defined isochronous mass spectrometry at the Cooling Storage Ring of the Heavy Ion Research Facility in Lanzhou to obtain direct mass measurements for both nuclei. They determined that the proton separation energy of sulfur-27 is 129 to 267 keV higher than earlier estimates and achieved an eightfold improvement in the precision of this quantity compared with previous work.
Using the updated masses, the team recalculated the reaction rate of 26P(p,gamma)27S under X ray burst conditions and found that it increases markedly between 0.4 and 2 Gigakelvin, reaching up to five times the earlier estimated rate at 1 Gigakelvin. The uncertainty in the reverse reaction rate is also significantly reduced, which tightens constraints on the balance between forward and reverse flows in this part of the network. The new rate enhances the abundance ratio of sulfur-27 relative to phosphorus-26, showing that nucleosynthesis in X ray bursts shifts more efficiently toward sulfur-27 along this branch.
"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.
Research Report:Precision Mass Measurement of 26P and 27S and Their Impact on the 26P(p,gamma)27S Reaction in Stellar X-Ray Bursts
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