Precision Mass Measurement of ²⁶ P and ²⁷ S and Their Impact on the ²⁶ P( <i>p</i> , <i>γ</i> ) ²⁷ S Reaction in Stellar X-Ray Bursts

Abstract Based on the experimental Cooler Storage Ring in Lanzhou, we directly determined the masses of 27 S and 26 P using Bρ -defined isochronous mass spectrometry. The mass excesses were determined to be 17,437(24) keV for 27 S and 10,997(10) keV for 26 P. These values yield a proton separation energy of 849(26) keV for 27 S, representing an eightfold precision improvement over the previous one of 581(214) keV. With precisely measured masses, we evaluated the thermonuclear 26 P( p , γ ) 27 S reaction rate under X-ray burst conditions. The updated rate is enhanced within the 0.4–2 GK temperature range, reaching up to 5 times the previous rate at 1 GK. The uncertainty of the reverse 27 S( γ , p) 26 P reaction rate is drastically reduced with the new high-precision mass results, from over 10 orders of magnitude to less than one. We further assessed the impact of the revised rates on the nucleosynthesis in Type I X-ray bursts using a one-zone postprocessing model. The results show that the final abundance ratio of 27 S to 26 P is increased by about 1 order of magnitude, indicating a more efficient reaction flow toward 27 S. Although the sequence 25 Si( p , γ ) 26 P( p , γ ) 27 S( β + ) 27 P is not on the main rp -process path, it emerges as a nonnegligible branch contributing to 27 P synthesis.