Image Credit: IMP
A research team from the Institute of Modern Physics (IMP) at the Chinese Academy of Sciences (CAS) has achieved a significant breakthrough by directly measuring the masses of two extremely unstable atomic nuclei—phosphorus-26 and sulfur-27. These nuclei exist for only fractions of a second, making them exceptionally difficult to study.
The team’s highly precise measurements now provide vital data for determining nuclear reaction rates involved in X-ray bursts, shedding new light on how elements are forged under some of the most extreme astrophysical conditions. The results of this study were published in The Astrophysical Journal on December 1.
Type I X-ray bursts are among the most frequent and powerful thermonuclear explosions in the galaxy. They typically occur in low-mass X-ray binary systems where a neutron star—one of the densest objects in the universe—pulls material from a companion star. As hydrogen and helium accumulate on the neutron star’s surface, they undergo unstable thermonuclear burning.
This explosive process is driven by a chain of rapid proton-capture reactions, collectively known as the rp-process. During this sequence, atomic nuclei quickly absorb protons to build heavier and heavier elements. The rate and direction of these reactions depend sensitively on the precise masses of specific nuclei involved.
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However, many of the nuclei within the rp-process lie close to the proton drip line, a boundary where nuclei can no longer bind an additional proton. Such nuclei often have extremely short lifetimes and lack accurate mass measurements, making it difficult for scientists to reliably calculate nuclear reaction pathways in astrophysical models.
Dr. Yan Xinliang of IMP, one of the study’s corresponding authors, explained that the role of a possible reaction branch involving phosphorus-26 and sulfur-27 has been debated for years due to the absence of precise mass data. This gap has long posed challenges for understanding nucleosynthesis in the phosphorus-sulfur region during X-ray bursts.
To address this, the team employed magnetic-rigidity-defined isochronous mass spectrometry at the Cooling Storage Ring of the Heavy Ion Research Facility in Lanzhou (HIRFL-CSR). Their measurements revealed that the proton separation energy of sulfur-27 is 129–267 keV higher than previously estimated, with an impressive eightfold improvement in precision.
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Using these updated mass values, the researchers recalculated the reaction rate of 26P(p,γ)27S under X-ray burst conditions. They discovered a significant enhancement in the rate across temperatures of 0.4–2 gigakelvin (GK), reaching as much as five times earlier estimates at 1 GK.
The uncertainty in the reverse reaction rate was also substantially reduced. These refined rates increase the predicted abundance of sulfur-27 relative to phosphorus-26, indicating a more efficient flow of nuclear reactions toward heavier elements in this region.
“Our high-precision mass measurements provide much more reliable input for astrophysical reaction networks,” said Dr. Hou Suqing, another corresponding author. “These findings help resolve long-standing uncertainties in nucleosynthesis pathways during X-ray bursts.”
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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 Saitama University in Japan. It 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.
