Breakthrough in rapid proton capture process unveils new reaction rate

Breakthrough in rapid proton capture process unveils new reaction rate
by Simon Mansfield
Sydney, Australia (SPX) Jul 26, 2023

In an important breakthrough for nuclear astrophysics, an international team led by HOU Suqing from the Institute of Modern Physics of the Chinese Academy of Sciences has successfully derived a new reaction rate for a crucial component of the rapid proton capture process (rp-process). Their findings were recently published in The Astrophysical Journal.

Type I X-ray bursts, the most common thermonuclear stellar explosions within our galaxy, feature the rp-process as a fundamental nucleosynthesis operation. The rp-process involves the rapid capture of protons by heavier atomic nuclei, leading to the formation of new, heavier atomic elements.

A key nuclear reaction within the rp-process is the 26P(p,y)27S reaction. This operation encapsulates the process of a proton being captured by a Phosphorus-26 (26P) atom, resulting in the formation of a Sulfur-27 (27S) atom and the emission of a gamma-ray photon. Accurate knowledge of this specific reaction is imperative for a comprehensive understanding of the rp-process during Type I X-ray bursts.

The international astrophysics team, which comprised members from the Hungarian Academy of Sciences, the University of Hull, Michigan State University, and Texas A and M University-Commerce, has advanced our understanding of this critical reaction rate. By using the most recent nuclear mass data for Sulfur-27, the team found that the 26P(p,y)27S reaction rate is largely driven by a direct capture reaction mechanism, contrary to the previously assumed resonant capture.

This discovery has a profound impact on the reaction rate values, with the newly derived rate being at least an order of magnitude smaller than prior rates derived from statistical models, particularly within the temperature range where X-ray bursts occur.

In addition, when the team applied their new reaction rates to rp-process calculations, they noted significant changes in the ratio of isotope abundances. Specifically, the ratio of Sulfur-27 to Phosphorus-26 isotopes became ten times smaller than ratios derived using prior rates from the Joint Institute for Nuclear Astrophysics reaction rate database (Reaclib). Furthermore, they discovered a greater accumulation of material on the Phosphorus-26 nucleus as compared to Sulfur-27 throughout the entire rp-process.

This research fundamentally redefines our understanding of the processes that drive Type I X-ray bursts. The new insights into the rp-process and the more accurate understanding of the nuclear reactions that fuel these stellar explosions could potentially revolutionize our models of these frequent and powerful thermonuclear events.

Research Report:New 26P(p,y)27S Thermonuclear Reaction Rate and Its Astrophysical Implications in the rp-process