Breakthrough Discovery of Astatine-188 Sheds Light on Proton Emission

In a significant advancement in nuclear physics, researchers at the University of Jyväskylä, Finland, have identified the heaviest proton-emitting nucleus known to date, astatine-188 (188At). This breakthrough, reported on June 13, 2025, marks a pivotal moment in understanding nuclear decay processes and the stability of atomic nuclei. The isotope, which consists of 85 protons and 103 neutrons, exhibits a rare form of radioactive decay by ejecting a proton, a process that allows it to approach a more stable state.

The discovery was made utilizing a fusion-evaporation reaction, where a beam of strontium-84 ions was directed at a silver-107 target. The RITU recoil separator was employed to isolate the newly formed isotope prior to its detection. Henna Kokkonen, a doctoral researcher at Jyväskylä, explained, "Proton emission is a rare form of radioactive decay, wherein the nucleus emits a proton to take a step toward stability."

Before this discovery, the heaviest known proton emitter was bismuth-185, recognized in 1996. While bismuth-185 had a decay time of approximately 2.8 microseconds, the half-life of 188At was determined to be roughly 190 microseconds. The detection of only two decay events underscores the precision required in tracking such fleeting processes.

Proton emission was initially documented in the 1970s, with isotopes like cobalt-53 and lutetium-151 leading the way. These early findings established that atomic nuclei could decay by shedding individual protons rather than only clusters such as alpha particles. The advancement of decay spectroscopy and detection technologies has allowed scientists to observe proton emissions from increasingly heavier and transient isotopes, with 188At representing the culmination of decades of research in experimental nuclear physics.

Identifying exotic isotopes like 188At necessitates extensive effort, sophisticated equipment, and a degree of serendipity. The fusion-evaporation method used in this study produced only a handful of atoms of the desired nucleus amidst billions of other particles. The team utilized the GREAT spectrometer and a silicon strip detector to capture decay events with sub-millisecond accuracy, highlighting the rarity and significance of the observed decay events.

This discovery was the result of collaborative efforts among experimental physicists, nuclear theorists, and engineers from various institutions, each contributing unique expertise. Such cross-disciplinary partnerships are increasingly vital in exploring phenomena at the forefront of physics, where no single institution possesses all the necessary tools or data.

Theoretical frameworks have been expanded to explain the decay characteristics of 188At. Scientists have integrated a non-adiabatic quasiparticle model to accommodate heavy nuclei, revealing that the proton emerges from a nucleus that is strongly prolate, or elongated in shape. The observed behavior of the proton in this isotope necessitates updates to existing nuclear models, enhancing their predictive power regarding rare isotopes and future discoveries.

The implications of this research extend beyond theoretical physics; they also inform practical applications in fields like nuclear medicine. Although 188At itself may not be utilized clinically, the insights gleaned from its structure and decay pathways contribute to the toolkit for predicting isotope behavior—essential for the development of targeted therapies and diagnostics. Furthermore, these insights support astrophysical models, particularly those that describe the formation of heavy elements during stellar explosions.

Kokkonen’s doctoral research, which builds upon her previous discovery of astatine-190 in 2023, will continue to focus on refining measurements of 188At's decay properties and potentially producing astatine-189, another isotope that may also exhibit proton emission. The outcomes of ongoing research in this domain could lead to significant advancements in our understanding of nuclear stability and the limits of matter. The findings were published in the journal Nature Communications, underscoring the critical nature of collaborative scientific inquiry in making groundbreaking discoveries.