For the first time in nearly 30 years, scientists have discovered the heaviest atomic nucleus ever observed that decays by emitting a proton—a rare and fascinating process in nuclear physics.
This new discovery, made at the Accelerator Laboratory of the University of Jyväskylä in Finland, pushes the boundaries of what we know about the building blocks of matter.
The newly found nucleus is a very light isotope of the element astatine, known as astatine-188 or 188At.
It contains 85 protons and 103 neutrons and is extremely unstable.
Because it exists for only a tiny fraction of a second and is produced in extremely small amounts, studying it requires cutting-edge technology and teamwork from researchers around the world.
The results were published in the journal Nature Communications on May 29, 2025.
Proton emission is a very rare type of radioactive decay. In this process, an unstable nucleus sheds a single proton to become more stable. Most radioactive elements lose energy by emitting alpha particles (two protons and two neutrons) or beta particles (electrons), but proton emission is much less common and harder to detect.
To create 188At, scientists used a method called fusion-evaporation. They fired ions of strontium-84 at a silver target, hoping the atoms would fuse and form new, exotic elements. Once formed, the particles were analyzed using a special detection system called the RITU recoil separator, which helps separate and identify very rare atoms.
Alongside the experimental work, scientists also updated theoretical models to help understand the behavior of this unusual nucleus. Based on their calculations, 188At has a strongly stretched, elongated shape—described as “watermelon-shaped” by the researchers. They also observed unexpected behavior in the way the outermost proton is held in place, suggesting a new kind of interaction that hasn’t been seen in such heavy elements before.
This discovery builds on earlier research by doctoral student Henna Kokkonen, who also played a key role in identifying a new type of astatine nucleus, 190At, back in 2023. That study was part of her master’s thesis, and now, her PhD research continues to push the boundaries of nuclear physics.
“Finding new isotopes is extremely rare,” Kokkonen said. “This is the second time I’ve been part of such a discovery, and it’s amazing to contribute to our understanding of the limits of matter and how atomic nuclei work.”
This finding not only expands our knowledge of nuclear structure but may also provide clues to how elements behave at the very edge of stability.
