The study is part of Henna Kokkonen’s doctoral thesis. Credit: Tommi Sassi
After more than 30 years, physicists in Finland have uncovered the heaviest nucleus ever seen to emit a proton, a rare process that sheds light on the limits of atomic matter.
The discovery of 188-astatine — an oddly “watermelon-shaped” nucleus — not only sets a new record but also hints at nuclear interactions never before observed.
Historic Breakthrough in Proton Emission Research
Radioactive decay has long been central to nuclear physics, serving as a key to understanding how atomic nuclei behave. At the University of Jyväskylä in Finland, researchers have now measured the heaviest nucleus ever observed to undergo proton emission.
“Proton emission is a rare form of radioactive decay, in which the nucleus emits a proton to take a step towards stability,” says Doctoral Researcher Henna Kokkonen from the University of Jyväskylä.
Studying Exotic Nuclei Is Difficult, But Not Impossible
The newly identified nucleus is the lightest isotope of astatine known so far, 188At, which contains 85 protons and 103 neutrons. Investigating nuclei like this is especially difficult because they exist for only a very short time and are produced in extremely small numbers, requiring highly precise experimental methods.
“The nucleus was produced in a fusion-evaporation reaction by irradiating a natural silver target with an 84Sr ion beam,” says Academy Research Fellow Kalle Auranen from the University of Jyväskylä. “The new isotope was identified using the detector setup of the RITU recoil separator.”
Study Reveals New Findings on Heavy Nuclei
In addition to the experimental results, the study expanded a theoretical model to interpret the measured data. Through the model, the nucleus can be interpreted as strongly prolate, i.e. “watermelon shaped.”
“The properties of the nucleus suggests a trend change in the binding energy of the valence proton,” says Kokkonen. “This is possibly explained by an interaction unprecedented in heavy nuclei.”
A Doctoral Journey Building on Past Discoveries
The study is part of Kokkonen’s doctoral thesis and a direct scientific follow-up to her master’s thesis, in which she discovered a new type of atomic nucleus, the 190-astatatine. The thesis article was published in the Physical Review C journal in 2023.
“Isotope discoveries are rare worldwide, and this is the second time I have had the opportunity to be part of making history,” Kokkonen rejoices. “Every experiment is challenging, and it feels great to do research that improves understanding of the limits of matter and the structure of atomic nuclei.”
The research article was written as part of an international research collaboration involving experts in theoretical nuclear physics. The study was published in the renowned Nature Communications on May 29, 2025.
Reference: “New proton emitter 188At implies an interaction unprecedented in heavy nuclei” by Henna Kokkonen, Kalle Auranen, Pooja Siwach, Paramasivan Arumugam, Andrew D. Briscoe, Sarah Eeckhaudt, Lidia S. Ferreira, Tuomas Grahn, Paul T. Greenlees, Pete Jones, Rauno Julin, Sakari Juutinen, Matti Leino, Ari-Pekka Leppänen, Enrico Maglione, Markus Nyman, Robert D. Page, Janne Pakarinen, Panu Rahkila, Jan Sarén, Catherine Scholey, Juha Sorri, Juha Uusitalo and Martin Venhart, 29 May 2025, Nature Communications.
DOI: 10.1038/s41467-025-60259-6
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