The study is part of Henna Kokkonen’s doctoral thesis. Credit: Tommi Sassi

Finnish Physicists Discover Record-Breaking ‘Watermelon-Shaped’ Nucleus, Redefining Atomic Science

After more than 30 years of research, scientists in Finland have uncovered the heaviest nucleus ever observed to emit a proton—a rare and revealing process that could reshape our understanding of atomic matter.

The newly discovered 188-astatine, nicknamed the “watermelon nucleus” due to its elongated shape, not only sets a record in nuclear physics but also hints at previously unseen nuclear interactions.

NASA and IBM Develop AI to Forecast Solar Flares Before They Strike Earth

A Historic Breakthrough in Proton Emission

Radioactive decay has long been a cornerstone of nuclear physics, offering crucial insights into how atomic nuclei behave. Researchers at the University of Jyväskylä have now measured 188-astatine, the heaviest nucleus recorded to undergo proton emission.

“Proton emission is a rare form of radioactive decay, where the nucleus ejects a proton to move toward stability,” explains Doctoral Researcher Henna Kokkonen from the University of Jyväskylä.

Investigating Exotic Nuclei: A Technical Challenge

188-astatine is the lightest isotope of astatine discovered to date, containing 85 protons and 103 neutrons. These nuclei exist for mere fractions of a second and are produced in extremely small quantities, demanding highly precise experimental techniques.

“We produced the nucleus using a fusion-evaporation reaction by bombarding a silver target with an 84Sr ion beam,” says Academy Research Fellow Kalle Auranen. “We then identified the new isotope using the RITU recoil separator.

New Insights Into Heavy Nuclei

Beyond the experimental findings, the team expanded a theoretical model to interpret the data, revealing that the nucleus is strongly prolate, resembling a watermelon in shape.

“The properties suggest a shift in the binding energy of the valence proton, possibly due to interactions never seen before in heavy nuclei,” says Kokkonen.

A Doctoral Journey of Discovery

This study is part of Kokkonen’s doctoral thesis, building on her master’s work, where she previously discovered 190-astatine.

“Isotope discoveries are rare globally, and this is the second time I’ve contributed to making history,” Kokkonen says. “Every experiment is a challenge, and it’s thrilling to expand our understanding of the limits of matter.”

The research is part of an international collaboration of theoretical nuclear physicists and was published in Nature Communications on May 29, 2025.