At a Glance
- An international team of physicists has discovered astatine-188 (188At), the heaviest known atomic nucleus that undergoes the rare decay process of proton emission.
- Researchers created this extremely short-lived isotope at the University of Jyväskylä by colliding a high-energy strontium-84 beam with a target made of natural silver.
- Analysis of the new nucleus revealed that it has a highly deformed prolate shape, similar to a watermelon, and decays in a way that defied theoretical predictions.
- The isotope’s unexpected decay behavior provides the first experimental evidence of a quantum mechanical interaction called the Thomas-Ehrman shift within a very heavy atomic nucleus.
- This groundbreaking research advances our understanding of nuclear structure at the limits of stability and provides a crucial new benchmark for testing and refining theoretical models.
An international team of physicists has announced the discovery of astatine-188 188At, a new atomic nucleus that is now the heaviest known to decay by emitting a proton. The breakthrough, achieved at the University of Jyväskylä in Finland, pushes the known limits of matter and provides new insight into the forces that hold atoms together. The findings were published in the journal Nature Communications. Most atomic nuclei decay by emitting other particles, such as electrons or alpha particles. Proton emission, however, is a much rarer process. “Proton emission is a rare form of radioactive decay, in which the nucleus emits a proton to take a step toward stability,” said Henna Kokkonen, a doctoral researcher at the University of Jyväskylä and a key member of the research team.
The new isotope, which consists of 85 protons and 103 neutrons, was created through a process known as a fusion-evaporation reaction. In the experiment, scientists bombarded a target made of natural silver with a high-energy beam of strontium-84 (84Sr) ions. The collision occasionally fused the two nuclei, which immediately evaporated three neutrons to form 188At. Because these exotic nuclei are incredibly short-lived and have a low production cross-section, they are challenging to create; to address this, the team used a highly sensitive detector setup called the RITU recoil separator to isolate and identify the new isotope just moments after its formation.
The researchers paired their experimental data with a sophisticated theoretical model to fully understand the new discovery. The model’s calculations suggest that the 188At nucleus is not spherical but is instead extremely “prolate,” meaning it is deformed into a shape resembling a watermelon. The measured decay rate revealed an unexpected deviation in the nucleus’s binding energy. The energy required to remove its outermost proton differed from what established trends predicted. The team interprets this as the first evidence of a quantum mechanical effect in a heavy nucleus known as the Thomas-Ehrman shift. This interaction can influence the stability of nuclei with loosely bound protons.
This discovery marks the second time in recent years that Kokkonen has been part of a team identifying a new isotope, following her master’s thesis work on the discovery of astatine-190 (190At). The research adds a new member to the chart of nuclides. It provides a crucial test for theoretical models that describe the complex behavior of atomic nuclei at the edge of stability. “Isotope discoveries are rare worldwide, and this is the second time I have had the opportunity to be part of making history,” Kokkonen said. “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.”
References
- Kokkonen, H., Auranen, K., Siwach, P., Arumugam, P., Briscoe, A. D., Eeckhaudt, S., Ferreira, L. S., Grahn, T., Greenlees, P. T., Jones, P., Julin, R., Juutinen, S., Leino, M., Leppänen, A.-P., Maglione, E., Nyman, M., Page, R. D., Pakarinen, J., Rahkila, P., … Venhart, M. (2025). New proton emitter 188At implies an interaction unprecedented in heavy nuclei. Nature Communications, 16(1), 4985. https://doi.org/10.1038/s41467-025-60259-6
- University of Jyväskylä. (2025, June 4). The heaviest proton emitter: New type of atomic nucleus discovered. Phys.Org; University of Jyväskylä. https://phys.org/news/2025-06-heaviest-proton-emitter-atomic-nucleus.html
