Using the focal-plane spectrometer of the gas-filled recoil separator at the Accelerator Laboratory of the University of Jyväskylä in Finland, physicists have observed two decay events of the new isotope astatine-188 (188At), which consists of 85 protons and 103 neutrons.
Kokkonen et al. report the discovery of the new atomic nucleus 188At, which is the heaviest proton-emitting isotope known to date.
“Proton emission is a rare form of radioactive decay, in which the nucleus emits a proton to take a step towards stability,” said Henna Kokkonen, a doctoral researcher at the University of Jyväskylä.
“The new nucleus is so far the lightest known isotope of astatine, 188At, consisting of 85 protons and 103 neutrons.”
“Exotic nuclei of this kind are extremely challenging to study due to their short lifetimes and low production cross sections, so precise techniques are needed.”
“The nucleus was produced in a fusion-evaporation reaction by irradiating natural silver target with 84Sr ion beam,” added Dr. Kalle Auranen, also from the University of Jyväskylä.
“The new isotope was identified using the detector setup of the RITU recoil separator.”
In addition to the experimental results, the physicists expanded a theoretical model to interpret the measured data.
According to the team, 188At can be interpreted as strongly prolate, i.e. ‘watermelon shaped.’
“The properties of the nucleus suggest a trend change in the binding energy of the valence proton,” Kokkonen said.
“This is possibly explained by an interaction unprecedented in heavy nuclei.”
“Isotope discoveries are rare worldwide, and this is the second time I have had the opportunity to be part of making history.”
“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 authors plan to further stimulate the theoretical studies of charged-particle decaying heavy nuclei, and their shape evolution, narrowing down the present uncertainties of the decay energy and half-life by observing more 188At decay events.
“Equally interesting would be to study the decay of presently unknown nucleus 189At, which might also be a proton-emitting nucleus, however, this remains to be seen in future experiments,” they concluded.
Their paper was published in the journal Nature Communications.
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H. Kokkonen et al. 2025. New proton emitter 188At implies an interaction unprecedented in heavy nuclei. Nat Commun 16, 4985; doi: 10.1038/s41467-025-60259-6
