A team of physicists from Germany, Finland, India and Japan has discovered a new isotope of the synthetic chemical element seaborgium.
Mosat et al. report on the discovery of the new isotope seaborgium-257. Image credit: P. Mosat, GSI/FAIR.
Seaborgium is a synthetic chemical element with symbol Sg and atomic number 106.
The element was first synthesized in 1974, and now has 14 isotopes; the most stable isotope, seaborgium-271, has a half-life of about 2.4 min.
The newly-discovered isotope, seaborgium-257, decays by spontaneous fission and alpha-particle emission with a half-life of 12.6 ms.
“For the production of seaborgium-257, an intense beam of chromium-52 from the GSI/FAIR linear accelerator UNILAC impinged onto high-quality lead-206 targets,” said GSI/FAIR physicist Pavol Mosat and colleagues.
Using the highly efficient detection system of the gas-filled recoil separator TASCA (TransActinide Separator and Chemistry Apparatus), the researchers observed a total of 22 decays of seaborgium-257.
“Our findings on seaborgium-257 provide exciting hints on the impact of shell effects on the fission properties of superheavy nuclei,” they said.
“As one consequence, it is possible that the next lighter, still unknown isotope may undergo fission in a very short time range of one nanosecond to six microseconds.
“The upper limit of this expected half-life range is near or even just below current experimental capabilities — unless a so-called K-isomeric state exists.”
“Such excited states, stabilized by quantum effects, exhibit longer fission lifetimes and open an indirect doorway to the short-lived nuclei.”
“Recently, significant progress toward the border of stability was made by discovering the 60-ns rutherfordium-252 via its longer-lived K-isomeric state.”
“The exploration of the isotopic border for the element seaborgium is a natural continuation of these experiments, mapping the coastline of the island of stability of the superheavy nuclei.”
“So far, no K-isomeric state has been observed in seaborgium isotopes.”
In the current experiment, however, the authors also irradiated a lead-208 target and observed strong evidence for the presence of a K-isomeric state in seaborgium-259.
“Our results on a K-isomeric state in seaborgium-259 open a doorway to explore the K-isomer phenomenon in other seaborgium isotopes and to enable the synthesis of the short-lived isotope seaborgium-256 isotope, if a long-lived K-isomeric state exists also in this nucleus,” said GSI/FAIR physicist Khuyagbaatar Jadambaa.
The team’s results were published in the journal Physical Review Letters.
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P. Mosat et al. 2025. Probing the Shell Effects on Fission: The New Superheavy Nucleus 257Sg. Phys. Rev. Lett 134, 232501; doi: 10.1103/s7hr-y7zq
