Land ahoy! — Experiments at GSI/FAIR reveal the shoreline of the island of stability of superheavy elements
15.01.2025 |
A team of researchers from GSI/FAIR, Johannes Gutenberg University Mainz and the Helmholtz Institute Mainz has succeeded in exploring the limits of the so-called island of stability within the superheavy nuclides more precisely by measuring the superheavy rutherfordium-252 nucleus, which is now the shortest-lived known superheavy nucleus. The results were published in the journal “Physical Review Letters” and highlighted as an “Editor's suggestion”.
The strong force ensures cohesion in atomic nuclei consisting of protons and neutrons. However, as the positively charged protons repel each other, nuclei with too many protons are at risk of splitting — a challenge in the production of new, superheavy elements. Certain combinations of protons and neutrons, the so-called “magic numbers”, give nuclei additional stability. When taking these magic combinations into account, theoretical works dating back to the 1960s predict an island of stability in the sea of unstable superheavy nuclei, where very long lifetimes could be achieved, even approaching the age of the Earth.
The concept of this island has since been confirmed, with the observation of increasing half-lives in the heaviest currently known nuclei as the predicted next magic number of 184 neutrons is approached. However, the location of the peak of this island, its height (reflecting the maximum expected half-life), and also the island’s extension are still unknown. Researchers at GSI/FAIR in Darmstadt, the Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM) have now come a step closer to mapping this island, by discovering the shortest-lived superheavy nucleus known thus far, which marks the position of the island’s shoreline in nuclei of rutherfordium (Rf, element 104).
To allow experimental detection, the minimum lifetime of superheavy nuclei is on the order of a millionth of a second, which renders extremely short-lived superheavy nuclei in the vicinity of sea of instability inaccessible. But there is a trick: Sometimes, excited states, stabilized by quantum effects, show longer lifetimes and open a doorway to the short-lived nuclei. “Such long-lived excited states, so-called isomers, are widespread in superheavy nuclei of deformed shape according to my calculations,” says Dr. Khuyagbaatar Jadambaa, first author of the publication from GSI/FAIR’s research department for superheavy element chemistry. “Thus, they enrich the picture of the island of stability with ‘clouds of stability’ hovering over the sea of instability.”
The research team from Darmstadt and Mainz succeeded in examining these predictions by searching for the hitherto unknown nucleus Rf-252. The researchers used an intense beam of titanium-50 available at the GSI/FAIR’s UNILAC accelerator to fuse titanium nuclei with lead nuclei supplied on a target foil. The fusion products were separated in the TransActinide Separator and Chemistry Apparatus TASCA. They implanted into a silicon detector after a flight-time of about 0.6 microseconds. This detector registered their implantation as well as their subsequent decay.
In total, 27 atoms of Rf-252 decaying by fission with a half-life of 13 microseconds were detected. Thanks to the fast digital data acquisition system developed by GSI/FAIR’s Experiment Electronics department, electrons emitted after the implantation of the isomer Rf-252m and released in its decay to the ground state, were detected. Three such cases were registered. In all cases, a subsequent fission followed within 250 nanoseconds. From these data, a half-life of 60 ns was deduced for the ground-state of Rf-252, which is now the shortest-lived superheavy nucleus currently known.
“The result decreases the lower limit of the known lifetimes of the heaviest nuclei by almost two orders of magnitude, to times that are too short for direct measurement in the absence of suitable isomer states. The present findings set a new benchmark for further exploration of phenomena associated with such isomer states, inverted fission-stability where excited states are more stable than the ground state, and the isotopic border in the heaviest nuclei.” says Professor Christoph E. Düllmann, head of the research department for superheavy element chemistry at GSI/FAIR.
In future experimental campaigns, the measurement of isomeric states with inverted fission stability in the next heavier element seaborgium (Sg, element 106) is envisioned and to be used for the synthesis of Sg istopes with lifetimes below a microseconds in order to further map the isotopic border. The result also opens new perspectives for the international facility FAIR (Facility for Antiproton and Ion Research), which is currently under construction in Darmstadt. (CP)
