A group of scientists has successfully explored the limits of what is referred to as the “island of stability” found within superheavy nuclides by investigating the rutherfordium-252 nucleus. This nucleus is now recognized as the shortest-lived superheavy nucleus identified to date.
Researchers from GSI/FAIR, Johannes Gutenberg University Mainz, and the Helmholtz Institute Mainz have made advancements in understanding the “island of stability” among superheavy nuclides by studying the rutherfordium-252 nucleus, now acknowledged as the shortest-lived superheavy nucleus known. Their results were published in the journal Physical Review Letters, where they were highlighted as an “Editor’s suggestion.”
The strong nuclear force binds protons and neutrons within atomic nuclei. Nevertheless, since protons carry a positive charge and repel one another, nuclei that have too many protons can become unstable—this complicates the creation of new superheavy elements. Certain combinations of protons and neutrons, termed “magic numbers,” enhance nuclear stability. Theoretical investigations dating back to the 1960s propose that an “island of stability” exists within a broader range of unstable superheavy nuclei, where some nuclei could have extraordinarily long lifetimes, potentially nearing the age of the Earth.
This theory has been supported by observations of increased half-lives in the heaviest known nuclei as they approach a theoretical magic number of 184 neutrons. However, the precise positioning of this island’s peak, its height (which indicates the maximum expected half-life), and its overall dimensions are still unclear. Researchers from GSI/FAIR, Johannes Gutenberg University Mainz, and the Helmholtz Institute Mainz have now progressed toward defining this island by identifying the shortest-lived superheavy nucleus to date, located on the island’s edge, rutherfordium (Rf, element 104).
To detect superheavy nuclei, they must have a minimum lifespan of about one millionth of a second, which makes it difficult to observe very short-lived superheavy nuclei near the unstable region. However, an alternative approach exists: some excited states can exhibit longer lifetimes due to quantum effects, allowing for the study of these fleeting nuclei. Dr. Khuyagbaatar Jadambaa, the lead author from the GSI/FAIR team focused on superheavy elements, states, “These longer-lived excited states, called isomers, are frequently observed in deformed superheavy nuclei, according to my calculations. They therefore enhance our understanding of the stability landscape with ‘clouds of stability’ set against a backdrop of instability.”
The team from Darmstadt and Mainz put these theories to the test by searching for the previously unobserved nucleus Rf-252. They utilized a robust titanium-50 beam at the GSI/FAIR UNILAC accelerator to combine titanium nuclei with lead nuclei on a target foil. After approximately 0.6 microseconds, the resulting fusion products were categorized in the TransActinide Separator and Chemistry Apparatus (TASCA) and implanted into a silicon detector. This detector recorded their implantations and subsequent decay.
The researchers successfully detected 27 atoms of Rf-252, each undergoing fission with a half-life of 13 microseconds. A rapid digital data system developed by GSI/FAIR’s Experiment Electronics department enabled the tracking of electrons emitted after the implantation of the isomer Rf-252m and its decay to the ground state, noting three occurrences. In each case, a fission event transpired within 250 nanoseconds. Based on this data, the team concluded that Rf-252’s ground state possesses a half-life of 60 nanoseconds, marking it as the shortest-lived superheavy nucleus known to date.
“This discovery significantly reduces the previously established lifetime limits of the heaviest nuclei by nearly two orders of magnitude, shortening intervals that are too brief for direct measurement without appropriate isomer states. Our results set a new benchmark for future investigations into phenomena related to such isomer states, inverted fission stability where excited states are more stable than ground states, and the isotopic limits for the heaviest nuclei,” explains Professor Christoph E. Düllmann, head of GSI/FAIR’s superheavy element chemistry research department.
Upcoming experimental efforts will look into isomeric states displaying inverted fission stability in seaborgium (Sg, element 106). This research will also aim to create Sg isotopes with lifetimes under a microsecond, sharpening the isotopic limits further. Additionally, these findings open up new possibilities for the international FAIR (Facility for Antiproton and Ion Research), which is currently under development in Darmstadt.
