Researchers from the University of Liverpool are part of a collaborative international project that has made significant advances in understanding the atomic structure of superheavy elements—those that lie at the far end of the periodic table. These elements, including fermium (element 100) and nobelium (element 102), do not occur naturally. Instead, they must be synthesized in laboratories using complex nuclear reactions in particle accelerators or reactors. The team’s findings, published in Nature, provide insight into what happens at the limits of neutron and proton numbers and offer clues to where the periodic table might ultimately end.
In this study, the research team focused on measuring the nuclear properties of different isotopes of nobelium and fermium, specifically examining how nuclear size changes with varying numbers of neutrons. This work was achieved through the use of laser spectroscopy, a technique allowing scientists to study the atomic structure and nuclear shape of individual isotopes by analyzing the way light interacts with them. By measuring the nuclear radii of several isotopes, the researchers gained a deeper understanding of how nuclear structure evolves as atomic mass increases, particularly in elements near the “superheavy” region of the periodic table.
A fundamental discovery from this study is that the nuclear size trends in these superheavy elements deviate from those observed in lighter regions of the periodic table. Normally, as neutrons are added across a nuclear shell, abrupt changes in nuclear radius are observed—a phenomenon known as a “kink.” This effect is associated with the nuclear shell structure, which stabilizes certain configurations of protons and neutrons in much the same way as electron shells influence the chemical properties of elements. However, in the case of nobelium and fermium, the researchers found that the nuclear radius changed more smoothly across the neutron shell closure, indicating that the nuclear shell effects become less pronounced. In this region, the nuclei begin to resemble a “deformed liquid drop,” meaning they behave more fluidly and are less influenced by discrete shell structures. This suggests that the added nucleons (neutrons and protons) exert less individual influence as the atomic number increases, which could have implications for the stability of future superheavy elements.
The University of Liverpool’s Department of Physics played a key role in this research. Professor Bradley Cheal and Dr. Charlie Devlin led the nobelium experiments, contributing their expertise in laser spectroscopy to probe nobelium isotopes and measure their nuclear hyperfine structures. To perform these measurements, the team produced nobelium isotopes by capturing decay products of another element, lawrencium. These atoms were isolated from a beam of reaction products and subsequently heated, ionized, and detected based on their unique alpha decay signatures. This innovative approach allowed the researchers to observe the atomic structure of nobelium in unprecedented detail, setting the foundation for future studies on even heavier elements.
Professor Cheal, who serves as co-spokesperson for the nobelium experiments, emphasized the importance of understanding what happens at the boundaries of neutron and proton numbers, as it can help scientists predict where the periodic table might end. Cheal notes that their expertise in laser spectroscopy, which has historically been used to study naturally occurring radioactive isotopes, proved invaluable for examining these artificially produced superheavy elements. This work builds on previous research published by Professor Cheal in Nature in 2016, marking the first time laser spectroscopy was successfully applied to nobelium. Over the years, this capability has expanded, allowing researchers to not only explore nuclear structure but also precisely determine atomic properties like ionization potential, an essential characteristic that had never before been measured in nobelium, which has no stable, naturally occurring isotopes.
By advancing our understanding of superheavy elements, this study could pave the way for future discoveries at the outer edges of the periodic table. The smooth trend observed in the nuclear radii of fermium and nobelium suggests that, as researchers approach even heavier elements, the nuclear shell effects may continue to diminish. This may imply that these superheavy nuclei could become increasingly unstable or take on entirely new properties that have not been observed in lighter elements.
The findings contribute to a broader field of research into nuclear structure and stability, helping scientists identify potential upper limits to the periodic table. By combining nuclear physics with cutting-edge laser spectroscopy, this study exemplifies how collaborative research can drive new insights into the fundamental properties of matter. The University of Liverpool and its partners are likely to continue exploring these boundaries, using the latest technology to probe the characteristics of superheavy elements and unravel the mysteries of atomic structure at the extremes.
Source: University of Liverpool