What is the endpoint of the periodic table of chemical elements, and how do heavy elements come into being? A team of international researchers has made strides in answering these questions through experiments at the GSI/FAIR accelerator facility and Johannes Gutenberg University Mainz. They explored the atomic nucleus structure of fermium (element 100) with varying neutron counts. Employing advanced laser spectroscopy techniques, they monitored changes in the nuclear charge radius, discovering a consistent increase with the addition of neutrons. This suggests that localized nuclear shell effects have diminished impact on the nuclear charge radius within these heavy nuclei. Their findings were published in the scientific journal Nature.
Elements heavier than uranium (element 92), like fermium (element 100), are not found naturally in the Earth’s crust and must be synthesized artificially to be studied. They act as a connection between the heaviest naturally occurring elements and the so-called superheavy elements, starting from element 104. The stability of superheavy elements is attributed to quantum mechanical shell effects, contributing about two thousandths of the total nuclear binding energy. This small fraction is crucial for counteracting the repulsive forces among positively charged protons.
The quantum mechanical phenomena influencing the atomic nucleus, composed of protons and neutrons, are described by the nuclear shell model. Just as complete electron shells provide chemical stability and inertness to atoms, nuclei that have filled nuclear shells (which consist of “magic” numbers of protons and neutrons) display greater stability. This results in higher nuclear binding energies and prolonged lifetimes. In lighter nuclei, filled nuclear shells are known to impact trends in nuclear charge radii as well.
Laser spectroscopy techniques enable the analysis of minor alterations in atomic structures, yielding insights into nuclear attributes such as the nuclear charge radius, which reflects the distribution of protons within the nucleus. Investigations on various atomic nuclei of the same element but with different neutron counts have suggested a gradual increase in this radius unless a magic number is crossed—where a noticeable change occurs in that trend at the shell closure. This phenomenon has been documented for lighter, spherical atomic nuclei up to lead.
New insights into heavy nucleus structure
“Through a laser-based approach, we examined fermium atomic nuclei consisting of 100 protons and between 145 and 157 neutrons. We aimed to understand the impact of quantum mechanical shell effects on the nuclei’s size. This research allowed us to gain fresh perspectives on the structure of these nuclei, particularly around the known shell effect at neutron number 152,” explains Dr. Sebastian Raeder, the experiment’s spokesperson at GSI/FAIR. “At this neutron number, we had previously recognized the signature of a neutron shell closure reflected in nuclear binding energy trends. The shell effect’s strength was ascertained through precise mass measurements conducted at GSI/FAIR in 2012. Given that mass is energy according to Einstein, these measurements hinted at the additional binding energy contributed by the shell effect. Nuclei near neutron number 152 serve as ideal subjects for further studies, as they tend to be more oblong rather than spherical, allowing the numerous protons within to be spaced farther apart compared to a spherical nucleus.”
An international collaboration involving 27 institutes from seven countries investigated fermium isotopes with lifespans ranging from seconds to hundreds of days. They utilized various production methods for these isotopes and advancements in the laser spectroscopy techniques used. Some short-lived isotopes were generated at the GSI/FAIR accelerator facility, yielding only a handful of atoms per minute for experimentation. A specialized laser spectroscopy method, initially developed for nobelium isotopes, was employed to analyze the produced nuclei. These nuclei were stopped in argon gas and gained electrons to form neutral atoms, which were then examined with laser light.
The neutron-rich, long-lived fermium isotopes (fermium-255, fermium-257) were synthesized in tiny amounts at Oak Ridge National Laboratory in Oak Ridge, USA, and at the Institut Laue-Langevin in Grenoble, France. Their samples were radiochemically prepared at Johannes Gutenberg University Mainz (JGU) and then evaporated in a reservoir for laser light analysis in a vacuum.
The appropriately wavelength laser light excites an electron in the fermium atom to a higher energy level, eventually removing it from the atom entirely, creating a fermium ion that can be efficiently detected. The specific energy required for this stepwise ionization process varies depending on the neutron number. These subtle changes in excitation energy were measured to gain information about the atomic nuclei’s size variations.
Dominance of macroscopic properties
The conducted investigations provided insights into the fluctuations of the nuclear charge radius in fermium isotopes across neutron number 152, revealing a consistent and gradual increase. A comparison of experimental results with calculations performed by international collaborators using contemporary theoretical nuclear physics models allowed for interpretations of the physical phenomena at play. Regardless of the different calculation approaches, all models aligned well with each other and with the experimental findings.
“Our experimental observations and their interpretations using modern theoretical frameworks indicate that nuclear shell effects exert limited influence on the nuclear charge radii of fermium nuclei, in contrast to their significant effects on the binding energies of these nuclei,” states Dr. Jessica Warbinek, who was a doctoral student at GSI/FAIR and JGU during the study and the publication’s lead author. “These results validate theoretical expectations that local shell effects from individual neutrons and protons diminish in importance as nuclear mass increases. Instead, influences predominant result from the collective behavior of all nucleons, viewing the nuclei more like a charged liquid drop.”
The methodological advancements achieved pave the way for further laser spectroscopic explorations of heavy elements around and beyond neutron number 152, marking progress toward a clearer understanding of the stabilization processes in heavy and superheavy elements. Ongoing advancements promise that forthcoming investigations will uncover subtle effects of nuclear shell structure that are central to the existence of the heaviest known elements.
