International collaboration sheds light on structure of superheavy nuclei

13 November 2024

Recently, a large research collaboration led by the GSI Helmholtz Centre for Heavy Ion Research in Germany united an international team of scientists to gain deeper insights into the role of nuclear shell effects in the heaviest elements. The team, which included Witek Nazarewicz, John A. Hannah Distinguished Professor of Physics and chief scientist at FRIB, and Alyssa Gaiser, assistant professor of chemistry at FRIB and in MSU’s Department of Chemistry, published a paper in Nature demonstrating the diminished roles of shell effects on superheavy nuclei with atomic numbers greater than 100, such as fermium. 

Read the full GSI press release.

Scientists use nuclear shell theory to chart how the number of electrons and neutrons in each isotope can influence its behavior. At certain numbers of protons and neutrons, these particles form closed “shells” that increase their stability. Physicists call these intervals magic numbers. Nuclei with a magic number of protons or neutrons are more stable than other nuclei. Currently, the most widely recognized magic numbers are 2, 8, 20, 28, 50, 82, and 126.

Other isotopes can be “doubly magic,” meaning they have magic numbers of both protons and neutrons, resulting in even more stability. Physicists value these isotopes for their enhanced stability, which allows for more detailed exploration of the nuclear many-body problem and a broader investigation into the nature of matter.

“When we study heavier elements, such as fermium or nobelium, we see that the charge density fluctuations that we see in lighter elements are very much reduced in these superheavy elements,” Nazarewicz said. “These elements act almost like a water droplet—their densities remain stable much longer inside the nucleus.” 

While magic and doubly-magic isotopes are a major area of interest for nuclear physicists, they are harder to study higher up the periodic table. Currently, lead-208 is the largest isotope confirmed to be doubly magic. But unlike lead-208, superheavy isotopes do not naturally occur and have to be created in laboratories. Researchers use particle accelerators to fire high-energy particles at certain isotopes, creating a short-lived, heavier element in the process. Facilities like GSI can produce these exotic elements. Theory and computational modelling complement the experiments.

Recently, physicists at GSI used their recently developed spectroscopy method, Radiation Detected Resonance Ionization Spectroscopy (RADRIS), to study charge radii of fermium nuclei. The team initially focused on creating fermium isotopes 245 and 246 and indirectly studied fermium-248, 249, 250, and 254. The team also ran complementary experiments using heavier nobelium isotopes to compare its findings with another similar isotope chain.

Gaiser worked with researchers from Helmholtz-Institut Mainz and GSI to perform separation and purification experiments to remove unwanted elements from the fermium sample used in the experiment. Similar to what will occur at FRIB with its isotope-harvesting program, the researchers ran the sample through a resin—a solid that has varying preference for different ions—in order to get a purer sample of the desired product, in this case, fermium.

“This was an awesome opportunity to work with the full fermium team, collaborating on the edge of the periodic table where nuclear and radiochemistries meet,” said Gaiser. “I am very excited to see this high-impact work being recognized.”

Nazarewicz and other collaborators paired these experiments with theoretical models that could predict shell effects. The team found consistent results between their models and the experimental data. 

“As we go to heavier nuclei, we see the pattern of shell effects getting weaker,” Nazarewicz said. “There are so many levels at these large numbers of particles that the energy gaps in these nuclei become smaller.”

According to Nazarewicz, the findings have implications for future superheavy nuclei research, and researchers’ investigations into even heavier, more exotic nuclei have only just begun. Nuclear theory predictions indicate that superheavy nuclei consisting of around 184 neutrons could transition from a region of highly unstable, short-lived isotopes to a region of enhanced stability. 

“One expects this peninsula of neutron-rich superheavy nuclei to show longer half-lives, and that is an exciting challenge for both theory and experiment,” Nazarewicz said. “This is a key line of inquiry for our research: going to more shell-stabilized superheavy species.”

Read the full GSI press release.

Eric Gedenk is a freelance science writer. 

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), with financial support from and furthering the mission of the DOE-SC Office of Nuclear Physics. Hosting the most powerful heavy-ion accelerator, FRIB enables scientists to make discoveries about the properties of rare isotopes in order to better understand the physics of nuclei, nuclear astrophysics, fundamental interactions, and applications for society, including in medicine, homeland security, and industry.

The U.S. Department of Energy Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of today’s most pressing challenges. For more information, visit energy.gov/science.

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