A scientific research team measured the nuclear charge radii of stable isotopes silicon-28, silicon-29, and silicon-30 at FRIB’s BEam COoler and LAser spectroscopy (BECOLA) facility. The team also measured the charge radii of the unstable isotope silicon-32 and compared it to that of its mirror nucleus, argon-32. These measurements will unlock new insights and expand our knowledge of nuclei and nuclear matter. The team recently published its findings in Physical Review Letters (“Nuclear charge radii of silicon isotopes”).
Ronald Fernando Garcia Ruiz, associate professor of physics at the Massachusetts Institute of Technology, and Kei Minamisono, senior scientist at FRIB and lead operator at the BECOLA facility, co-led the team. The lead author of the study was Kristian König. König, a researcher at the University of Darmstadt in Germany, was a postdoctoral researcher at FRIB. FRIB is the only accelerator-based DOE-SC user facility on a university campus. FRIB is operated by Michigan State University (MSU) to support the mission of the DOE-SC Office of Nuclear Physics as one of 28 DOE-SC user facilities.
Measurements key to nuclear theories
The nuclear force binds protons and neutrons in an atomic nucleus. It plays a critical role in the formation of stars and elements found in the universe. Yet, this force continues to challenge physicists from around the world. Its complex behavior makes it difficult to develop a broad nuclear theory that precisely predicts crucial observed properties of nuclei. One such crucial nuclei property is the nuclear charge radius. The nuclear charge radius is a measurement of the size and structure of an atomic nucleus, its proton distribution.
It is unknown whether nuclear theories that precisely describe nuclei can also describe the properties of nuclear matter in extreme conditions, like neutron stars. To answer these questions, scientists must measure the charge radii for nuclei with large proton-to-neutron imbalances.
Team’s findings match previous studies
The team extracted silicon monoxide molecules and fostered a molecular breakup. This allowed them to perform precision laser spectroscopy on silicon atoms. Using FRIB’s BECOLA facility, the team measured the nuclear charge radius of the unstable isotope silicon-32. This served as a test for several abstract predictions, including ab initio calculations. Ab initio calculations aim to calculate nuclear properties, starting from the underlying microscopic interactions of protons and neutrons. In this case, the experiment's findings matched the predictions of the ab initio lattice effective field theory approach. This study was carried out by a global research team that included Dean Lee, professor of physics at FRIB and in MSU’s Department of Physics and Astronomy and head of the Theoretical Nuclear Science department at FRIB. The team also included Yuan-Zhuo Ma, postdoctoral research associate at FRIB.
The charge radii of silicon-32 was compared to that of its mirror nucleus, argon-32. Argon-32 has protons and neutrons opposite to those of silicon-32. The charge radii difference between the two was used to constrain a parameter crucial for explaining the physics of astrophysical objects such as neutron stars. The constraint was found using a theory introduced by Alex Brown, professor of physics at FRIB and in MSU's Department of Physics and Astronomy. The team’s findings agree with results from independent experiments such as gravitational wave observations.
This material is based upon work supported by the National Science Foundation and the U.S. Department of Energy.
Michigan State University operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. Hosting what is designed to be 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.