Researchers investigate isospin symmetry breaking in exotic nuclei using silicon-23

A team of researchers recently used the Low Energy Beam and Ion Trap (LEBIT) at FRIB to make a high-precision mass measurement of silicon-23, a short-lived, proton-rich atomic nucleus. Franziska Maier, a research associate at FRIB, led the experiment, coordinating a collaborative effort that included scientists from FRIB and Michigan State University’s Department of Physics and Astronomy. The team’s results were published in Physical Review C (Exploring isospin symmetry breaking in exotic nuclei: High-precision mass measurement of ²³Si and shell-model calculations of 𝑇=5/2 nuclei).

Using LEBIT, Maier and her team measured the mass of silicon-23 in the sd-shell—a region of the nuclear chart defined by the numbers of protons and neutrons in a nucleus. They determined a mass excess of 23,362.9 kiloelectron-volts (keV), with a margin of error of 5.8 keV, achieving a level of precision 20 times greater than in previous measurements.

This new data provides the most accurate measurement yet of the mass of certain atomic nuclei with more protons than neutrons. This helps improve the understanding of isospin, a concept that treats protons and neutrons as similar under the strong force, which holds atomic nuclei together. Isospin helps scientists study how these particles interact, especially in mirror nuclei—pairs with switched numbers of protons and neutrons.

The experiment also looked at binding energy and the Thomas-Ehrman shift, where proton energy levels are slightly higher than neutron ones due to repulsion between protons. By comparing their results with predictions from advanced nuclear models, the researchers found strong agreement. This supports the accuracy of current theories in explaining small but important differences between protons and neutrons inside atomic nuclei.

This material is based upon work supported by the U.S. Department of Energy (DOE) Office of Science, Office of Nuclear Physics. The work was conducted with the support of Michigan State University; the U.S. National Science Foundation; and the DOE Office of Science, Office of Nuclear Physics.

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. User facility operation is supported by the DOE-SC Office of Nuclear Physics as one of 28 DOE-SC user facilities.

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.