New Insights into Nuclear Shell Structure via Beta-Decay Half-Life Measurements

An international team of researchers has systematically measured the β-decay half-lives of 40 nuclei near calcium-54, providing key experimental data for understanding the structure of extremely neutron-rich nuclei. The study, published in Physical Review Letters, was led by researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences, in collaboration with institutions including RIKEN in Japan and Peking University. Atomic nuclei exhibit exceptional stability when the proton (Z) or neutron (N) number reaches certain "magic numbers," such as 2, 8, 20, 28, 50, 82, or 126. The shell model successfully explained these magic numbers by introducing spin-orbit coupling, a contribution for which M. Mayer and J. Jensen were awarded the Nobel Prize in Physics in 1963. However, recent studies have revealed that traditional magic numbers may vanish and new ones may emerge in regions far from the stability line. For instance, in neutron-rich calcium isotopes, N=32 and 34 have been experimentally confirmed to exhibit subshell effects. Investigating subshell closures and experimental challenges One important question is how these subshell closures at N=32 and 34 evolve in lighter isotopic chains, such as those of potassium and chlorine. "Due to the extremely low production yields of nuclei in this region, it is extremely challenging to investigate the evolution of the nuclear shell structure using traditional methods based on mass measurement or gamma (γ)-ray spectroscopy," said Zeng Quanbo, a Ph.D. student from IMP and first author of this study. To overcome this challenge, the researchers proposed that β-decay half-lives could serve as a sensitive experimental probe for the evolution of single-particle orbitals in this region. They conducted their experiment at the Radioactive Isotope Beam Factory (RIBF) at RIKEN in Japan. Key findings and implications The researchers precisely measured the β-decay half-lives of 40 nuclei around calcium-54. Among these, the half-lives of 10 nuclei were measured for the first time, and the precision for five others was significantly improved. They discovered two notable features for the first time. First, a significant drop in the half-life of potassium-54 indicates a subshell effect at N=34, an interpretation supported by shell model calculations. Second, the half-life of chlorine-48 was notably shorter than those of its neighboring isotopes. The researchers suggested that this anomaly stems from the significant neutron excitations across the N=32 subshell in chlorine-48. Further theoretical analysis revealed that this cross-shell excitation primarily results from strong configuration mixing, rather than a significant weakening of the N=32 subshell gap. Future prospects with new accelerator facilities