In two recent experiments at the accelerator facility at GSI Darmstadt, groups led by Reiner Krücken of the Technical University Munich and Rituparna Kanungo of St Mary’s University, Halifax, in collaboration with international teams, revealed further evidence for new magic shell closures at the limit of nuclear existence in the neutron-rich isotopes 24O and 54Ca.
The shell structure of atomic nuclei with its magic numbers (2, 8, 20, 28, 50, 82, 126) for protons and neutrons corresponding to an enhanced binding is a cornerstone in understanding the structure and dynamics of nuclei. The explanation of the magic numbers in 1949 as a result of the strong spin-orbit interaction was awarded the Nobel Prize in 1963. Until recently these magic numbers were assumed to remain universal across the whole nuclear chart, but mounting experimental evidence and theoretical predictions indicate that the shell gaps associated with the numbers are not universal. Instead they can change locally under the influence of variations in the effective interaction of the nucleons in the nucleus. Such changes in the shell structure can have dramatic effects on the production of elements in stellar explosions.
The experiments used precise momentum measurements to study the dynamics of reactions where a single neutron is knocked out from a neutron-rich nucleus. The results provide crucial information about the energies and occupation of the neutron single-particle orbitals in the respective nuclei. In the experiment with 24O (8 protons and 16 neutrons), the measurements revealed the spherical nature of the shell closure for the 16 neutrons, thus establishing 24O as a doubly magic nucleus, with a new magic number of 16 (R Kanungo et al. 2009). The second experiment studied one-neutron knockout in 56Ti (22 protons and 34 neutrons). It confirmed that shell-model calculations predicting a new shell closure in 54Ca (20 protons and 34 neutrons) correctly describe the single-particle structure in the neighbouring nucleus 55Ti (P Maierbeck et al. 2009).
The experiments were highly challenging because 24O and 56Ti form unstable radioactive beams, which can only be produced with a yield of a few particles a second, compared with the 109 ions a second that is typical of experiments with stable nuclei. The results also demonstrate the capability of the fragment separator, FRS, at GSI for high-precision momentum measurement with such extremely rare isotopes. This capability will be developed further in the near future at the Facility for Antiproton and Ion Research in Darmstadt.