Rigidity of the Doubly-Magic 100Sn Core

Abstract: The doubly magic nucleus 100Sn represents a crucial test for the nuclear shell model, which is a widely used model for a quantitative description of nuclei. Excited states in 100Sn are experimentally not accessible with present day techniques, but it is possible to gain insight into its structure by studying the properties of excited states of neighboring nuclei. These nuclei are by themselves very interesting since they enable the determination of single-particle energies and two-body matrix elements, which are the basic parameters of the shell model. Excited states of nuclei in the vicinity of 100Sn have been studied in a series of experiments using three different experimental setups. The first experiment utilized the NORDBALL detector array where for the first time excited states in the Tz=-3/2 nuclei 99Cd and 101In were identified. In addition, the level schemes of several nuclei with previously known excited states were significantly extended. The nuclei 103In, 105In, 107In and 109In are described in this thesis. The following experiments used a recoil catcher setup in combination with two EUROBALL cluster detectors, where for the first time excited states in the Tz=-1 nuclei 98Cd and 102Sn were identified. These two nuclei are now the nearest neighbors of 100Sn with known excited states. The last three experiments used the Fragment Mass Analyzer at Argonne National Laboratory to study 102Sn in more detail. In all experiments a beam of 58Ni ions was used to bombard targets of 46Ti and 50Cr. The experimental level schemes of 98Cd, 99Cd, 102Sn and 103In are well reproduced by the shell model calculations. Isomeric states were found in 98Cd, 99Cd and 102Sn and their half-lives were measured. The isomeric states in all three nuclei decay via low energy E2 transitions. The deduced E2 transition rates lead to controversial results for the proton and neutron effective charges, which are also basic shell model parameters. In 102Sn the measured B(E2;6+-4+) value leads to a neutron effective charge of 2.0+0.5-0.3e, which is a fairly large value. The analysis of the isomeric 17/2+ state in 99Cd suggests a proton effective charge of 1.4(1) e, whereas the measured B(E2;8+-6+) value in 98Cd requires a proton effective charge of only 0.93+0.14-0.10e. The small proton effective charge in 98Cd is very surprising, since it is even smaller than the bare proton charge. In all other nuclei studied so far, the effective charges are larger than the bare nucleon charges, and usually the neutron effective charge is lower than the proton effective charge. The large neutron effective charge points to a softness of the 100Sn core with respect to quadrupole shape changes, whereas the two proton holes weakly polarize the core in 99Cd and not at all in 98Cd.