Octupole collectivity and E1 moments in neutron rich nuclei

Sammanfattning: Spectroscopy of short-lived fission and reaction products has allowed a detailed structural study of neutron rich nuclei just above above 132Sn and 208Pb, respectively. The low-energy structures of 140Xe and 143Ba as well as a handful of neutron rich nuclei in the actinide region were examined. The excited levels were populated in β-decays of ISOL beams at the OSIRIS facility at Studsvik, and at the ISOLDE facility at CERN. At OSIRIS, a novel method for production of highly pure beams of iodine and bromium was implemented. Studies of the decays of these nuclei have until now been severely hampered by the strong isobaric contamination.The choice of nuclei to be studied was driven by the present lack of knowledge concerning octupole collectivity in general, and the associated electric dipole moments in particular. In order to be able to deduce correct electric dipole moments, the accurate βγγ(t) fast timing method was applied formeasurement of level half-lives in addition to the Multi Spectrum Scaling, γγ coincidence techniques and the measurement of internal conversion coefficients.In 140Xe predictions of a local quenching of the dipole moment were verified. Such quenching has been observed fortuitiously twice before, but never, as in the present case, in terms of a verification of theoretical calculations. The 3' state of this nucleus was found at a comparatively hi energy, indicating the rather sudden ending of the region of octupole collectivity above 132Sn. At 2457 keV a 5- state due to the coupling of an octupole phonon with a γ vibration was found. Previous evidence for the existence of such a state is scarce, or even inconclusive. The nearby nucleus 143Ba was also studied in detail. For the first time it was possible to rigorously assign positive parities to states in this nucleus, and the details of these states indicated an octupole deformed ground state. An enhanced dipole moment, typical for nuclei with octupole collectivity, was deduced from the measured level half-lives. However, comparison of β2 values from isotope shift measurements and from B(E2) values from the present study show a discrepancy which has to be explained in terms of enhancement of the values from the isotope shift measurements due to octupole deformation, Thus, the evidence is in conflict.Radium nuclei with mass 223,227 and 228, as well as 227Fr and 231Th were studied. For 227Fr a detailed decay-scheme, indicating the presence of octupole correltaions was established. Previously, no excited states were known. The results on 227Ra and 231Th showed that calculations with the quasi-particle plus phonon model can repdoduce the configuration mixing present. In addition, calculations based on a more rigid deformation failed to reproduce the absence of a major drop of E1 strength as deduced from the presently measured values in 228Ra.

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