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[en] In the context of collectivity in the A=100 mass region N=52 isotones were investigated in detail and phonon excitations - especially the mixed-symmetry (MS) states - were identified. In order to investigate how states with this MS character evolve with increasing valence neutron number N=54 isotones were studied. While in ⁹⁶Mo the one-phonon MS state 2⁺_1,ms was identified, ⁹⁸Ru shows a breakdown of vibrational structure above the two-phonon triplet and no candidate for the 2⁺_1,ms state was assigned. The recently investigated N=54 isotone ¹⁰⁰Pd was explained well with an U_πν(5) fit within the IBM-2. Furthermore, a candidate for the one-phonon MS2⁺_1,ms state was determined. To do further investigations on the N=54 isotone ⁹⁸Ru - in particular regarding the breakdown of vibrational symmetry and the one-phonon MS excitation - an experiment was performed at the Cologne Tandem Accelerator using the HORUS spectrometer. The nuclei were populated by the reaction ⁹⁷Mo(³He,2n)⁹⁸Ru. By analysing the coincidence and γγ angular correlation data the level scheme was extended and clarified by determining spins, multipole mixing ratios and branching ratios. The low-energy excitations of positive parity are discussed and compared to theoretical expectations. The results are compared with those for other N=54 isotones.

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[en] Low-energy excitations of heavy even-even nuclei can be described well using the Interacting Boson Model (IBM). The supersymmetric extension of the IBM uses a symmetry between fermions and bosons and has already been successfully applied to describe low-energy excitations in the Au-Pt mass region. Using the Interacting Boson Fermion Fermion Model (IBFFM) predictions for ¹⁹⁸Hg have been derived in, considering two j=3/2 proton fermions and four neutron bosons with respect to the ²⁰⁸Pb shell closure. Referring to, this IBFFM configuration should describe excited two-quasiparticle and not low-lying ¹⁹⁸Hg states. We show that an equivalent description for ¹⁹⁸Hg can be achieved within the IBM-2 and that new experimental data shows quite a good matching to the theoretical predictions - also for the low-energy states.

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[en] The investigation of ¹⁹⁶Hg - especially the determination of level spins and multipole mixing ratios of γ transitions between low-energy states - is of great interest for testing the existence of a relation between the nucleus ¹⁹⁶Hg and the so-called magical quartet around ¹⁹⁴Pt. Within the U_ν(6/12) x U_π(6/4) extended supersymmetry, this quartet is supposed to be the supermultiplet being described the best. Similar to the quartet members ^194,195Pt and ^195,196Au, the nucleus ¹⁹⁶Hg can be described theoretically by seven IBA-2 particles to the shell closure, in particular by two j = 3/2 proton fermions and five sd neutron bosons as a two-fermions - five-bosons supermultiplet member. In order to obtain experimental data on ¹⁹⁶Hg a γγ angular correlation experiment was performed at the Tandem accelerator of the Institute for Nuclear Physics in Cologne. Using a ⁴He beam with an energy of 28 MeV impinging a ¹⁹⁴Pt target the reaction ¹⁹⁴Pt(α, 2n)¹⁹⁶Hg was induced. To analyze the γ decays of yrast and non-yrast ¹⁹⁶Hg states 13 high purity germanium detectors were mounted in the Horus cube γ-ray spectrometer, which allows the determination of level spins and multipole mixing ratios by the analysis of γγ angular correlations. We discuss first results of the experiment.