In-beam γ-ray spectroscopy of N=84 nuclei above Gd and the impact of the (πh11/2νh9/2)1+ attraction on their yrast lines

In in-beam experiments using the γ-spectrometers Nordball at the Niels Bohr Institute and Osiris at the Hahn-Meitner Institute we populated the four N = 84 isotones 151Ho, 152Er, 153Tm and 154Yb through compound evaporation reactions induced with medium HI beams of masses ranging from 28 to 56. All observed γ-transitions with intensities above 1% (5% in 154Yb) of the respective exit channel were placed in the level schemes, with spins established up to 10 MeV (8 MeV for 154Yb) and parties up to between 5 and 7 MeV. We have found that in these N = 84 nuclei the multi-valence particle configurations πhn11/2νf27/2 and πhn11/2νf7/2h9/2 are strongly populated in the yrast cascades, and we have observed them up to their maximum spins of 43/2- in Ho (at 4.8 MeV), 24+ in Er (7.5 MeV), 51/2- in Tm (6.9 MeV), and only up to 24+ in Yb, 0.5 MeV below the fully aligned 26+ state expected at 9.1 MeV. The πhnνf2 configuration forms smooth sections of the yrast line, while the more irregular yrast line of the πhnνfh character reflects the strong (νfh)8+ two-body attraction which gives rise to pronounced yrast line dips. We have also carried out full shell model calculations of these yrast lines, which are in excellent agreement with the experimental data. Since we take all dynamic input values, i.e. the two-body matrix elements and the single particle energies, from experiment, the calculations are free of any adjusted parameter. An interesting new result is the systematic identification in the πhn11/2νfh configuration of yrast states formed by specific proton couplings, where the strongly attractive (πh11/2νh9/2)1+ interaction is activated. As a consequence these states drop down to the yrast lines and in energy below the maximum aligned state of the next lower proton seniority. These seniority inverted yrast states were systematically identified in the four N = 84 nuclei, including two such states in 153T, the first N 84 nucleus where a second seniority inverted yrast state can be formed. (orig.)