Yrast Properties of Dysprosium Isotopes in the Double Mid-Shell Region

The yrast bands of the even-even dysprosium isotopes ^164-174Dy are calculated up to high angular momentum 34h based on the projected shell model. Our calculations are in good agreement with the experiments quantitatively. They predict that an energy minimum of the first 2⁺ state in these isotopes exists around the neutron mid-shell N=104, implying the maximum collectivity in ₆₆¹⁷⁰Dy₁₀₄, however, the energy ratio R₄=E(4⁺₁)/E(2⁺₁) reaches a saturation at N=102. Meanwhile, the back-bending plots of these yrast bands are examined carefully. It is found that the sharpness of the back-bending has a gradual shift along the dysprosium isotopic chain, namely, the slope of the back-bending becomes steeper as the neutron number increases except for an irregularity of a decreasing dip angle in the double mid-shell nucleus ¹⁷⁰Dy. We suggest that ¹⁷⁰Dy undergoes a dual alignment with midshell high-j protons and neutrons aligning simultaneously at spin I≈16h, which probably results in the distinctive back-bending behavior.