] 2 0 Ju n 20 07 Low - energy Dipole Excitations in Nuclei at the N = 50 , 82 and Z = 50 Shell Closures as Signatures for a Neutron Skin

Abstract. Low-energy dipole excitations have been investigated theoretically in N=50 Sr and Zr, several N=82 isotones and the Z = 50 Sn isotopes. For this purpose a method incorporating both HFB and multi-phonon QPM theory is applied. A concentration of one-phonon dipole strength located below the neutron emission threshold has been calculated in these nuclei. The analysis of the corresponding neutron and proton dipole transition densities allows to assign a genuine pattern to the low-energy excitations and making them distinct from the conventional GDR modes. Analyzing also the QRPA wave functions of the states we can identify these excitations as Pygmy Dipole Resonance (PDR) modes, recently studied also in Sn and N=82 nuclei. The results for N = 50 are exploratory for an experimental project designed for the bremsstrahlung facility at the ELBE accelerator.