No abstract

The very neutron-rich oxygen isotopes 25

The degree of collectivity in the neutron-rich nuclei 32 Mg and 34 Mg has been determined via intermediateenergy Coulomb excitation in inverse kinematics. Measured energies of the first excited 2 + states and reduced electric quadrupole transition probabilities B(E2; 0 + g.s. → 2 + 1 ) are presented for 32 Mg and 34 Mg. The results agree with previous measurements and confirm the placement of both nuclei within the "island of inversion."

The 112,120 Sn(γ, γ ) reactions below the neutron separation energies have been studied at the superconducting Darmstadt electron linear accelerator S-DALINAC for different endpoint energies of the incident bremsstrahlung spectrum. Dipole strength distributions are extracted for 112 Sn up to 9.5 MeV and for 120 Sn up to 9.1 MeV. A concentration of dipole excitations is observed between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays to excited states is estimated in a statistical model. A fluctuation analysis is applied to the photon scattering spectra to extract the amount of the unresolved strength hidden in background due to fragmentation. The strength distributions are discussed within different model approaches such as the quasiparticle-phonon model and the relativistic time blocking approximation allowing for an inclusion of complex configurations beyond the initial particle-hole states. While a satisfactory description of the fragmentation can be achieved for sufficently large model spaces, the predicted centroids and total electric dipole strengths for stable tin isotopes strongly depend on the assumptions about the underlying mean field.