Dipole excitations to bound states in116Snand124Sn

Dipole transitions to bound states in the Sn nuclei ${}^{116}\mathrm{Sn}$ and ${}^{124}\mathrm{Sn}$ have been investigated by means of nuclear resonance fluorescence (NRF) using 12 MeV linearly polarized bremsstrahlung and unpolarized bremsstrahlung with different end point energies (4.1, 7.5, and 10 MeV). The measurements enable the determination in a completely model-independent way of reduced transition probabilities, multipolarities, and parities of the observed transitions. More than 150 new dipole ground state transitions have been identified. The observed dipole strength distribution displays for both isotopes a clear concentration around 6.5 MeV. For about half of the observed dipole excitations parities could be extracted. They all turned out to be $E1\mathrm{}$ excitations except for three tentative $M1\mathrm{}$ assignments. The NRF results are compared with data from tagged photon scattering experiments and quasiparticle phonon model calculations.