Dipole response ofSr88up to the neutron-separation energy

Abstract

The dipole response of the magic $N=50$ nucleus $^{88}\mathrm{Sr}$ was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 9.0, 13.2, and 16.0 MeV. We identified 160 levels up to an excitation energy of 12 MeV. By using polarized photons linear polarizations of about 50 \ensuremath{\gamma} transitions were measured that enabled parity assignments to the corresponding states. In the energy range of 6--12 MeV we identified only one $M1$ transition; all other transitions have $E1$ character. Thus, $E1$ character was proven for 63% of the total dipole strength of the observed levels in the given energy range. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from $(\ensuremath{\gamma},n)$ data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of $E1$ strength at 6--11 MeV may be considered as an indication for a pygmy dipole resonance.