Low-lying dipole excitations in vibrational nuclei: The Cd isotopic chain studied in photon scattering experiments

Abstract

High-resolution nuclear resonance fluorescence experiments (NRF) were performed on $^{110,111,112,114,116}\mathrm{Cd}$ at the bremsstrahlung facility of the 4.3-MV Dynamitron accelerator in Stuttgart to study the low-lying dipole strength distributions in these vibrational nuclei. Numerous excited states, most of them previously unknown, were observed in the excitation energy range up to 4 MeV. Detailed spectroscopic information has been obtained on excitation energies, spins, decay widths, decay branchings, and transition probabilities. For states in the even-even isotopes $^{110,112,114,116}\mathrm{Cd}$, parities could be assigned from linear polarization measurements. Together with our previous results for $^{108,112,113,114}\mathrm{Cd}$ from NRF studies without polarization measurements, systematics was established for the dipole strength distributions of the stable nuclei within the Cd isotopic chain. The results are discussed with respect to the systematics of $E1$ two-phonon excitations and mixed-symmetry states in even-even nuclei near the $Z=50$ shell closure and the fragmentation of these excitation modes in the odd-mass Cd isotopes.