Search for: All records

Background: Little data is available for the pygmy dipole resonance (PDR) in axially deformed nuclei. Photon scattering experiments are complicated by high level densities in the PDR region and the small energy difference of transitions to the ground state and to excited states. Purpose: We report on an experimental study of the low-energy dipole strength distribution of the well-deformed nucleus 164Dy between 4.0–7.7 MeV. Methods: The low-lying photoresponse of 164Dy has been investigated using the method of nuclear resonance fluorescence using a quasimonochromatic linearly polarized γ -ray beam in the energy range of 4.0–7.7 MeV in steps of 0.2 MeV. Results: For excitation energies between 4 MeV and 5 MeV, sufficiently low level densities allow for the identification of individual states, including level energies, reduced transition widths and branching ratios. Energy-averaged mean decay branching ratios, mean population ratios and partial absorption cross sections were determined above 5 MeV up to the neutron-separation threshold at 7.7 MeV. A Lorentzian-shaped enhancement of the partial photo absorption cross section followed by decays back to the ground-state band is found at 6.10(5) MeV with a width of 0.77(23) MeV. A comparison with results from complementary measurements is performed using the framework of the statistical model. Conclusions: The experimental results for the mean population ratios deviate systematically from the statistical model simulation by 30(6)%. However, they are in agreement within one standard deviation of the simulation. 

The size of a ΔK ¼ 0 M1 excitation strength has been determined for the first time in a predominantly axially deformed even-even nucleus. It has been obtained from the observation of a rare K-mixing situation between two close-lying Jπ ¼ 1þ states of the nucleus 164Dy with components characterized by intrinsic projection quantum numbers K ¼ 0 and K ¼ 1. Nuclear resonance fluorescence induced by quasimonochromatic linearly polarized γ-ray beams provided evidence for K mixing of the 1þ states at 3159.1(3) and 3173.6(3) keV in excitation energy from their γ-decay branching ratios into the ground-state band. The ΔK ¼ 0 transition strength of BðM1; 0 þ 1 → 1 þ K¼0 Þ ¼ 0.008ð1Þμ2 N was inferred from a mixing analysis of their M1 transition rates into the ground-state band. It is in agreement with predictions from the quasiparticle phonon nuclear model. This determination represents first experimental information on the M1 excitation strength of a nuclear quantum state with a negative R-symmetry quantum number.