Photo response of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Dy</mml:mi><mml:mprescripts/><mml:none/><mml:mn>164</mml:mn></mml:mmultiscripts></mml:math>
O. Papst, V. Werner, J. Isaak, N. Pietralla, T. Beck, C. Bernards, M. Bhike, N. Cooper, B. P. Crider, U. Friman-Gayer, J. Kleemann, Krishichayan, B. Löher, F. Naqvi, E. E. Peters, F. M. Prados-Estévez, R. S. Ilieva, Thomas J. Ross, D. Savran, W. Tornow, J. R. Vanhoy
Abstract
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 $^{164}\mathrm{Dy}$ between 4.0--7.7 MeV.Methods: The low-lying photoresponse of $^{164}\mathrm{Dy}$ has been investigated using the method of nuclear resonance fluorescence using a quasimonochromatic linearly polarized $\ensuremath{\gamma}$-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.