Pygmy resonance and low-energy enhancement in the γ-ray strength functions of Pd isotopes.

Background: An unexpected enhancement in the γ-ray strength function, as compared to the low-energy tail of the giant dipole resonance (GDR), has been observed for Sc, Ti, V, Fe, and Mo isotopes for Eγ < 4 MeV. This enhancement was not observed in subsequent analyses on Sn isotopes, but a pygmy dipole resonance (PDR) centered at Eγ ≈ 8 MeV was however detected. The γ-ray strength functions measured for Cd isotopes exhibit both features over the range of isotopes, with the low-energy enhancement decreasing and PDR strength increasing as a function of neutron number. This suggests a transitional region for the onset of low-energy enhancement, and also that the PDR strength depends on the number of neutrons. Purpose: The γ-ray strength functions of 105-108Pd have been measured in order to further explore the proposed transitional region. Method: Experimental data were obtained at the Oslo Cyclotron Laboratory by using the charged particle reactions (³He,³He'γ) and (³He,αγ) on 106,108Pd target foils. Particle-γ coincidence measurements provided information on initial excitation energies and the corresponding γ-ray spectra, which were used to extract the level densities and γ-ray strength functions according to the Oslo method. Results: The γ-ray strength functions indicate a sudden increase in magnitude for Eγ > 4 MeV, which is interpreted as a PDR centered at Eγ ≈ 8 MeV. An enhanced γ-ray strength at low energies is also observed for 105Pd, which is the lightest isotope measured in this work. Conclusions: A PDR is clearly identified in the γ-ray strength functions of 105-108Pd, and a low-energy enhancement is observed for Pd105. Further, the results correspond and agree very well with the observations from the Cd isotopes, and support the suggested transitional region for the onset of low-energy enhancement with decreasing mass number. The neutron number dependency of the PDR strength is also evident. [ABSTRACT FROM AUTHOR]