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 E_γ<4 MeV. This enhancement was not observed in subsequent analyses on Sn isotopes, but a pygmy dipole resonance (PDR) centered at E_γ≈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,0.16em0ex³He^'γ) and (³He,0.16em0exαγ) 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 E_γ>4 MeV, which is interpreted as a PDR centered at E_γ≈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 ¹⁰⁵Pd. 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.