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

Background: An unexpected enhancement in the $\ensuremath{\gamma}$-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 ${E}_{\ensuremath{\gamma}}l4$ MeV. This enhancement was not observed in subsequent analyses on Sn isotopes, but a pygmy dipole resonance (PDR) centered at ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}8$ MeV was however detected. The $\ensuremath{\gamma}$-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 $\ensuremath{\gamma}$-ray strength functions of $^{105--108}\mathrm{Pd}$ 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 $(^{3}\mathrm{He},\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{He}{}^{\ensuremath{'}}\ensuremath{\gamma})$ and ($^{3}\mathrm{He},\phantom{\rule{0.16em}{0ex}}\ensuremath{\alpha}\ensuremath{\gamma}$) on $^{106,108}\mathrm{Pd}$ target foils. Particle-$\ensuremath{\gamma}$ coincidence measurements provided information on initial excitation energies and the corresponding $\ensuremath{\gamma}$-ray spectra, which were used to extract the level densities and $\ensuremath{\gamma}$-ray strength functions according to the Oslo method.Results: The $\ensuremath{\gamma}$-ray strength functions indicate a sudden increase in magnitude for ${E}_{\ensuremath{\gamma}}g4$ MeV, which is interpreted as a PDR centered at ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}8$ MeV. An enhanced $\ensuremath{\gamma}$-ray strength at low energies is also observed for $^{105}\mathrm{Pd}$, which is the lightest isotope measured in this work.Conclusions: A PDR is clearly identified in the $\ensuremath{\gamma}$-ray strength functions of $^{105--108}\mathrm{Pd}$, and a low-energy enhancement is observed for $^{105}\mathrm{Pd}$. 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.