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An unexpected enhancement in the $\gamma$-ray strength function, as comparedto the low energy tail of the Giant Dipole Resonance (GDR), has been observedfor Sc, Ti, V, Fe and Mo isotopes for $E_\gamma<4$ MeV. This enhancement wasnot observed in subsequent analyses on Sn isotopes, but a Pygmy DipoleResonance (PDR) centered at $E_\gamma\approx8$ MeV was however detected. The$\gamma$-ray strength functions measured for Cd isotopes exhibit both featuresover the range of isotopes, with the low-energy enhancement decreasing- and PDRstrength increasing as a function of neutron number. This suggests atransitional region for the onset of low-energy enhancement, and also that thePDR strength depends on the number of neutrons. The $\gamma$-ray strengthfunctions of $^{105-108}$Pd have been measured in order to further explore theproposed transitional region. Experimental data were obtained at the OsloCyclotron Laboratory by using the charged particle reactions ($^{3}$He,$^{3}$He$^{\prime}\gamma$) and ($^{3}$He, $\alpha$$\gamma$) on $^{106,108}$Pdtarget foils. Particle$-\gamma$ coincidence measurements provided informationon initial excitation energies and the corresponding $\gamma$-ray spectra,which were used to extract the level densities and $\gamma$-ray strengthfunctions according to the Oslo method. The $\gamma$-ray strength functionsindicate a sudden increase in magnitude for $E_{\gamma}>4$ MeV, which isinterpreted as a PDR centered at $E_{\gamma}\approx8$ MeV. An enhanced$\gamma$-ray strength at low energies is also observed for $^{105}$Pd, which isthe lightest isotope measured in this work. Further, the results correspond andagree very well with the observations from the Cd isotopes, and support thesuggested transitional region for the onset of low-energy enhancement withdecreasing mass number. The neutron number dependency of the PDR strength isalso evident.

Pygmy resonance and low-energy enhancement in the $\gamma$-ray strength functions of Pd~isotopes