Level densities and γ-ray strength functions in ^170,171,172Yb

Level densities and radiative strength functions in ¹⁷¹Yb and ¹⁷⁰Yb nuclei have been measured using the ¹⁷¹Yb(³He,³He^′γ)¹⁷¹Yb and ¹⁷¹Yb(³He,αγ)¹⁷⁰Yb reactions. New data on ¹⁷¹Yb are compared to previous measurements for ¹⁷¹Yb from the ¹⁷²Yb(³He,αγ)¹⁷¹Yb reaction. The systematics of level densities and radiative strength functions in ^170,171,172Yb are established. The entropy excess in ¹⁷¹Yb relative to the even-even nuclei ^170,172Yb due to the unpaired neutron quasiparticle is found to be approximately 2k_B. Results for the radiative strength function from the two reactions lead to consistent parameters characterizing the “pygmy” resonances. Pygmy resonances in the ^170,172Yb populated by the (³He,α) reaction appear to be split into two components for both of which a complete set of resonance parameters is obtained.