Level densities and $\gamma$-ray strength functions in ${}^{170,171,172}$Yb

Abstract

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

8 pages, 7 figures