$\gamma$-ray strength function and pygmy resonance in rare earth nuclei

The $\gamma$-ray strength function for $\gamma$ energies in the 1–7 MeV region has been measured for ${}^{161,162}\mathrm{Dy}$ and ${}^{171,172}\mathrm{Yb}$ using the ${(}^3\mathrm{He},\alpha \gamma )$ reaction. Various models are tested against the observed $\gamma$-ray strength functions. The best description is based on the Kadmenskiĭ, Markushev, and Furman $E1\mathrm{}$ model with constant temperature and the Lorentzian $M1\mathrm{}$ model. A $\gamma$-ray bump observed at $E_{\gamma }\sim 3$ MeV is interpreted as the so-called pygmy resonance, which has also been observed previously in $(n,\gamma )$ experiments. The parameters for this resonance have been determined and compared to the available systematics.