Entropy in hot ${}^{161,162}\mathrm{Dy}$ and ${}^{171,172}\mathrm{Yb}$ nuclei

The density of accessible levels at low spin in the ${(}^3\mathrm{He},\alpha \gamma )$ reaction has been extracted for the ${}^{161,162}\mathrm{Dy}$ and ${}^{171,172}\mathrm{Yb}$ nuclei. The entropy of the even-odd and even-even nuclei has been deduced as a function of excitation energy. The entropy of one quasiparticle outside an even-even core is found to be 1.70(15) $k_B.$ This quasiparticle picture of hot nuclei is well accounted for within a simple pairing model. The onset of two, four, and six quasiparticle excitations in the ${}^{162}\mathrm{Dy}$ and ${}^{172}\mathrm{Yb}$ nuclei is discussed and compared to theory. The number of quasiparticles excited per excitation energy is a measure for the ratio of the level energy spacing and the pairing strength.