Entropy in hot ^161,162Dy and ^171,172Yb nuclei

The density of accessible levels at low spin in the (³He,αγ) reaction has been extracted for the ^161,162Dy and ^171,172Yb 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 ¹⁶²Dy and ¹⁷²Yb 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.