Entropy in hot Dy-161,Dy-162 and Yb-171,Yb-172 nuclei - art. no. 024306

The density of accessible levels at low spin in the (He-3, alpha gamma) rea ction has been extracted for the Dy-161,Dy-162 and Yb-171,Yb-172 nuclei. Th e entropy of the even-odd and even-even nuclei has been deduced as a functi on of excitation energy. The entropy of one quasiparticle outside an even-e ven core is found to be 1.70(15) k(B). This quasiparticle picture of hot nu clei is well accounted for within a simple pairing model. The onset of two, four, and six quasiparticle excitations in the Dy-162 and Yb-172 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 a nd the pairing strength.