  Experimental level densities for 171,172Yb, 166,167Er, 161,162Dy, and
148,149Sm are analyzed within the microcanonical ensemble. In the even isotopes
at excitation energies E < 2 MeV, the Helmholtz free energy F signals the
transition from zero to two quasiparticles. For E > 2 MeV, the odd and even
isotopes reveal a surprisingly constant F at a critical temperature Tc of appr.
0.5 MeV, indicating the continuous melting of nucleon Cooper pairs as function
of excitation energy.
