Free energy and criticality in the nucleon pair breaking process
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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.
11 pages, including 6 figures, to be published in Physical Review C
11 pages, including 6 figures, to be published in Physical Review C