Entropy of thermally excited particles in nuclei

Information on level density for nuclei with mass numbers A∼20–250 is deduced from discrete low-lying levels and neutron resonance data. The odd-mass nuclei exhibit in general 4–7 times the level density found for their neighboring even-even nuclei at the same excitation energy. This excess corresponds to an entropy of ∼1.7k_B for the odd particle. The value is approximately constant for all midshell nuclei and for all ground state spins. For these nuclei it is argued that the entropy scales with the number of particles not coupled in Cooper pairs. A simple model based on the canonical ensemble theory accounts qualitatively for the observed properties.