S-shaped heat capacity in an odd–odd deformed nucleus

Abstract

We examine the thermodynamic properties of mass A∼200 nuclei utilizing angular momentum (J) gated nuclear level densities (NLDs) extracted in the excitation energy range of 2–15 MeV. Interestingly, the experimental NLDs are in good agreement with the results of a microscopic approach, which is derived based on the exact pairing plus the independent-particle model at finite temperature (EP+IPM), whereas the conventional Hartree–Fock BCS (HFBCS) and Hartree–Fock–Bogoliubov plus combinatorial method (HFBC) fail to describe these data. Consequently, the thermodynamic properties of those nuclei at finite angular momentum have been extracted using the EP+IPM NLDs. While the heat capacities of 200Tl, 211Po and 212At (near spherical nuclei) follow the trend as expected in odd–odd and even–odd masses, surprisingly an S-shaped heat capacity is found in odd–odd deformed nucleus 184Re. It has been shown that this S-shaped heat capacity observed in 184Re is caused by not only the breaking of nucleon Cooper pairs but also the change of pairing induced by deformation. Keywords: Nuclear thermodynamics, Statistical model calculation, Pairing phase transition