Investigation of pairing phase transition and exotic nuclear shapes

A long standing problem in the experimental nuclear physics research has been the characterization of the thermodynamic properties of atomic nucleus and the transition from paired states at zero temperature to unpaired states at finite temperature. This phase transition depends on the pairing correlations in atomic nuclei, which is successfully explained by the BCS theory of superconductivity. An experimental signature of these pairing phase transition is the local increase in the heat capacity as a function of temperature. In the recent past, Oslo group of Norway measured the level density for different nuclei and extracted S-shaped canonical heat capacity as a function of temperature indicating pairing phase transition with a critical temperature of Tc = 0.7 -1.0 MeV. But, the experimental data are not sufficient to confirm the second order pairing phase transition and also could not explore thermodynamic quantities beyond the particle threshold energy. I, along with our group, proposed a new method to calculate the thermodynamic quantities such as free energy, entropy, average energy and heat capacity for studying the pairing correlations. We have performed self-consistent measurement of the angular momentum gated nuclear level densities for the reaction 4He+93Nb at different excitation energies 6 - 26 MeV and temperature 1 - 1.5 MeV. The light ion alpha beam provides us the scope of extracting those thermodynamic quantities from the experimental nuclear level density in compound nuclear reaction at lower angular momentum and temperature. The microscopic calculation using the exact solutions of the pairing Hamiltonian for a given number of single-particle levels around the Fermi surface has also been performed to explain the experimental data. It is observed that all the four experimental thermodynamic quantities agree well with he microscopic calculation. These new experimental data explore the excitation energy well above the particle threshold, where no data were available so far, and could as well be utilized as a testing ground of different theoretical calculations on pairing correlations in the near future. In another experiment, the evolution of exotic shape of excited nuclei has been investigated via the measurement of high energy giant dipole resonance (GDR) gamma rays. It is observed that the clustering structure of atomic nuclei has an important role in the evolution of exotic shape of atomic nuclei. The appearance of Jacobi shape transition of non-collective oblate to collective prolate via tri-axial shape in non-alpha clusters and its disappearance in alpha-clusters may provide important insight in the studies on nuclear structure.