Thermal pairing and giant dipole resonance in highly excited nuclei

Abstract

Recent results are reported showing the effects of thermal pairing in highly excited nuclei. It is demonstrated that thermal pairing included in the phonon damping model (PDM) is responsible for the nearly constant width of the giant dipole resonance (GDR) at low temperature $T <$ 1 MeV. It is also shown that the enhancement observed in the recent experimentally extracted nuclear level densities in $^{104}$Pd at low excitation energy and various angular momenta is the first experimental evidence of the pairing reentrance in finite (hot rotating) nuclei. In the study of GDR in highly excited nuclei, the PDM has been extended to include finite angular momentum. The results of calculations within the PDM are found in excellent agreement with the latest experimental data of GDR in the compound nucleus $^{88}$Mo. Finally, an exact expression is derived to calculate the shear viscosity $\eta$ as a function of $T$ in finite nuclei directly from the GDR width and energy at zero and finite $T$. Based on this result, the values $\eta/s$ of specific shear viscosity in several medium and heavy nuclei were calculated and found to decrease with increasing $T$ to reach $(1.3 - 4)\times\hbar/(4\pi k_B)$ at $T =$ 5 MeV, that is almost the same value obtained for quark-gluon-plasma at $T >$ 170 MeV.