Application of relativistic continuum random phaseapproximation to giant dipole resonance of ²⁰⁸Pb and ¹³²Sn

The relativistic continuum random phase approx-imation (RCRPA) is applied to describe the properties ofisovector giant dipole resonances (IVGDR) in Pb-208 and Sn-132 with NL3 effective interaction. We analyze the strengthdistribution, various sum rules, centroid energies and the inte-gral photoabsorption cross sections of the pygmy dipole res-onance (PDR) and giant dipole resonance (GDR), the resultsare compared to the values obtained by discretized RRPA andavailable experimental data. Difference between the resultsobtained by RRPA and RCRPA is found, which may due tothe different ways in treating the contribution of continuum.The calculated centroid energies of GDR can reproduce theexperimental data well for Pb-208 and Sn-132. A better agree-ment with photoabsorption cross section data is obtained for Sn-132 in the RCRPA calculation. By correlating the exci-tation energy (the electric dipole polarizability) of GDR tothe nuclear matter properties, we could constrain the densitydependence of symmetry energy. The deduced nuclear sym-metry energy is located in the range 30.3-36.5 MeV (30.8-32.8 MeV), and the slope parameterLessentially covers therange of 40.1-106.1 MeV (40.0-63.0 MeV) at the saturationdensity.