Symmetry energy from two-nucleon separation energies of Pb and Ca isotopes

We investigate the symmetry energy in relation with the two-proton and two-neutron separation energies using different nuclear mass data. For this aim, we exploit the deformed relativistic Hartree-Bogoliubov theory in the continuum (DRHBc), FRDM2012, and AME2020 data. First, we study the two-proton and two-neutron separation energies in Pb and Ca isotopes by subtracting the contribution of Coulomb energy. They show a strong correlation with neutron number as well as with the neutron skin thickness. By taking the relative difference of both separation energies, we derive the symmetry energy from Ca and Pb isotopes. Since the nuclear surface contributes to the symmetry energy, we deduce the volume symmetry energy by subtracting the surface contribution using several mass models. The obtained symmetry energy coefficient, $a_{\mathrm{sym}}$, is 20.0–22.7 MeV for Pb isotopes and 18.7–19.3 MeV for Ca isotopes from the DRHBc mass table data, while the results from other mass tables are 19.6–22.1 (20.7–22.3) MeV for Pb isotopes and 18.9–19.0 (19.6–19.7) MeV for Ca isotopes from AME2020 (FRDM2012) data. The volume contribution to the asymmetry coefficient, $a_{\mathrm{sym}}^v$, which depends on the ratio of the surface to the volume energy coefficients, $a_s/a_v$, is also provided for each mass model. Since the ratio $a_s/a_v$ is determined neither by nuclear theory nor by experimental data, we have investigated $a_{\mathrm{sym}}^v$ by using the ratio $a_s/a_v$ as a free parameter, and have obtained $a_{\mathrm{sym}}^v=$ 27.0 MeV, almost irrespective of nuclear model and isotopic chain, with the ratio $a_s/a_v$ constrained as $a_s/a_v=1.10–1.13$.