Surface and Volume Symmetry Energy Coefficients of a Neutron-Rich Nucleus

Using an isobaric method, the symmetry-energy coefficient (a_sym) of a neutron-rich nucleus is obtained from experimental binding energies. The shell effects are shown in a_sym^*/A≡4a_sym/A of nuclei. A (sub)magic neutron magic number N=40 is suggested in a very neutron-rich nucleus, and a_sym^*/A of a nucleus is found to decrease when its mass increases. The a_sym^*/A of a very neutron-rich nucleus with large mass saturates. The volume-symmetry coefficients (b_v) and surface-symmetry coefficients (b_s) of a neutron-rich nucleus are extracted from a _sym^*/A by a correlation a_sym^*/A=b_v/A?b _s/A^4/3. It is found that b_v and b_s decrease when the nucleus becomes more neutron-rich, and tend to saturate in the very neutron-rich nucleus. A linear correlation between b _v and b_s is obtained in nuclei with different neutron-excess I, and b_v of I>7 nuclei is found to coincide with the results of infinite nuclear matter a _sym=32 ±4 MeV, and b_s/b_v of the nucleus is found to coincide with the results of the finite-range liquid-drop model results.