New formulae for the $(-2)$ moment of the photo-absorption cross section, $\sigma_{_{-2}}$

Abstract

Two new formulae for the $(-2)$ moment of the photo-absorption cross section, $\sigma_{_{-2}}$, have been determined, respectively, from the 1988 compilation of Dietrich and Berman and a mass-dependent symmetry energy coefficient, $a_{sym}(A)$. The data for $A\gtrsim50$ follow, with a {\small $RMS$} deviation of 6\%, the power-law $\sigma_{_{-2}}=2.4 A^{5/3}$$\mu$b/MeV, which is in agreement with Migdal's calculation of $\sigma_{_{-2}}=2.25A^{5/3}$$\mu$b/MeV based on the hydrodynamic model and the $\sigma_{_{-2}}$ sum rule. The additional inclusion of $a_{sym}(A)$ provides a deeper insight to the nuclear polarization of $A\geqq10$ nuclei.