Nuclear Dipole Vibrations and the Surface Symmetry Energy

The frequency of nuclear electric dipole vibrations of the type suggested by Goldhaber and Teller is derived on the basis of the semi-empirical nuclear model. The resulting formula, taken together with the ($\gamma ,n$) and ($\gamma ,f$) cross-section data of Baldwin and Klaiber, determines a lower bound for the term in the surface energy depending on $N-Z$ (surface symmetry energy). This is close to an upper bound estimated by Feenberg, and one may tentatively assume that the nuclear surface tension contains a factor $1-0.4{\mathrm{}(N-Z)\mathrm{}}^2{A}^{-2\mathrm{}}$.