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Empirical drops in ground-state nuclear polarizabilities indicate deviations from the effect of giant dipole resonances and may reveal the presence of shell effects in semi-magic nuclei with neutron magic numbers $N=50$, 82 and 126. Similar drops of polarizability in the quasi-continuum of nuclei with, or close to, magic numbers $N=28$, 50 and 82, could reflect the continuing influence of shell closures up to the nucleon separation energy. These findings open a new avenue to investigating magic numbers at high-excitation energies and strongly support recent large-scale shell-model calculations in the quasi-continuum region, which describe the origin of the low-energy enhancement of the photon strength function as induced paramagnetism. The nuclear-structure dependence of the photon-strength function asserts the generalized Brink-Axel hypothesis as more universal than originally expected.

This is a Reply to a Comment on my paper: New formulas for the (-2) moment of the photo-absorption cross section, sigma-2., Comment: 2 pages, 1 figure

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., Comment: 4 pages, 4 figures

New polarization potentials have been determined based on: 1) the latest photo-neutron cross section evaluation and a missing factor of two in previous work, and 2) the mass dependency of the symmetry energy, $a_{sym}(A)$. The magnitude of the first one is 35\% stronger than the currently accepted polarization potential. The second one opens up the possibility for a parameter-free polarization potential. Both polarization potentials are essentially the same for heavy nuclei. The polarization effect on quadrupole collectivity is more substantial than previously assumed for light nuclei. Particular cases are discussed where long-standing discrepancies between high-precision Coulomb-excitation and lifetime measurements still remain. A solution to the long-standing discrepancy between $B(E2; 0^+_1\rightarrow 2^+_1)$ values determined in $^{18}$O by several Coulomb-excitation studies and a high-precision lifetime measurement is provided in favor of the latter. Polarization effects in light nuclei also influence the determination of spectroscopic quadrupole moments in Coulomb-excitation measurements. The hindrance of polarizability observed in the photo-neutron cross section for single-closed shell nuclei is calculated to have a negligible effect on quadrupole collectivity, within the existing experimental uncertainties., Comment: 5 pages, 4 figures

I explore weakly-collective singly-closed shell nuclei with high-j shells where active valence neutrons and particle-particle correlations may be the dominant collective degree of freedom. The combination of large and close-lying proton and neutron pairing gaps extracted from experimental masses seems to charaterize the origin of the weak collectivity observed in Ni and Sn superfluids with $N\approx Z$. The trend of $E2$ transition strengths, i.e., $B(E2; 2^+_1\rightarrow 0^+_1)$ values, in these nuclei is predicted from proton and neutron pairing-gap information. The agreement with the Ni isotopes is excellent and recent experimental results support the trend in the Sn isotopes. This work emphasizes the importance of atomic masses in elucidating nuclear-structure properties. In particular, it indicates that many-body microscopic properties such as nuclear collectivity could be directly inferred from more macroscopic average properties such as atomic masses., Comment: 4 pages, 5 figures