Abstract
The nuclear dipole polarizability is mainly governed by the dynamics of the giant dipole resonance and, assuming validity of the brink-Axel hypothesis, has been investigated along with the effects of the low-energy enhancement of the photon strength function for nuclides in medium- and heavy-mass nuclei. Cubic-polynomial fitsto both data sets extrapolated down to a gamma-ray energy of 0.1 MeV show a significantreduction of the nuclear dipole polarizability for semi-magic nuclei, with magic numbers N =28, 50 and 82, which supports shell effects at high-excitation energies in the the quasi-continuum region. This work assigns σ-2 values as sensitive measures of long-range correlations of the nuclear force and provides a new spectroscopic probe to search for “old” and “new” magic numbers at high-excitation energies.
Highlights
We propose an alternative spectroscopic probe, namely the nuclear polarizability, to investigate the rise of magic numbers at high-excitation energies
These measurements are performed in the quasi-continuum energy region and assume the validity of the Brink-Axel hypothesis [27, 28], which states that f (Eγ) is independent of the particular structure of the initial and final state [29] and only depends on Eγmax σtotal (Eγ), i.e. giant dipole resonance (GDR) properties are similar for all nuclear states
Shell effects arising from σ−2 values are evident in nuclei with or near magic numbers N = 28, 50, 82 and 126
Summary
Atomic masses are useful measures to study nuclear structure via nuclear binding energies. Deviations from the smooth trend presented by two-neutron separation energies can be associated with a sudden onset of deformation [1] or the rise of a magic number [2] This information depends on high-precision measurements of atomic masses and is limited to ground and isomeric states of nuclei [3]. This work expands on the data analysis of our recent publications [4, 5] by doing a systematics study for the limited available information and assuming that the enhancement of the photon-strength function continues at lower gamma-ray energies of Eγ ≧ 0.1 MeV Overall, this cutoff shows a greater sensitivity of the nuclear polarizability to the long-range correlations in the nuclear wave functions, as realized from relative sharp drops of the nuclear polarizability in semi-magic nuclei. Because of the 1/Eγ2 weighting in Eq 3, σ−2 values are not significantly affected by contributions above the GDR energy (e.g. recent discrepancies found between the Livermore and Saclay photoneutron data with monochromatic photon beams [20, 21] or nucleon resonances at Eγ ≳ 140 MeV [22]), and are extremely sensitive measures – unlike σtotal – of long-range correlations in the nuclear wave functions
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