Electric dipole polarizability of low-lying excited states in atomic nuclei
Journal of Physics G: Nuclear and Particle Physics(2023)
摘要
Novel equations for the electric dipole polarizability α__E1 of
low-lying excited states in atomic nuclei – and the related (-2) moment of
the total photo-absorption cross section, σ__-2 – are inferred in
terms of electric dipole and quadrupole matrix elements. These equations are
valid for arbitrary angular momenta of the initial/ground and final/excited
states and have been exploited in fully converged 1ħω shell-model
calculations of selected p- and sd-shell nuclei that consider
configuration mixing; advancing previous knowledge from ^17O to ^36Ar,
where thousands of electric dipole matrix elements are computed from isovector
excitations which include the giant dipole resonance region. Our results are in
reasonable agreement with previous shell-model calculations and follow –
except for ^6,7Li and ^17,18O – Migdal's global trend provided by the
combination of the hydrodynamic model and second-order non-degenerate
perturbation theory. Discrepancies in ^6,7Li and ^17O arise as a result
of the presence of α-cluster configurations in odd-mass nuclei, whereas
the disagreement in ^18O comes from the mixing of intruder states, which is
lacking in the shell-model interactions. More advanced ab initio
calculations of the dipole polarizability for low-lying excited states covering
all the isovector states within the giant dipole resonance region are missing
and could be very valuable to benchmark the results presented here and shed
further light on how atomic nuclei polarize away from the ground state
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