Instantaneous Shape Sampling - a model for the $\gamma$-absorption cross section of transitional nuclei

TL;DR

This paper introduces the Instantaneous Shape Sampling Model, combining the Interacting Boson Model and RPA, to accurately predict gamma-absorption cross sections in transitional nuclei, accounting for shape fluctuations and damping effects.

Contribution

The model uniquely integrates shape fluctuations and damping to improve predictions of gamma-absorption in transitional nuclei, including semi-magic cases with pygmy resonances.

Findings

Accurately reproduces experimental gamma-absorption cross sections.

Shows shape fluctuations and Landau fragmentation influence low-energy cross sections.

Identifies pygmy resonances in semi-magic nuclei.

Abstract

The influence of the quadrupole shape fluctuations on the dipole vibrations in transitional nuclei is investigated in the framework of the Instantaneous Shape Sampling Model, which combines the Interacting Boson Model for the slow collective quadrupole motion with the Random Phase Approximation for the rapid dipole vibrations. Coupling to the complex background configurations is taken into account by folding the results with a Lorentzian with an energy dependent width. The low-energy energy portion of the $\gamma$- absorption cross section, which is important for photo-nuclear processes, is studied for the isotopic series of Kr, Xe, Ba, and Sm. The experimental cross sections are well reproduced. The low-energy cross section is determined by the Landau fragmentation of the dipole strength and its redistribution caused by the shape fluctuations. Collisional damping only wipes out fluctuations of the absorption cross section, generating the smooth energy dependence observed in experiment. In the case of semi-magic nuclei, shallow pygmy resonances are found in agreement with experiment.