Electric dipole strength in $sd$-shell nuclei from small-angle proton scattering

TL;DR

This study measures and analyzes the electric dipole strength in sd-shell nuclei using small-angle proton scattering, comparing experimental results with shell-model calculations and neural network predictions to understand nuclear structure and reactions.

Contribution

First measurements of photoabsorption cross sections for several sd-shell nuclei, with comparison to theoretical models and neural network predictions, advancing understanding of nuclear dipole responses.

Findings

Reasonable agreement with previous data for some nuclei

Shell-model calculations generally describe the data well

Discrepancies observed in certain nuclei and energy regions

Abstract

The present work reports new total photoabsorption cross sections for the $N=Z$ nuclei in the $sd$-shell $^{20}$Ne, $^{24}$Mg, $^{28}$Si, $^{32}$S, $^{36}$Ar, and for $^{26}$Mg. The results are compared to predictions of a data-driven artificial neural network application and to configuration-interaction shell-model calculations covering the excitation energy region of the isovector giant dipole resonance. Double-differential cross sections of the $(p,p^\prime)$ reaction at 295 MeV have been measured between $0^\circ$ and $14^\circ$. The angular distributions of the $E1$ parts due to Coulomb excitation have been extracted with a multipole decomposition analysis for excitation energies 12 to 24 MeV and converted to equivalent photoabsorption cross sections with the virtual photon method. Reasonable agreement of the photoabsorption cross sections with previous experiments is found for $^{24}$Mg and $^{28}$Si, while the present results diverge for $^{32}$S. For the first time, data are presented for $^{20}$Ne, $^{26}$Mg and $^{36}$Ar. Configuration-interaction shell-model calculations provide an overall satisfactory description of the fragmented $E1$ strength distributions. The same holds for absolute cross sections except for $^{26}$Mg and $^{36}$Ar, where the experimental results significantly exceed the expected exhaustion of the Thomas-Reiche-Kuhn energy-weighted sum rule. Fot light nuclei, there is a larger model dependence compared to previous analyses in heavy nuclei, in particular for excitation energies above 20 MeV, due to the need to constrain the continuum background with additional assumptions. The overall success of the shell-model approach to describe the features of the experimental photoabsorption cross sections motivates its application in large-scale reaction network calculations aiming at an understanding of the mass composition of ultrahigh-energy cosmic rays.