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

R. W. Fearick1, O. Le Noan2,3, H. Matsubara4,5, P. von Neumann-Cosel6,*, K. Sieja2,3, and A. Tamii4

  • *Contact author: vnc@ikp.tu-darmstadt.de
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Phys. Rev. C 113, 064311 – Published 12 June, 2026

DOI: https://doi.org/10.1103/4yjj-ybpj

Abstract

Background: Total photoabsorption cross sections are important for an understanding of nuclear structure and for many applications, in particular in astrophysics. The majority of data stems from measurements of neutron emission after photoexcitation, which, in heavy nuclei, represent a good approximation to the total cross sections. However, this does not hold for light nuclei, where charged-particle emission thresholds are typically lower in energy. Accordingly, data in light nuclei are scarce.

Purpose: The present work reports new total photoabsorption cross sections for the N=Z nuclei in the sd-shell Ne20, Mg24, Si28, S32, Ar36, and for Mg26. 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.

Methods: Double-differential cross sections of the (p,p′) reaction at an incident proton energy of 295 MeV have been measured between 0∘ and 14∘. 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.

Results: Reasonable agreement of the photoabsorption cross sections with previous experiments is found for Mg24 and Si28, while the present results diverge for S32. For the first time, data are presented for Ne20, Mg26, and Ar36. 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 Mg26 and Ar36, where the experimental results significantly exceed the expected exhaustion of the Thomas-Reiche-Kuhn energy-weighted sum rule.

Conclusions: Despite a reduction of Coulomb relative to nuclear cross sections in light nuclei with respect to previous analyses of the same kind in medium-mass and heavy nuclei, forward-angle proton scattering at energies of several hundred MeV remains a valuable tool to extract the nuclear E1 response. However, there is a larger model dependence, 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.

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Phys. Rev. C 113, 064311– Published 12 June, 2026
  • Received 19 January 2026
  • Revised 8 April 2026
  • Accepted 21 May 2026
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