Background: The dipole response of a nuclear system, characterized by its photon strength function (PSF), is a key ingredient of many applications of nuclear structure, ranging from nuclear reactor design and nuclear waste transmutation to astrophysical models of nucleosynthesis and stellar evolution. While the majority of those applications require knowledge of the PSF of midmass and heavy nuclei, there is now renewed interest in $E1$ strength distributions of light nuclei in the framework of the PANDORA project, which aims at an understanding of the mass distribution of ultrahigh-energy cosmic radiation (UHECR). UHECR is of extragalactic origin, and its interaction along the travel path is dominated by photoabsorption of cosmic background radiation boosted to the Giant Dipole Resonance energy region in the center-of-mass system. Thus, systematic knowledge of the photoabsorption cross sections in light nuclei and of their subsequent particle decay is required.

Purpose: The purpose of this work is to enhance the database of available theoretical evaluations of the PSFs of light nuclei that are necessary in the studies of UHECR propagation.

Methods: We employ the Configuration Interaction Shell Model (CI-SM) approach to provide predictions of $E1$ dipole response for $p$- and $sd$-shell nuclei, with mass number $A$ between 7 and 40. Theoretical predictions are compared with available data and with existing predictions from phenomenological and microscopic models. Finally, the impact of using CI-SM PSFs on the predicted propagation of a ${}^{40}\mathrm{Ca}$ UHECR source is studied.

Results: As the CI-SM approach captures all the nucleonic correlations in a given configuration space, the CI-SM predictions of the centroids and widths of PSFs exhibit stronger variation and fragmentation, respectively, than those from linear-response and from phenomenological models. When used in the nuclear reaction network, they lead to an outcome that is compatible with the recently developed relativistic quasiparticle finite amplitude method, which is at stronger variance with the predictions employing the $\mathrm{D}1\mathrm{M}+\mathrm{QRPA}$ (quasiparticle random-phase approximation) approach.

Conclusions: The CI-SM method is shown to be a valuable alternative to provide PSFs for the modeling of the propagation of the UHECR, so far only achieved with linear-response models and phenomenological models of nuclear strength functions. While this work covers a considerable fraction of nuclei necessary in such applications (namely, $p$- and $sd$-shell nuclei), future work will also treat $pf$-shell systems and hence complete the CI-SM systematics of dipole PSF necessary for such applications.

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• Cosmic ray propagation
• Electromagnetic transitions
• Gamma-ray strength functions
• Giant resonances
• Nuclear astrophysics
• Nuclear structure & decays
• Nuclear many-body theory
• Shell model