arXiv subjects

O. Le Noan

Publications and source records attributed to O. Le Noan.

3 recordsLinked to original sources

arXiv2026

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

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.

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arXiv2025

Modeling Ultra-High-Energy Cosmic Rays propagation using the input from Configuration Interaction Shell Model

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 the knowledge of PSF of mid-mass 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 (GDR) 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. The purpose of this work is to enhance the database of available theoretical evaluations of PSF of light nuclei that are necessary in the studies of UHECR propagation. 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 to available data and to existing predictions from phenomenological and microscopic models. Finally, the impact of using of CI-SM PSF on the predicted propagation of a $^{40}$Ca UHECR source is studied.

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arXiv2025

Electric dipole response of sd-shell nuclei within the Configuration-Interaction Shell Model approach

Reliable theoretical predictions of nuclear dipole excitations are crucial for various nuclear applications, particularly in nuclear astrophysics. Calculations of radiative capture cross sections often rely on theoretical gamma strength functions, with the electric dipole response being the dominant component. We aim at a systematic description of the E1 strength of nuclei with mass numbers between 17 and 40 of interest for various applications and to better understand the nature of the low-energy dipole strength in neutron-rich nuclei, known as pygmy dipole resonances. We use the Configuration Interaction shell-model framework in the p-sd-pf valence space with a previously established empirical Hamiltonian. Systematic results of photoabsorption strength show good agreement with experimental data, provided a renormalization of the dipole operator is applied. Transition densities are computed in 26Ne and confirm the pure isovector character of the Giant Dipole Resonance. The strength at 7-10MeV is shown to have a distinct structure with largely fragmented wave functions and transition densities of isovector character at the edge of the nucleus. The Configuration Interaction shell model is proved to be a valuable tool in the description of the photoresponse of light nuclei, providing more accurate results than the usually employed approaches.

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