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The nuclear structural properties of dipole excitations in nuclei of different mass regions are investigated using a theoretical approach based on energy-density-functional theory (EDF), quasiparticle-random-phase approximation (QRPA) and the quasiparticle-phonon model (QPM) [1]. The systematic comparison between QRPA and multiphonon QPM calculations in different nuclei shows that the behavior of the dipole strength at low energies is influenced by the competition between static and dynamic effects. The first effect is related to the mean field (MF) and the excitation of a pygmy dipole resonance (PDR) associated with neutron skin oscillations, while the second represents the coupling of the single-particle states with more complex excitations related to nuclear polarization and giant dipole resonance (GDR). These effects lead to a redistribution and fragmentation of the electric dipole strength (E1) at low energies, with particular emphasis on the neutron one-particle-one-hole (1p-1h) components of the QPM dipole state vectors [1-3]. The latter have been identified as doorway states common to neutron- and gamma-channels in (n,gamma) reactions [4]. As doorway states, the 1- QRPA excited states of the PDR contain mainly 1p-1h neutron configurations, which are expected to strongly influence the (n,gamma) cross sections and thereby affect isotope production in explosive stellar environments [1]. In these theoretical studies, the comparison of dipole spectral distributions obtained from Nuclear Resonance Fluorescence (NRF) experiments plays a special role [1]. Recent developments of our theoretical method, including a reaction theory [4,5], have been successfully applied to studies of (d,p) and (d,p gamma) reaction cross sections in neutron-excess nuclei. The theoretical observations show an increase in the neutron transfer reaction cross section in the PDR energy range, which was also observed experimentally, thus confirming the neutron origin of the PDR [4,5]. Furthermore, coincidence studies of dipole spectral distributions at low energies with various probes such as alphas, photons, protons, etc. are important tools to investigate the isospin dynamics of the nucleus. Experimentally, the determination of the dipole strength distribution and the associated photoabsorption cross section requires knowledge of the intensity distribution of the ground state transitions and their branching ratios. These quantities cannot be derived directly from the measured spectra. However, they can be determined theoretically from our microscopic calculations. An interesting new result observed in our studies of direct and cascade decay channels in 56Fe is an enhanced E1 below the neutron threshold in both direct and cascade spectral distributions, which, due to their spectroscopic properties, resemble PDR modes based on nuclear ground and excited states.
This work was carried out under the contract PN 23.21.01.06 sponsored by the Romanian Ministry of Research, Innovation and Digitalization and partially supported by ELI-RO-RDI-2024-AMAP of the Romanian Government.
[1] N. Tsoneva, H. Lenske, Energy-density functional plus quasiparticle-phonon model theory as a powerful tool for nuclear structure and astrophysics, Physics of Atomic Nuclei 79, 885–903 (2016) and refs. therein.
[2] T. Shizuma, S. Endo, A. Kimura, R. Massarczyk, R. Schwengner, R. Beyer, T. Hensel, H. Hoffmann, A. Junghans, K. Römer, S. Turkat, A. Wagner, and N. Tsoneva, Low-lying dipole strength distribution in 204Pb, Phys. Rev. C 106, 044326 (2022).
[3] P. -A. Söderström, M. Markova, N. Tsoneva, et al., Statistical properties and photon strength functions of the 112,114Sn isotopes below the neutron separation threshold, Phys. Rev. (2025).
[4] M. Spieker, A. Heusler, B.A. Brown, T. Faestermann, R. Hertenberger, G. Potel, M. Scheck, N.Tsoneva, M. Weinert, H.-F. Wirth, and A. Zilges, Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in 208Pb, Phys. Rev. Lett. 125, 102503 (2020).
[5] M. Weinert, M. Spieker, G. Potel, N. Tsoneva, M. Müscher, J. Wilhelmy, and A. Zilges, Microscopic Structure of the Low-Energy Electric Dipole Response of 120Sn, Phys. Rev. Lett. 127, 242501 (2021).
