Fine Structure of the Isovector Giant Dipole Resonance in $^{142-150}$Nd and $^{152}$Sm

Abstract

Background: Inelastic proton scattering at energies of a few hundred MeV and very-forward scattering angles including 0 • has been established as a tool for the study of electric-dipole strength distributions in nuclei. The present work reports a systematic investigation of the chain of stable even-mass Nd isotopes representing a transition from spherical to quadrupole-deformed nuclei.Purpose: Extraction of the equivalent photo-absorption cross sections and analysis of their fine structure in the energy region of the IsoVector Giant Dipole Resonance (IVGDR).Method: Proton inelastic scattering reactions of 200 MeV protons were measured at the iThemba Laboratory for Accelerator Based Sciences in Cape Town, South Africa. The scattering products were momentum-analysed by the K600 magnetic spectrometer positioned at θ Lab = 0 • . Using dispersion-matching techniques, energy resolutions of ∆E ≈ 40 − 50 keV (Full Width at Half Maximum) were obtained. After subtraction of background and contributions from other multipoles, the spectra were converted to photo-absorption cross sections using the equivalent virtual-photon method.Results: Wavelet-analysis techniques are used to extract characteristic energy scales of the fine structure of the IVGDR from the experimental data. Comparisons with the Quasiparticle-Phonon Model (QPM) and Skyrme Separable Random Phase Approximation (SSRPA) predictions provide insight into the role of different giant resonance damping mechanisms.Conclusions: Fine structure is observed even for the most deformed nuclei studied. Fragmentation of the one particle-one hole (1p1h) strength seems to be the main source of fine structure in both spherical and deformed nuclei. Some impact of the spreading due to coupling of the two particle-two hole (2p2h) states to the 1p1h doorway states is seen in the spherical/transitional nuclei, where calculations beyond the 1p1h level are available.