Background: Inelastic α-particles scattering at energies of a few hundred MeV and very-forward scattering angles including 0◦ has been established as a best tool for the study of the isoscalar giant monopole (IS0) strength distributions in nuclei. The present study describes a systematic investigation of the ISGMR in nuclei ranging from light-deformed to heavy-mass across the periodic table.
Objective: In an attempt to understand the apparent discrepancies between results from studies at the Research Center for Nuclear Physics (RCNP) and Texas A&M University (TAMU) has led us to extract the monopole strength to have yet another independent data set. However, it is worth noting that the primary objective of this work was to investigate the fine structure in the energy region of the isoscalar giant monopole resonance (ISGMR).
Methods: The isoscalar giant monopole resonance (ISGMR) was excited in 24Mg, 28Si, 58Ni, 90Zr, 120Sn and 208Pb using α-particle inelastic scattering measurements acquired with an Eα = 196 MeV beam at scattering angles θLab = 0◦ and 4◦. The K600 magnetic spectrometer at iThemba LABS was used to detect and momentum analyze the inelastically scattered α particles. The IS0 strength distributions in the nuclei studied were obtained with the Difference-of-Spectrum (DoS) technique. Further, characteristic energy scales are extracted from the fine structure by means of wavelet analysis. The experimental scales are compared to different theoretical approaches allowing for the inclusion of complex configurations beyond the mean-field level. Calculations are performed in the framework of QRPA and beyond QRPA in a relativistic approach based on an effective meson-exchange interaction.
Results: IS0 strength distributions for 24Mg, 28Si, 58Ni, 90Zr, 120Sn and 208Pb are extracted and compared to previously published results from experiments with a lower energy resolution. While the structural character of the IS0 strength distributions vary, a reasonable agreement is obtained between the different studies. For the fine structure analysis, all models highlight the role of Landau fragmentation for the damping of the ISGMR especially in the medium-mass region, while the coupling strength between one particle-one hole (1p-1h) and two particle-two hole (2p-2h) correlated (relativistic) and non-correlated (non-relativistic) configurations becomes more pronounced in the heavy-mass region indicating the importance of the spreading width.