Fine structure of the isovector giant dipole resonance in neutron-rich calcium isotopes using the (p,p') reaction at zero-degrees

Abstract

In the present study, the fine structure of the IsoVector Giant Dipole Resonance (IVGDR) of stable neutron-rich isotopes of calcium, namely 42Ca, 44Ca and 48Ca, has been investigated in a systematic way by including readily available data of 40Ca. Measurements were carried out at the cyclotron facility of iThemba Laboratory for Accelerator Based Sciences (iThemb LABS) which provided a high quality proton beam of 200 MeV, together with the state-of-the-art K600 magnetic spectrometer in zero-degree mode. Excellent energy-resolution of up to ∆E ≈ 33 keV was attained which allowed for a clear observation of fine structure in the excitation energy region of the IVGDR which lies between 11 and 25 MeV. Double differential cross-sections were extracted from the experimental energy spectra and subsequently converted to equivalent photo-absorption cross-sections. The obtained results agree well with the photo-absorption cross-section data avail able in the literature, within limits of experimental errors. Centroid energies and widths of the IVGDR of the calcium isotopes were extracted by fitting the ex perimental data with both Lorentzian and Gaussian functions over the excitation energy range of 15 - 23 MeV. The obtained values of centroids and width agree well with the predictions of the theoretical macroscopic model. Characteristic energy scales were extracted from the experimental spectra and compared to those of theoretical microscopic models of the Relativistic Quasi particle Time Blocking Approximation (RQTBA), the Relativistic Quasi-particle Random Phase Approximation (RQRPA), the Random Phase Approximation

(RPA), the Quasi-particle RPA (QRPA), the Second RPA (SRPA) and the Con tinuum RPA (CRPA). The RQTBA was found to best describe the experimentally extracted energy scales. The level density of the Jπ = 1− states was also extracted from the high energy resolution spectra and the results show a good agreement with the theoretical phenomenological models of the Back-shifted Fermi Gas in the excitation energy range of 13 and 20 MeV for all four calcium isotopes.