Study of electric isovector giant nuclear resonances by the charge exchange reaction. Search for a monopole mode

Charge exchange reactions (13C, 13N) and (13C, 13B) at 50 MeV/n are studied with the purpose of investigating the electric isovector modes of giant nuclear resonances in 12C, 40Ca, 58Ni, 60Ni, 90Zr, 120Sn and 208Pb, and especially the monopole mode. Measurements have been performed at GANIL, using the magnetic spectrometer SPEG. The main characteristics of giant nuclear resonances are reviewed, and the ability of the chosen reactions to excite electric isovector modes is examined. Experimental setup and data reduction are described. Results are commented and analyzed. The analog state of the dipole resonance is strongly excited in nuclei with A ≤ 60; excitation energies and widths are obtained with a good accuracy. For nuclei with A ≥ 60, a large peak is observed in the spectra. Its excitation energy is closed to the excitation energy of the giant isovector monopole resonance, measured with the (π-, π0) reaction, except in 208Pb. Experimental angular distributions do not quite show the features expected from DWBA calculations. The low-lying states of 12N and 12B are used to test the calculations and the reaction mechanisms