Excitation of giant monopole resonances in 144Sm and 208Pb by (α, α') reactions at Eα = 84–119 MeV

Abstract

The giant monopole resonance (GMR) in 144Sm and 208Pb was studied by (α, α') reactions at incident energies from 84 to 119MeV. Scattered α-particles were detected in an angular range from 1.6° to 7.5° by using a coupled system of a dipole magnet and a triplet quadrupole magnet lens. The resonance shape and strength were carefully investigated by comparing the angular distributions obtained at different incident energies. The observed angular distributions of the GMR were analyzed by DWBA calculations. It was found that an appreciable amount (~ 20 % of the energy-weighted sum-rule fraction) of the L = 2 component coexists in the GMR region. This component had a significant influence at low incident energy on the extraction of the monopole cross section. The observed excitation function of the GMR was found to decrease more steeply than the DWBA predictions if the incident energy was lowered from 100 to 84 MeV. This strongly contrasts with the excitation function of the neighboring giant quadrupole resonance, which is well described by the DBWA predictions in the framework of the collective model. Anomalous behavior of the excitation function of the GMR may suggest a need for a more refined description of the GMR.