Excitation of dipole resonances in 4He and in the α clusters of 6Li and 7Li

The isovector dipole resonances (DR's) consisting of the giant dipole resonance and spin dipole resonance (SDR) in He-4 were investigated by using the (p,p(')) reaction at an incident energy of 300 MeV. We compared the excitation energy, width, resonance shape and the differential cross sections for the DR in He-4 with those for the DR's in Li-6 and Li-7 which had been observed and interpreted in earlier work as the DR's in the alpha clusters of Li-6 and Li-7. The DR's in He-4 and in the alpha clusters of Li-6,Li-7 were observed at E-x=25.1 +/- 0.5,28.0 +/- 0.5, and 30.5 +/- 0.5 MeV with the widths of 11.0 +/- 2.0,11.7 +/- 1.5, and 11.3 +/- 2.0 MeV, respectively. The peaks corresponding to the 2(-) SDR in the alpha cluster were not as dominant as that observed in He-4 at E-x=22.1 MeV. The resonance shapes for the alpha cluster in Li-6,Li-7 were reproduced well with the DR shape without including the 2(-) SDR in He-4. The excitation energies for the DR's in the alpha clusters slightly increase with the number of the surrounding nucleons. The cross sections for the excitation of the DR's in the alpha clusters of Li-6,Li-7, normalized to the spectroscopic factors for the alpha particle in Li-6,Li-7, are comparable to the cross section for the excitation of the DR in He-4.