Analogs of the giant dipole and spin-dipole resonances inHe4and in α clusters ofLi6,7studied by theHe4,Li6,7(Li7,Be7γ) reactions

We studied analogs of the giant dipole resonance (GDR) and spin-dipole resonance (SDR) in $^{4}\mathrm{He}$ and in the \ensuremath{\alpha} clusters of $^{6,7}\mathrm{Li}$ via the ($^{7}\mathrm{Li}$,$^{7}\mathrm{Be}$\ensuremath{\gamma}) reactions on $^{4}\mathrm{He}$, $^{6}\mathrm{Li}$, and $^{7}\mathrm{Li}$ at an incident energy of 455 MeV and at a scattering angle of $0\ifmmode^\circ\else\textdegree\fi{}$ by measuring spin-nonflip and spin-flip spectra. The reaction $Q$-values for the analogs of the GDR and SDR in the \ensuremath{\alpha} clusters of $^{6,7}\mathrm{Li}$ were found to be more negative than those in $^{4}\mathrm{He}$ by $2.0\ifmmode\pm\else\textpm\fi{}0.5$ MeV. The ratios of the cross section for the analog of the GDR to that for the analog of the SDR in $^{4}\mathrm{He}$ and in the \ensuremath{\alpha} clusters of $^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}$ were found to be the same within errors, $0.5\ifmmode\pm\else\textpm\fi{}0.1$. The cross sections for the analogs of the GDR as well as those for the analogs of the SDR in the \ensuremath{\alpha} clusters of $^{6,7}\mathrm{Li}$ were 0.6\ensuremath{\sim}0.8 times smaller than those in $^{4}\mathrm{He}$. These results suggest that excitations of \ensuremath{\alpha} clusters embedded in nuclei are suppressed as compared with excitations of free \ensuremath{\alpha} particles.