Effect of the nuclear medium onα-cluster excitation inLi6

Abstract

The giant dipole resonance (GDR) in $^{6}\mathrm{Li}$ was investigated via the $^{6}\mathrm{Li}(\ensuremath{\gamma},xn)$ reactions by using quasi-mono-energy $\ensuremath{\gamma}$ rays in an energy range from 4.9 to 53.6 MeV. The $\ensuremath{\gamma}$ rays were generated via Compton backscattering of Nd laser photons with relativistic energy electrons in an electron storage ring, NewSUBARU. The energy resolution in a full width at half maximum of $\ensuremath{\gamma}$ ray was simulated to be $5%$ at 50 MeV. Photoneutrons were detected with a $4\ensuremath{\pi}$-type neutron detector consisting of 41 $^{3}\mathrm{He}$-gas proportional counters. The $(\ensuremath{\gamma},n)$ cross sections were dominant, while the $(\ensuremath{\gamma},2n)$ and $(\ensuremath{\gamma},3n)$ cross sections were negligibly small. The energy integral of photoneutron cross sections up to 53.6 MeV was 59 $\mathrm{MeV}\phantom{\rule{0.16em}{0ex}}\mathrm{mb}$, which exhausted $65%$ of the Thomas-Reiche-Kuhn sum rule. The GDR in $^{6}\mathrm{Li}$ was found to consist of mainly two components. The peak energy and the width for the low-energy component were ${E}_{r}=12\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and $\mathrm{\ensuremath{\Gamma}}=21\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. Those for the high-energy component were ${E}_{r}=33\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and $\mathrm{\ensuremath{\Gamma}}=30\ifmmode\pm\else\textpm\fi{}2$ MeV. The low-energy component corresponded to the GDR in $^{6}\mathrm{Li}$. The high-energy component was inferred to be the GDR owing to an $\ensuremath{\alpha}$-cluster excitation in $^{6}\mathrm{Li}$. The existence of this component was recently proposed and was suggested by the experimental studies of the $(p,{p}^{\ensuremath{'}})$, $(^{3}\mathrm{He},t)$, and $(^{7}\mathrm{Li},^{7}\mathrm{Be})$ reactions. The observed resonance shape of the high-energy component was well reproduced by modifying the GDR shape of a theoretical prediction for $^{4}\mathrm{He}$ at ${E}_{r}=26\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ with $\mathrm{\ensuremath{\Gamma}}=20\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$; with increasing the excitation energy by 7 MeV ($Q$ value was more negative), widening the width by $1.5\ifmmode\pm\else\textpm\fi{}0.1$ times, and decreasing a peak height by $0.29\ifmmode\pm\else\textpm\fi{}0.02$ times. As a result, the magnitude of the energy integral of the cross sections for the high-energy component observed in the present work was $0.86\ifmmode\pm\else\textpm\fi{}0.06$ times that in the theoretical prediction of the $^{4}\mathrm{He}(\ensuremath{\gamma},n)$ reaction. It is a well-known fact that a frequency of a vibrating system is inversely proportional to the size of the system. We suggest that in excitation of the $\ensuremath{\alpha}$ cluster in $^{6}\mathrm{Li}$, the mass of the $\ensuremath{\alpha}$ cluster increases by $7\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$, the size of the $\ensuremath{\alpha}$ cluster in $^{6}\mathrm{Li}$ is smaller than that of the free $^{4}\mathrm{He}$ by $\ensuremath{\sim}20%$, and the width of the GDR is broader than that of $^{4}\mathrm{He}$ by 1.5 times owing to the nuclear medium effect.