Evidence for charged-particle decay of dipole-excited H 4 clusters embedded in He 6 and He 7

Charged-particle decay from the dipole resonances at ${E}_{x}=24$ MeV in $^{6}\mathrm{He}$ and at ${E}_{x}=18$ MeV in $^{7}\mathrm{He}$ was studied via the $^{6,7}\mathrm{Li}$($^{7}\mathrm{Li}$,$^{7}\mathrm{Be}$ $x$) coincidence experiment at 455 MeV and at ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$. These resonances are those that were interpreted as the analogs of the dipole resonances in the \ensuremath{\alpha} clusters of $^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}$ in our previous work. The dominant decay channels (branching ratios) of the dipole resonances are found to be $d+t+n$ ($26~32%$) and $\ensuremath{\alpha}+2n$ ($47~69%$) for $^{6}\mathrm{He}$ and $t+t+n$ ($47~55%$) and $\ensuremath{\alpha}+3n$ ($38~47%$) for $^{7}\mathrm{He}$. The branching ratios measured for the $d+t+n$ and $t+t+n$ channels in $^{6}\mathrm{He}$ and $^{7}\mathrm{He}$, respectively, are larger than those calculated in a simple statistical model by a factor of $~100$. These results are consistent with the picture that the observed dipole resonances are analogs of the corresponding dipole resonances in the \ensuremath{\alpha} clusters of $^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}$.