9.
如图,直三棱柱\(ABC\)\(-\)\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)中,\(AB\)\(=\)\(AC\)\(=\)\(AA\)\({\,\!}_{1}=4\),\(BC\)\(=2 \sqrt{2}\).\(BD\)\(⊥\)\(AC\),垂足为\(D\),\(E\)为棱\(BB\)\({\,\!}_{1}\)上一点,\(BD\)\(/\!/\)平面\(AC\)\({\,\!}_{1}\)\(E\).
\((I)\)求线段\(B\)\({\,\!}_{1}\)\(E\)的长;
\((II)\)求二面角\(C\)\({\,\!}_{1}-\)\(AC\)\(-\)\(E\)的余弦值.