\begin{table}%t2
%\centering
\par
\caption{$r$-process production ratios.\label{pr}}
\begin{tabular}{lllllll}
%
\hline\hline
\noalign{\smallskip}
Reference & \multicolumn{5}{c}{Production ratio~$\beta$} \\
     & Th/Eu & Th/Hf & Th/Os & Th/Ir & U/Th\\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\citet{kratz07} & 0.453 & 0.862 & 0.093 & 0.089 & 0.638\\
\citet{sneden03}            & 0.420\\
\citet{schatz02}            & 0.468 &       & 0.071 & 0.066\\
\citet{cowan02}             & 0.507 &       &       & 0.087\\
\hline
\end{tabular}\vspace*{2mm}
\end{table}