\begin{table}%t2
\caption{Fraction (in \%) of positrons forming positronium in flight, 
in a completely neutral medium.\label{tab:f1}}
   $$\begin{array}[b]{lccc}
\hline
\hline
\mbox{References}  & \mbox{H} & \mbox{H$_{2}$} & \mbox{He} \\
\hline
\mbox{BRD79} & 95 & 93 & - \\
\mbox{Brown \& Leventhal} \qquad & - & \quad 89.7 \pm 0.3 \quad & 80.7 \pm 0.5 \\
\mbox{Wallyn et~al. (\cite{Wallyn1994})} & 98 & 90 & - \\
\mbox{Chapuis et~al. (\cite{Chapuis1994})} \qquad & - & - & 78 \\
\mbox{This paper} & 95.5 & 89.6 & 81.7 \\
  \hline
\end{array}$$
\end{table}