\section{Fast rotation histogram for different positron energy thresholds}
\label{app:frs_eth}

Figure~\ref{fig:frs_eth} shows the fast rotation signal between 4-5 $\mu s$ after injection for different positron energy thresholds.

\begin{figure}[bt]
\centering
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_500}}
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_700}}
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_900}}
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_1100}}
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_1300}}
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_1500}}
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_1700}}
\subfigure[]{\includegraphics[width=0.42\textwidth]{fig/eth_1900}}
\caption{Fast rotation histograms from 4 to 5 $\mu s$ after injection for 8 positron energy thresholds: (a) 500, (b) 700, (c) 900, (d) 1100, (e) 1300, (f) 1500, (g) 1700, 
and (h) 1900 MeV.}
\label{fig:frs_eth}
\end{figure}

