CERN provides researchers with \textbf{test beam} facilities to test detectors before letting them down into the LHC cavern. Our trial period was scheduled between 12\superscript{th} and 22\superscript{nd} August 2010, and it took place at the \textbf{H4} beam line in the CERN site of Pr\'{e}vessin.\\
% CHECK DATES and beam line

Bunches of protons are accelerated in the SPS, a circular particle accelerator, up to a momentum of about $450GeV/c$. In order to feed more than one test beam facility at the same time, the beam extracted from the SPS is then branched into several channels, each of them terminating in a target where the incident protons create secondary particles. In our case, the target (\textbf{T2}) produced pions $ \pi^- $, with a momentum of $ 150 GeV/c $.\\

Both hadron and lepton beams are provided, so that all the possible test requests made by users are covered. Pions have a mean lifetime $\tau = (2,6033 \pm 0,0005) * 10^{-8}s$; their primary decay branch (with \textit{branching ratio} $\Gamma_i / \Gamma = (99,98770 \pm 0,00004) \%$ \cite{pdg}) is leptonic:
$$
\pi^- \longrightarrow \mu^- + \overline{\nu}_\mu
$$
\textbf{H4} beam consists of both pions and their decay products, muons. It is possible to switch to a pure lepton beam by closing the beam collimators: at this energy muons are minimum ionizing particles (\textbf{MIP}s) and pass through the collimators, while pions are stopped.\\

We tested the large prototype GEM detector on:
\begin{itemize}
	\item a $0,8kHz$ $\mu^-$ beam;
	\item a $38kHz$ $\pi^-$ beam;
	\item several $\pi^-$ beams at intermediate intensities (in order to check if it is possible to experience charging up effects).
\end{itemize}