%The first run of the AlCap experiment was performed at the $\pi$E1 beam line
%area, PSI from November 26 to December 23, 2013. The goal of the run was to
%measure protons rate and their spectrum following muon capture on aluminium.

The low energy muons from the $\pi$E1 beam line were stopped in thin aluminium
and silicon targets, and charged particles emitted were measured by two pairs
of silicon detectors inside of a vacuum vessel
(\cref{fig:alcap_setup_detailed}).  A stopped muon event is defined by
a group of upstream detectors and a muon veto plastic scintillator. 
The number of stopped muons is monitored by a germanium detector placed outside
of the vacuum chamber. In addition, several plastic scintillators were used to
provide veto signals for the silicon and germanium detectors.
\begin{figure}[btp] 
	\centering
	\includegraphics[width=0.55\textwidth]{figs/alcap_setup_detailed} 
	\caption{AlCap detectors: two silicon packages inside the vacuum vessel, 
    muon beam detectors including plastic scintillators and a wire chamber, 
    germanium detector and veto plastic scintillators.}
	\label{fig:alcap_setup_detailed} 
\end{figure} 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\subsubsection{Muon beam}
\label{ssub:muon_beam}
%\Cref{fig:alcap_setup_detailed} shows the experimental setup. The muon
%beam enters from the right of the image and hits the target, which is
%placed at the center of the vacuum chamber and orientated at 45
%degrees to the beam axis.

  %In order to define
%stopped muon events, four muon counters are used: a 500~\rmmu m thick
%scintillator muon trigger counter (\rmmu SC); a muon anti-coincidence
%counter (\rmmu SCA) surrounding the trigger counter with a hole of 35
%mm diameter to define the beam radius; and a multiwire proportional
%chamber (\rmmu PC) that uses 24 X wires and 24 Y wires with at 2 mm
%intervals. This detector system belongs to the MuSun experiment and
%was well tuned in advance.  A muon veto counter (\rmmu Ve) is placed
%at the downstream end of the chamber and is used to reject muons that
%pass through the stopping target.
One of the main requirements of the AlCap experiment was a muon beam
with narrow momentum bite in order to achieve a high fraction of
stopping muons in the very thin targets. The actual set up used in the Run
2013 was: muon momentum from \SIrange{28}{45}{\MeV\cc} and momentum spread of
3\% FWHM.