%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 (wire chamber, plastic scintillator) and a downstream muon veto (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
	\subfigure{
	\includegraphics[width=0.55\textwidth]{figs/Chamber_layout} 
	}
	\subfigure{
	\includegraphics[width=0.4\textwidth,trim= 0 -20 0 0]{figs/Chamber_dimensions} 
	}
	\caption{Layout of the AlCap experiment for R2013. Left: Photograph final setup.  Muons entered from the right of the image and after passing through the muon trigger system and collimator stopped in the target in the centre.  Charged particles could then be detected in the Silicon detector packages(top and bottom), X-rays in the germanium (lower-right corner, out of image) and neutrons in the liquid scintillator (lower-left corner, out of image). Right: The dimensions of the R2013 setup.  All measurements are in \si{mm}.
%The two silicon packages inside the vacuum vessel (SiL and SiR) measure the charged particles emitted from the target, the upstream muon counters consist of plastic scintillators and a wire chamber for defining the input muon beam ($\mu$PC), 
%germanium detector measures the emitted X-rays and gamma rays and the veto plastic scintillator vetos muons that don't stop in target ($\mu$Ve).
}
	\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\per c} and momentum spread of
3\% FWHM.

\subsubsection{Silicon detectors}
\label{ssub:silicon_detectors}
The main detectors for charged particles measurement are four large area 
silicon detectors. The silicon detectors were grouped into two detector
packages located symmetrically at 90 degrees relative to
the nominal muon beam path, SiL
and SiR in \cref{fig:alcap_setup_detailed}. Each arm consists of: one
$\Delta$E counter, a \SI{65}{\micro\meter}-thick silicon detector, divided into
4 quadrants; one E counter made from \SI{1500}{\micro\meter}-thick silicon; and
one plastic scintillator to identify electrons or high energy protons that
pass through both silicon detectors.
The area of each of these silicon detectors and the
scintillators is $50\times50 \textrm{mm}^2$. 

Due to the large capacitance of the thin detectors, noise and
pickup suppression had to be carefully optimised in the real PSI accelerator
environment. The achievable electronic resolution was between 55 and 76
keV FWHM in the thin silicon detectors, and between 35 and 40 keV for the
thick detectors.

\subsubsection{Germanium detector}
\label{ssub:germanium_detectors}
We used a germanium detector to normalise the number of stopped muons by
measuring characteristic muonic X-rays from the target material. The primary
X-rays of interest are the \atrn{2p}{1s} transitions,
the 346.828~keV line for aluminium targets, and the
400.177~keV line for silicon targets.

The germanium detector is
a model GMX20P4-70-RB-B-PL, n-type, coaxial high purity germanium detector produced
by ORTEC. The detector was optimised for low energy gamma and X-ray
measurements, with an ultra-thin entrance window of 0.5-mm-thick beryllium and
a 0.3-\si{\micro\meter}-thick ion implanted contact. The germanium crystal is
\SI{52.5}{\mm} in diameter and \SI{55.3}{\mm} in length. The axial well has
a diameter of \SI{9.9}{\mm} and is \SI{47.8}{\mm} deep.

The detector was energy and acceptance calibrated with the many lines
from a Eu-152 source of known activity.
Energy resolutions are better than 2 keV for all calibrated peaks. Absolute
efficiencies at the energies of interest are listed in \cref{tab:xray_ref}.
\begin{table}[btp]
  \centering
  \caption{Reference values of major muonic X-rays from aluminium and silicon. Energy and intensity values are taken from~\cite{MeasdayAl}.}
    \begin{tabular}{c l l l l c}
    \addlinespace
      \toprule
      \textbf{Elements}  & \textbf{Transition}
      & \textbf{Energy (keV)} & \textbf{Intensity (\%)} & \textbf{Calibrated eff.}\\
      \midrule 
      $^{27}\textrm{Al}$ &  \atrn{2p}{1s} & $346.828 \pm 0.002$ & $79.8\pm 0.8$ &$4.95\times10^{-4}$\\
         &  \atrn{3p}{1s} & $412.87  \pm 0.05$ &  $7.62\pm 0.15$ & $4.41\times10^{-4}$\\
      \addlinespace
      $^{28}\textrm{Si}$ &  \atrn{2p}{1s} & $400.177 \pm 0.005$ & $80.3\pm 0.8$
      &$4.40\times10^{-4}$\\
       &  \atrn{3p}{1s} & $476.80 \pm 0.05$ & $7.40 \pm 0.20$ &$3.81\times10^{-4}$\\
      \bottomrule
    \end{tabular}
  \label{tab:xray_ref}
\end{table}


\subsubsection{Neutron detectors}
\label{ssub:neutron_detectors}


The main emphasis of R2013 was on the measurement of charged
particle emission after muon capture, however, some neutron
emission data were collected and used to develop analysis
tools, and to study efficiencies and rates. Two BC501 neutron
counters ($5''$ diameter  and $5''$ depth) were borrowed from the
MuSun experiment, and placed outside the vacuum chamber on
both sides of the muon beam and centred on the stopping
target.  Data collection used a 12-bit, 170-MHz FADC and readout
was by means of the MIDAS framework.

%346.828,4.95E-04,1.216E-05
%412.87,4.41E-04,9.780E-06
%400.177,4.40E-04,9.747E-06
%476.800,3.81E-04,7.680E-06

