\section{Experimental setup}
\label{sec:experimental_set_up}
This measurement is part of the AlCap experiment done at PSI, Switzerland.
The 2015 summer run focused on the detection of neutral particles: low energy
X-ray, gamma ray and neutron emission after the muon is captured by the
nucleus. 

The X-rays and gamma rays of interest are:
\begin{itemize}
  \item muonic $2p-1s$ transition in aluminum: \SI{346.8}{\kilo\eV}
  \item \SI{843.7}{\kilo\eV} gamma from the $\beta^-$ decay of \ce{^{27}Mg}
    (half-life: \SI{9.46}{\min})
  \item \SI{1808.7}{\kilo\eV} gamma from the first excited state of
    \ce{^{26}Mg}
\end{itemize}

Low momentum muons (less than \SI[]{40}{\mega\eVperc}) were stopped in
a target after passing a muon counter
(\SI{60}{\mm}$\times$\SI{60}{\mm}$\times$\SI{0.5}{\mm} plastic scintillator). 
% Upstream from the muon counter, a 
% \SI{10}{\cm} $\times$ \SI{10}{\cm}  $\times$ \SI{0.6}{\cm} scintillator with
% a \SI{40}{\mm} diameter hole cut in the center acted as a  beam defining
% veto counter to the incoming muon beam.
There were two 5"$\times$2" liquid scintillator BC501a detectors setup on the
beam right to detect neutrons. For gamma spectrum analysis and normalization
we used an HPGe detector installed on the beam left. In addition, a \ce{LaBr3}
scintillator was also tested if it would be suitable to use in the STM. A 25
LYSO crystal array was placed downstream of the target beam left to observe
high energy photons emitted.

Two identical preamplifier outputs from the HPGe detector were fed into: (a)
a timing filter amplifier for timing information, and (b) a spectroscopy
amplifier for energy information. The timing pulses were read out by a 14-bit
500-MS/s desktop digitizer(CAEN DT5730). In order to accommodate both low
energy X-rays and relatively high energy gamma rays, we used two channels from
the spectroscopy amplifier with different gain settings: (a) a lower gain
channel for photons up to \SI{6.5}{\mega\eV}; and (b) a higher gain channel for
photons up to \SI{2.5}{\mega\eV}. These channels were read out by a 14-bit
100-MS/s VME digitizer (CAEN V1724).

The \ce{LaBr3} crystal is coupled with a photomultiplier, of which output
pulses were large enough so no further amplification was needed. This channel
is read out with the DT5730.

% Detectors' outputs were read out using waveform digitizers. We used a 14-bit
% 100-MS/s VME digitizer (CAEN V1724) to record energy signals from
% HPGe and \ce{LaBr3} detectors. There were two energy outputs from the HPGe
% detector with different gain settings: (a) low gain channel for photons up to
% \SI{6.5}{\mega\eV}; and (b) high gain channel for photons up to
% \SI{2.5}{\mega\eV}. The timing signals from these detectors, and signals from
% plastic and liquid scintillators were fed into a faster digitizer, namely
% a 14-bit 500-MS/s desktop digitizer (CAEN DT5730).
% These fast timing channels
% were also read out using a multihit TDC (CAEN V1290A) as a back up solution.
% All digitizers and TDC were synchronized by an external master clock.

Experimental layout  is shown  in \cref{fig:R2015a_setup}.
\begin{center}
  \begin{figure}[tbp]
    \centering
    \includegraphics[width=0.70\textwidth]{figs/r15a_setup_photo}
    \caption{Layout of the AlCap 2015 summer run. Muons entered from the top of
      the image. The LYSO detector is not visible in this image, which is
      located further out in the bottom of the image.} 
    \label{fig:R2015a_setup}
  \end{figure}
\end{center}

There were several runs with different targets made of aluminum, titanium,
lead, water. All targets were sufficiently thick to stop the muon beam with
momenta up to \SI{40}{\mega\eVperc}.


