339 lines
17 KiB
Groff
339 lines
17 KiB
Groff
%The first run of the AlCap experiment was performed at the $\pi$E1 beam line
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%area, PSI from November 26 to December 23, 2013. The goal of the run was to
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%measure protons rate and their spectrum following muon capture on aluminium.
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The low energy muons from the $\pi$E1 beam line were stopped in thin aluminium
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and silicon targets, and charged particles emitted were measured by two pairs
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of silicon detectors inside of a vacuum vessel
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(\cref{fig:alcap_setup_detailed}). A stopped muon event is defined by
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a group of upstream detectors and a muon veto plastic scintillator.
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The number of stopped muons is monitored by a germanium detector placed outside
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of the vacuum chamber. In addition, several plastic scintillators were used to
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provide veto signals for the silicon and germanium detectors. Two liquid
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scintillators for neutron measurements were also tested in this run.
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\begin{figure}[btp]
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\centering
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\includegraphics[width=0.55\textwidth]{figs/alcap_setup_detailed}
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\caption{AlCap detectors: two silicon packages inside the vacuum vessel,
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muon beam detectors including plastic scintillators and a wire chamber,
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germanium detector and veto plastic scintillators.}
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\label{fig:alcap_setup_detailed}
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\end{figure}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsubsection{Muon beam and vacuum chamber}
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The muon beam of low energy at \SIrange{28}{45}{\MeV\per\cc}, and narrow
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momentum spread of 3\% were used.
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\Cref{fig:Rates} shows the measured muon rates
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as a function of momentum for two different momentum bites.
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\Cref{fig:Beam} shows an example of the resulting energy spectra recorded by
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our silicon detector.
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\begin{figure}[btp]
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\centering
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\includegraphics[width=0.65\textwidth]{figs/Rates.png}
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\caption{Measured muon rates at low momenta during the Run 2013. Beam rates
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at 1 \% FWHM momentum bite were about 3 times smaller than the rates at
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3 \% FWHM.}
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\label{fig:Rates}
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\end{figure}
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\begin{figure}[btp]
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\centering
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\includegraphics[width=1.00\textwidth]{figs/beam.pdf}
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\caption{Energy deposition at \SI{36.4}{/c} incident muon beam in an
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\SI{1500}{\micro\meter}-thick active target. The peak at low energy is due
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to beam electrons, the peaks at higher energies are due to muons. Momentum
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bite of 1 and 3\% FWHM on left and right hand side, respectively. The
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electron peak are the same in both plots as beam electrons are minimum
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ionisation particles and passed though the detector easily. The muon peak
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at the 3 \% FWHM momentum bite is notably broader than that at 1 \% FWHM
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setting.}
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\label{fig:Beam}
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\end{figure}
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The targets and charged particle detectors are installed inside the vacuum
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chamber as shown in \cref{fig:alcap_setup_detailed}. The muon beam enters
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from the right of \cref{fig:alcap_setup_detailed} and hits the target, which is
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placed at the centre of the vacuum chamber and orientated at 45 degrees to the
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beam axis.
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The side walls and bottom flange of the vessel provide several
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vacuum-feedthroughs for the high voltage and signal cables for the silicon and
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scintillator detectors inside the chamber.
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In addition, the chamber is equipped with several lead collimators
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to quickly capture muons that do not stop in the actual target.
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For a safe operation of the silicon detector, a vacuum of \SI{e-4}{\milli\bar}
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was necessary. With the help of the vacuum group of PSI, we could consistently
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reach the required vacuum level within 45 minutes after closure of the
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chamber's top flange.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsubsection{Silicon detectors}
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The main detectors for charged particles measurement are four large area
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silicon detectors. The silicon detectors were grouped into two detector
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packages located symmetrically at 90 degrees of the nominal muon beam path, SiL
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and SiR in \cref{fig:alcap_setup_detailed}. Each arm consists of: one
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$\Delta$E counter, a \SI{65}{\micro\meter}-thick silicon detector, divided into
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4 quadrants; one E counter made from \SI{1500}{\micro\meter}-thick silicon; and
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one plastic scintillator to identify electrons or high energy protons that
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pass through the silicon. The area of each of these silicon detectors and the
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scintillators is $50\times50 \textrm{mm}^2$.
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%There is a dead layer of
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%\SI{0.5}{\micro\meter} on each side of the silicon detectors according to the
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%manufacturer Micron Semiconductor
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%\footnote{\url{http://www.micronsemiconductor.co.uk/}}.
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%The detectors were named according to their positions relative to the muon
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%view: the SiL package contains the thin
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%detector SiL1 and thick detector SiL2; the SiR package has SiR1 and SiR2
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%accordingly. Each quadrant of the thin detectors were also numbered from 1 to
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%4, i.e. SiL1-1, SiL1-2, SiL1-3, SiL1-4, SiR1-1, SiR1-2, SiR1-3,
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%SiR1-4.
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Bias for the four silicon detectors was supplied by an ORTEC 710 NIM module,
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which has a vacuum interlock input to prevent biasing before the safe vacuum
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level has been reached. Typical voltage to fully depleted the detectors were
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\SI{-300}{\volt} and \SI{-10}{\volt} for the thick and thin silicon detectors
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respectively. The leakage currents at the operating voltages are less than
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\SI{1.5}{\micro\ampere} for the thick detectors, and about
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\SI{0.05}{\micro\ampere} for the thin ones (see \cref{fig:si_leakage}).
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\begin{figure}[btp]
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\centering
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\includegraphics[width=0.85\textwidth]{figs/si_leakage}
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\caption{Leakage currents of the silicon detectors under bias.}
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\label{fig:si_leakage}
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\end{figure}
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The fact that a detector were fully depleted was checked by putting
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a calibration source $^{241}\textrm{Am}$ at its ohmic side, and observing the
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output
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pulse height on an oscilloscope. One would expect that the maximum pulse height
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increases as the bias is raised until the voltage of fully depleted. The effect
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can also be seen on the pulse height spectrum as in
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\cref{fig:sir2_bias_alpha}.
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\begin{figure}[btp]
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\centering
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\includegraphics[width=0.75\textwidth]{figs/sir2_bias_alpha}
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\caption{$^{241}\textrm{Am}$ spectra in cases of fully depleted (top), and
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partly depleted (bottom).}
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\label{fig:sir2_bias_alpha}
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\end{figure}
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% subsubsection silicon_detectors (end)
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsubsection{Upstream counters}
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\label{sub:upstream_counters}
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The upstream detector consists of three counters: a \SI{500}{\micro\meter}-thick
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scintillator muon trigger counter ($\mu$SC); a muon anti-coincidence counter
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($\mu$SCA) surrounding the trigger counter with a hole
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of 35 \si{\milli\meter}\ in diameter to define the beam radius; and a multi-wire
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proportional chamber ($\mu$PC) that uses 24 X wires and 24 Y wires at
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2~\si{\milli\meter}~intervals.
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This set of detectors along with their read-out system
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belong to the MuSun experiment, which operated at the same beam line just
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before our run. Thanks to the MuSun group, the detectors were well-tuned and
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ready to be used in our run without any modification.
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% subsubsection upstream_counters (end)
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsubsection{Germanium detector}
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%\begin{figure}[btp]
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%\centering
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%\includegraphics[width=0.9\textwidth]{figs/neutron.png}
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%\caption{Setup of two
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%liquid scintillators outside the vacuum envelope for neutron detection.}
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%\label{fig:neutron}
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%\end{figure}
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We used a germanium detector to normalise the number of stopped muons by
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measuring characteristics muon X-rays from the target material. The primary
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X-rays of interest are the 346.828~keV line for aluminium targets, and the
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400.177 line for silicon targets. The energies and intensities of the X-rays
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listed in \cref{tab:xray_ref} follow measurement results from
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Measday and colleagues~\cite{MeasdayStocki.etal.2007}.
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\begin{table}[btp]
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\begin{center}
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\begin{tabular}{c l l l l }
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\toprule
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\textbf{Elements} & \textbf{Transition}
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& \textbf{Energy} & \textbf{Intensity}\\
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\midrule
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$^{27}\textrm{Al}$ & $2p-1s$ & $346.828 \pm 0.002$ & $79.8\pm 0.8$\\
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& $3p-1s$ & $412.87 \pm 0.05$ & $7.62\pm 0.15$\\
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\midrule
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$^{28}\textrm{Si}$ & $2p-1s$ & $400.177 \pm 0.005$ & $80.3\pm 0.8$\\
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& $3p-1s$ & $476.80 \pm 0.05$ & $7.40 \pm 0.20$\\
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\bottomrule
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\end{tabular}
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\end{center}
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\caption{Reference values of major muonic X-rays from aluminium and silicon.}
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\label{tab:xray_ref}
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\end{table}
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The germanium detector is
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a GMX20P4-70-RB-B-PL, n-type, coaxial high purity germanium detector produced
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by ORTEC. The detector was optimised for low energy gamma and X-rays
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measurement with an ultra-thin entrance window of 0.5-mm-thick beryllium and
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a 0.3-\si{\micro\meter}-thick ion implanted contact. The germanium crystal is
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\SI{52.5}{\mm} in diameter, and \SI{55.3}{\mm} in length. The axial well has
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a diameter of \SI{9.9}{\mm} and \SI{47.8}{\mm} deep.
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%(\cref{fig:ge_det_dimensions}).
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%\begin{figure}[btp]
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%\centering
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%\includegraphics[width=0.9\textwidth]{figs/ge_det_dimensions}
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%\caption{Dimensions of the germanium detector}
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%\label{fig:ge_det_dimensions}
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%\end{figure}
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The detector was installed outside of the vacuum chamber at 32 cm from the
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target, viewing the target through a 10-mm-thick aluminium window, behind
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a plastic scintillator counter used to veto electrons. Liquid nitrogen
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necessary for the operation of the detector had to be refilled every 8 hours.
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\subsubsection{Plastic and liquid scintillators}
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\label{sub:plastic_scintillators}
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Apart from the scintillators in the upstream group, there were four other
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plastic scintillators used as veto counters for:
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\begin{itemize}
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\item punch-through-the-target muons, ScVe
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\item electrons and other high energy charged particles for germanium
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detector (ScGe) and silicon detectors (ScL and ScR)
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\end{itemize}
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The ScL, ScR and ScVe were installed inside the vacuum vessel and were
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optically connected to external PMTs by light-guides at the bottom flange.
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We also set up two liquid scintillation counters for neutron measurements in
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preparation for the next beam time where the neutron measurements will be
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carried out.
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% subsubsection plastic_scintillators (end)
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsubsection{Front-end electronics and data acquisition system}
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The front-end electronics of the AlCap experiment was simple since we employed
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a trigger-less read out system with waveform digitisers and flash ADCs
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(FADCs). As shown in \cref{fig:alcapdaq_scheme}, all plastic
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scintillators signals were amplified by PMTs, then fed into the digitisers. The
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signals from silicon and germanium detectors were preamplified, and
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subsequently shaped by spectroscopy amplifiers and timing filter amplifiers
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(TFAs) to provide energy and timing information.
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\begin{figure}[btp]
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\centering
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\includegraphics[width=0.99\textwidth]{figs/alcapdaq_scheme}
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\caption{Schematic diagram of the electronics and DAQ used in the Run 2013}
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\label{fig:alcapdaq_scheme}
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\end{figure}
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The germanium detector has its own transistor reset preamplifier
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installed very close to the germanium crystal. Two ORTEC Model 142
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preamplifiers were used for the thick silicon detectors. The timing outputs of
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the preamplifiers were fed into three ORTEC Model 579 TFAs.
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We used an ORTEC Model 673 to shape the germanium signal with 6~\si{\micro\second}
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shaping time.
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A more modern-style electronics was used for thin silicon detectors where the
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preamplifier, shaping and timing amplifiers were implemented on one compact
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package, namely a Mesytec MSI-8 box. This box has 8 channels, each channel
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consists of one preamplifier board and one shaper-and-timing filter board which
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can be fine-tuned independently. The shaping time was set to 1~\si{\micro\second}\
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for all channels.
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The detector system produced signals that differs significantly in time scale,
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ranging from very fast (about 40~\si{\nano\second}\ from scintillators) to very slow
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(several \si{\micro\second}\ from shaping outputs of semiconductor detectors). This
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lead to the use of several sampling frequencies from 17~\si{\mega\hertz}\ to
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250~\si{\mega\hertz}, and three types of digitisers were employed:
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\begin{itemize}
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\item custom-built 12-bit 170-MHz FADCs which was designed for the
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MuCap experiment. Each FADC board has the same dimensions as those of
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a single-width 6U VME module, but is hosted in a custom built crate due to
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its different power supply mechanical structure. The FADC communicates with
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a host computer through a 100-Mb/s Ethernet interface using a simple
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Ethernet-level protocol. The protocol only allows detecting
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incomplete data transfers but no retransmitting is possible due to the
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limited size of the module's output buffer. The FADCs accept clock signal
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at the frequency of 50~\si{\mega\hertz}\ then multiply that internally up to
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170~\si{\mega\hertz}. Each channel on one board can run at different sampling
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frequency not dependent on other channels. The FADC has 8 single-ended
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LEMO inputs with 1~\si{\volt} pp dynamic range.
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\item a 14-bit 100-MS/s CAEN VME FADC waveform digitiser model V1724. The
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module houses 8 channels with 2.25~Vpp dynamic range on single-ended MCX
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coaxial inputs. The digitiser features an optical link for transmission of
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data to its host computer. All of 8 channels run at the same sampling
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frequency and have one common trigger.
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\item a 12-bit 250-MS/s CAEN desktop waveform digitizer model DT5720. This
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digitiser is similar to the V1724, except for its form factor and maximum
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sampling frequency. Although there is an optical link available, the module
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is connected to its host computer through a USB 2.0 interface where data
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transfer rate of 30 MB/s was determined to be good enough in our run
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(actual data rate from this digitiser was typically about 5 MB/s during the
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run). Communication with both CAEN digitisers was based on CAEN's
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proprietary binary drivers and libraries.
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\end{itemize}
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All digitisers were driven by external clocks which were derived from the same
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500-\si{\mega\hertz}\ master clock, a high precision RF signal generator Model SG382
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of Stanford Research System.
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The silicon detectors were read out by FADC boards feature network-based data
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readout interface. To maximize the data throughput, each of the four FADC
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boards was read out through separate network adapter.
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The CAEN digitisers were used to read out
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the germanium detector (timing and energy, slow signals) or scintillator
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detectors (fast signals). For redundancy, all beam monitors ($\mu$SC, $\mu$SCA
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and $\mu$PC) were also read out by a CAEN time-to-digital converter (TDC)
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model V767 which was kindly provided by the MuSun experiment.
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The Data Acquisition System (DAQ) of the AlCap experiment, so-called AlCapDAQ,
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provided the readout of front-end electronics, event assembling, data logging,
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hardware monitoring and control, and the run database of the experiment
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(\cref{fig:alcapdaq_pcs}). It was based on the MIDAS framework~\footnote{
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MIDAS is a general purpose DAQ software system developed at PSI and TRIUMF:\\
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\url{http://midas.triumf.ca}} and consisted of two circuits, {\em i})
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a detector circuit for synchronous data readout from the front-end electronics
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instrumenting detectors, and {\em ii}) a slow control circuit for asynchronous
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periodic hardware monitoring (vacuum, liquid nitrogen
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filling). The detector circuit consisted of three computers, two front-end
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computers and one computer serving both as a front-end and as a back-end
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processor. The slow circuit consisted of one computer. All computers were
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running Linux operating system and connected into a private subnetwork.
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%\hl{TODO: storage and shift monitor}
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\begin{figure}[btp]
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\centering
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\includegraphics[width=0.95\textwidth]{figs/alcapdaq_pcs}
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\caption{AlCapDAQ in the Run 2013. The {\ttfamily fe6} front-end is
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a VME single board computer belongs to the MuSun group, reads out the
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upstream detectors.}
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\label{fig:alcapdaq_pcs}
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\end{figure}
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The data were collected as dead-time-free time segments of 110~ms, called
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``block'', followed by about 10-ms-long time intervals used to complete data
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readout and synchronize the DAQ. Such data collection approach was chosen to
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maximize the data readout efficiency. During each 110-ms-long period, signals
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from each detector were digitized independently by threshold crossing. The data
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segment of each detector data were first written into on-board memories of
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front-end electronics and either read out in a loop (CAEN TDCs and CAEN
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digitizers) or streamed (FADCs) into the computer memories. The thresholds were
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adjusted as low as possible and individually for each detector. The time
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correlation between detectors would be established in the analysis stage.
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At the beginning of each block, the time counter in each digitiser is reset to
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ensure time alignment across all modules. The period of 110~ms was chosen to be:
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{\em i}) long enough compared to the time scale of several \si{\micro\second}\
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of the physics of interest, {\em ii}) short enough so that there is no timer
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rollover on any digitiser (a FADC runs at its maximum speed of
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\SI{170}{\mega\hertz} could handle up to about \SI{1.5}{\second} with its
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28-bit time counter).
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To ease the task of handling data, the data collecting period was divided into
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short runs, each run stopped when the logger had recorded 2 GB of data.
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The data size effectively made each run last for about 5 minutes. The DAQ
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automatically started a new run with the same parameters after about 6 seconds.
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The short period of each run also allows the detection, and helps to reduce the
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influence of effects such as electronics drifting, temperature fluctuation.
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