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nam
2014-10-12 00:37:16 +09:00
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thesis/chapters/chap5_alcap_setup.tex
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@@ -19,7 +19,7 @@ provide veto signals for the silicon and germanium detectors. Two liquid
scintillators for neutron measurements were also tested in this run.
\begin{figure}[btp]
\centering
\includegraphics[width=0.55\textwidth]{figs/alcap_setup_detailed}
\includegraphics[width=0.65\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.}
@@ -28,22 +28,22 @@ scintillators for neutron measurements were also tested in this run.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Muon beam and vacuum chamber}
Muons in the $\pi$E1 beam line are decay products of pions created
as a \SI{590}{\mega\electronvolt} proton beam hits a thick carbon target
(E-target in \cref{fig:psi_exp_hall_all}). The beam line was designed to
deliver muons with momenta ranging from
\SIrange{10}{500}{\mega\electronvolt\per\cc} and
momentum spread from \SIrange{0.26}{8.0}{\percent}. These parameters can be
as a \SI{590}{\mega\electronvolt} proton beam hits a thick carbon target. The
beam line was designed to deliver muons with momenta ranging from
\SIrange{10}{500}{\mega\electronvolt\per\cc} and momentum spread from
\SIrange{0.26}{8.0}{\percent}~\cite{Foroughli.1997}. These parameters can be
selected by changing various magnets and slits shown in
\cref{fig:psi_piE1_elements}~\cite{Foroughli.1997}.
\cref{fig:psi_piE1_elements}.
\begin{figure}[p]
\centering
\includegraphics[height=0.85\textheight]{figs/psi_exp_hall_all}
\caption{Layout of the PSI experimental hall, $\pi$E1 experimental area is
marked with the red circle. \\Image taken from
\url{http://www.psi.ch/num/FacilitiesEN/HallenplanPSI.png}}
\label{fig:psi_exp_hall_all}
\end{figure}
%(E-target in \cref{fig:psi_exp_hall_all}).
%\begin{figure}[p]
%\centering
%\includegraphics[height=0.85\textheight]{figs/psi_exp_hall_all}
%\caption{Layout of the PSI experimental hall, $\pi$E1 experimental area is
%marked with the red circle. \\Image taken from
%\url{http://www.psi.ch/num/FacilitiesEN/HallenplanPSI.png}}
%\label{fig:psi_exp_hall_all}
%\end{figure}
\begin{figure}[btp]
\centering
@@ -402,8 +402,8 @@ correlation between detectors would be established in the analysis stage.
At the beginning of each block, the time counter in each digitiser is reset to
ensure time alignment across all modules. The period of 110~ms was chosen to be:
{\em i} long enough compared to the time scale of several \si{\micro\second}\
of the physics of interest, {\em ii} short enough so that there is no timer
{\em i}) long enough compared to the time scale of several \si{\micro\second}\
of the physics of interest, {\em ii}) short enough so that there is no timer
rollover on any digitiser (a FADC runs at its maximum speed of
\SI{170}{\mega\hertz} could handle up to about \SI{1.5}{\second} with its
28-bit time counter).
@@ -430,8 +430,8 @@ The energy calibration for the silicon detectors were done routinely during the
run, by:
\begin{itemize}
\item a \SI{79.5}{\becquerel} $^{241}\textrm{Am}$ alpha source. The most
prominent alpha particles have energies of \SI{5.484}{\si{\MeV}} (85.2\%)
and \SI{5.442}{\si{\MeV}} (12.5\%). The alpha particles from the source
prominent alpha particles have energies of \SI{5.484}{\MeV} (85.2\%)
and \SI{5.442}{\MeV} (12.5\%). The alpha particles from the source
would lose about \SI{66}{\kilo\eV} in the \SI{0.5}{\um}-thick dead layer,
and the peak would appear at \SI{5418}{\kilo\eV} (\cref{fig:toyMC_alpha});
@@ -753,12 +753,12 @@ sets are shown in \cref{tb:stat}.
Al 100 \si{\micro\meter}& 1.09& 14.37&$2.94 \times 10^8$\\
& 1.07& 2.56& $4.99 \times 10^7$\\
\midrule
Al 50 \si{\micro\meter} m & 1.07& 51.94& $8.81 \times 10^8$\\
Al 50 \si{\micro\meter} & 1.07& 51.94& $8.81 \times 10^8$\\
\bottomrule
\end{tabular}
\end{center}
\caption{Run statistics. Momentum scaling
normalized to 28 MeV/c.}
\caption{Run statistics. Momentum scaling factors are normalised to
\SI{28}{\MeV\per\cc}.}
\label{tb:stat}
\end{table}
@@ -913,12 +913,126 @@ example, the X-ray spectrum analysis was done to confirm that we could observe
the muon capture process and to help in choosing optimal momenta which
maximised the number of stopped muons.
Although the offline analyser is still not fully developed yet, several modules
are ready. They are described in detailed in the next chapter.
Although the offline analyser is still not fully available yet, several modules
are ready(\cref{tab:offline_modules}). An initial analysis is possible using
the existing modules thanks to the modularity of the analysis framework.
\begin{table}[htb]
\begin{center}
\begin{tabular}{l p{8cm}}
\toprule
\textbf{Module name} & \textbf{Functions}\\
\midrule
MakeAnalysedPulses & make a pulse with parameters extracted from
a waveform\\
MaxBinAPGenerator & simplest algorithm to get pulse information\\
TSimpleMuonEvent & sort pulses occur in a fixed time window around the
muon hits\\
ExportPulse \& PulseViewer & plot waveforms for diagnostics\\
PlotAmplitude & plot pulse height spectra\\
PlotAmpVsTdiff & plot pulse correlations in timing and amplitude\\
EvdE & plot \sdEdx histograms\\
\bottomrule
\end{tabular}
\end{center}
\caption{Available offline analysis modules.}
\label{tab:offline_modules}
\end{table}
The MakeAnalysedPulses module takes a raw waveform, calculates the pedestal
from a predefined number of first samples, subtracts this pedestal taking
pulse polarity into account, then calls another module to extract pulse
parameters. At the moment, the simplest module, so-called MaxBinAPGenerator,
for pulse information calculation is in use. The module looks for the
sample that has the maximal deviation from the baseline, takes the deviation as
pulse amplitude and the time stamp of the sample as pulse time. The procedure
is illustrated on \cref{fig:tap_maxbin_algo}. This module could not handle
pile-up or double pulses in one \tpulseisland{} in \cref{fig:tap_maxbin_bad}.
\begin{figure}[htb]
\centering
\includegraphics[width=0.85\textwidth]{figs/tap_maxbin_algo}
\caption{Pulse parameters extraction with MaxBinAPGenerator.}
\label{fig:tap_maxbin_algo}
\end{figure}
\begin{figure}[htb]
\centering
\includegraphics[width=0.47\textwidth]{figs/tap_maxbin_bad}
\includegraphics[width=0.47\textwidth]{figs/tap_maxbin_bad2}
\caption{Double pulse and pile up are taken as one single pulse by the
MaxBinAPGenerator}
\label{fig:tap_maxbin_bad}
\end{figure}
The TSimpleMuonEvent first picks a muon candidate, then loops through all
pulses on all detector channels, and picks all pulses occur in
a time window of \SI{\pm 10}{\si{\us}} around each candidate to build
a muon event. A muon candidates is a hit on the upstream plastic scintillator
with an amplitude higher than a threshold which was chosen to reject MIPs. The
period of \SI{10}{\si{\us}} is long enough compared to the mean life time of
muons in the target materials
(\SI{0.758}{\si{\us}} for silicon, and \SI{0.864}{\si{\us}}
for aluminium~\cite{SuzukiMeasday.etal.1987}) so practically all of emitted
charged particles would be recorded in this time window.
%\begin{figure}[htb]
%\centering
%\includegraphics[width=0.85\textwidth]{figs/tme_musc_threshold}
%\caption{Pulse height spectrum of the $\mu$Sc scintillator}
%\label{fig:tme_musc_threshold}
%\end{figure}
A pile-up protection mechanism is employed to reject multiple muons events: if
there exists another muon hit in less than \SI{15}{\us} from the
candidate then both the candidate and the other muon are discarded. This
pile-up protection would cut out less than 11\% total number of events because
the beam rate was generally less than \SI{8}{\kilo\hertz}.
%In runs with active silicon targets, another requirement is applied for the
%candidate: a prompt hit on the target in $\pm 200$ \si{\ns}\ around the
%time of the $\mu$Sc pulse. The number comes from the observation of the
%time correlation between hits on the target and the $\mu$Sc
%(\cref{fig:tme_sir_prompt_rational}).
%\begin{figure}[htb]
%\centering
%\includegraphics[width=0.85\textwidth]{figs/tme_sir_prompt_rational}
%\caption{Correlation in time between SiR2 hit and muon hit}
%\label{fig:tme_sir_prompt_rational}
%\end{figure}
To make sure that we will analyse good data, a low level data quality checking
was done on the whole data sets. The idea is plotting the variations of basic
parameters, such as noise level, length of raw waveforms, pulse rate, time
correlation to hits on the muon counter on each channel during the data
collecting period. Runs with significant difference from the averaging
values were further checked for possible causes, and would be discarded if such
discrepancy was too large or unaccounted for. Examples of such trend plots are
shown in \cref{fig:lldq}.
\begin{figure}[htb]
\centering
\includegraphics[width=0.47\textwidth]{figs/lldq_noise}
\includegraphics[width=0.47\textwidth]{figs/lldq_tdiff}
\caption{Example trend plots used in the low level data quality checking:
noise level in FWHM (left) and time correlation with muon hits (right). The
noise level was basically stable in in this data set, except for one
channel. On the right hand side, this sanity check helped find out the
sampling frequency was wrongly applied in the first tranche of the data
set.}
\label{fig:lldq}
\end{figure}
% subsection offline_analyser (end)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% section analysis_strategy (end)
\section{Monte Carlo simulation}
\label{sec:monte_carlo_simulation}
A full Monte Carlo (MC) simulation of the experimental set up has been developed
based on Geant4~\cite{Agostinelli.etal.2003}. The geometrical implementation
was as detailed as possible and could be modified via configuration script at
run time. Descriptions of the muon beam came from the beam line optic
calculation provided by the accelerator experts at PSI.
The MC model greatly assisted the design of the experiment, such as alignment
of the detectors with respect to the target, and shielding of scattered muons.
It also helps make sense of observed results during the run and data
analysing.
% chapter the_alcap_run_2013 (end)