\chapter{The AlCap Run 2013} \label{cha:the_alcap_run_2013} \thispagestyle{empty} The first run of the AlCap experiment was performed at the $\pi$E1 beam line area, PSI (Figure~\ref{fig:psi_exp_hall_all}) from November 26 to December 23, 2013. The goal of the run was to measure protons rate and spectrum following muon capture on aluminium. \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} \section{Experimental set up} \label{sec:experimental_set_up} 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 (Figure~\ref{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. Two liquid scintillators for neutron measurements were also tested in this run. \begin{figure}[htbp] \centering \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.} \label{fig:alcap_setup_detailed} \end{figure} \subsection{Muon beam and vacuum chamber} Muons in the $\pi$E1 beam line are decay products of pions created as a 590~\mega\electronvolt\ proton beam hit a thick carbon target (E-target in Figure~\ref{fig:psi_exp_hall_all}). The beam line was designed to deliver muons with momenta ranging from 10 to 500~\mega\electronvolt\per\cc\ and momentum spread from 0.26 to 8.0\%. These parameters can be selected by changing various magnets and slits shown in Figure~\ref{fig:psi_piE1_elements}~\cite{Foroughli.1997}. \begin{figure}[htb] \centering \includegraphics[width=0.7\textwidth]{figs/psi_piE1_elements} \caption{The $\pi$E1 beam line} \label{fig:psi_piE1_elements} \end{figure} One of the main requirements of the AlCap experiment was a low energy muon beam with narrow momentum bite in order to achieve a high fraction of stopping muons in the very thin targets. In this Run 2013, muons from 28 to 45~\mega\electronvolt\per\cc\ and momentum spread of 1\% and 3\%were used. For part of the experiment the target was replaced with one of the silicon detector packages allowed an accurate momentum and range calibration %(via range-energy relations) of the beam at the target. Figure~\ref{fig:Rates} shows the measured muon rates as a function of momentum for two different momentum bites. Figure~\ref{fig:Beam} shows an example of the resulting energy spectra. \begin{figure}[htbp] \centering \includegraphics[width=0.6\textwidth]{figs/Rates.png} \caption{Measured muon rate (kHz) at low momenta. Momentum bite of 3 and 1 \% FWHM, respectively.} \label{fig:Rates} \end{figure} \begin{figure}[htbp] \centering \includegraphics[width=0.9\textwidth]{figs/beam.pdf} \caption{Energy deposition at 36.4 MeV/c incident muon beam in an 1500-\micron-active target. The peak at low energy is due to beam electrons, the peaks at higher energies are due to muons. Momentum bite of 1 and 3\% FWHM on left and right hand side, respectively.} \label{fig:Beam} \end{figure} The targets and charged particle detectors are installed inside the vacuum chamber as shown in Figure~\ref{fig:alcap_setup_detailed}. The muon beam enters from the right of the image and hits the target, which is placed at the centre of the vacuum chamber and orientated at 45 degrees to the beam axis. The side walls and bottom flange of the vessel provide several vacuum-feedthroughs for the high voltage and signal cables for the silicon and scintillator detectors inside the chamber. In addition, the chamber is equipped with several lead collimators %so that muons that are not captured in the target would quickly decay. to quickly capture muons that do not stop in the actual target. %\begin{figure}[htbp] %\centering %\includegraphics[width=0.55\textwidth]{figs/SetupOverview.jpg} %\caption{Vacuum chamber in beam line} %\label{fig:SetupOverall} %\end{figure} %It is known fact that there is a risk of sparkling between the electrodes of %a silicon detector in the low vacuum region of $10^{-3}$ mbar. %An interlock mechanism was installed to prevent the bias of the %silicon detectors from being applied before the safe vacuum level. For a safe operation of the silicon detector, a vacuum of $<10^{-4}$\,mbar was necessary. With the help of the vacuum group of PSI, we could consistently reach $10^{-4}$\,mbar within 45 minutes after closure of the chamber's top flange. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Silicon detectors} The main detectors for proton measurement in the Run 2013 were four large area silicon detectors. The silicon detectors were grouped into two detector packages located symmetrically at 90 degrees of the nominal muon beam path, SiL and SiR in Figure~\ref{fig:alcap_setup_detailed}. Each arm consists of: one $\Delta$E counter, a 65-\micro\meter-thick silicon detector, divided into 4 quadrants; one E counter made from 1500-\micron-thick silicon; and one plastic scintillator to identify electrons or high energy protons that pass through the silicon. The area of each of these silicon detectors and the scintillators is $50\times50 \textrm{mm}^2$. The detectors were named according to their positions relative to the muon view: the SiL package contains the thin detector SiL1 and thick detector SiL2; the SiR package has SiR1 and SiR2 accordingly. Each quadrant of the thin detectors were also numbered from 1 to 4, i.e. SiL1-1, SiL1-2, SiL1-3, SiL1-4, SiR1-1, SiR1-2, SiR1-3, SiR1-4. Bias for the four silicon detectors was supplied by an ORTEC 710 NIM module, which has a vacuum interlock input to prevent biasing before the safe vacuum level has been reached. Typical voltage to fully depleted the detectors were -300~\volt\ and -10~\volt\ for the thick and thin silicon detectors respectively. The leakage currents at the operating voltages are less than 1.5~\micro\ampere\ for the thick detectors, and about 0.05~\micro\ampere\ for the thin ones (see Figure~\ref{fig:si_leakage}). \begin{figure}[htb] \centering \includegraphics[width=0.85\textwidth]{figs/si_leakage} \caption{Leakage currents of the silicon detectors under bias.} \label{fig:si_leakage} \end{figure} The fact that a detector were fully depleted was checked by putting a calibration source $^{241}\textrm{Am}$ at its ohmic side, and observing the output pulse height on an oscilloscope. One would expect that the maximum pulse height increases as the bias is raised until the voltage of fully depleted. The effect can also be seen on the pulse height spectrum as in Figure~\ref{fig:sir2_bias_alpha}. \begin{figure}[htb] \centering \includegraphics[width=0.75\textwidth]{figs/sir2_bias_alpha} \caption{$^{241}\textrm{Am}$ spectra in cases of fully depleted (top), and partly depleted (bottom).} \label{fig:sir2_bias_alpha} \end{figure} %It is known that the noise level of a silicon detector increases linearly with %its capacity. So both noise and pick-up suppression had been carefully %optimised in the real PSI accelerator environment, particularly for the thin %silicon detectors which have a large capacity of 1~\nano\farad~in each %quadrant. %After improving the feed-through flanges during the set-up phase of the %experiment with isolated ground connections, good electronic resolution of %55--76~\kilo\electronvolt\ FWHM was achieved in the thin silicon detectors. %So achieving good energy resolution was particularly challenging %for the thin silicon detector, as each quadrant had a large capacity of %1~\nano\farad. Both %noise and pick-up suppression had been carefully optimized in the real PSI %accelerator environment. %Optimization of the fast timing signals proved another challenge. %The energy calibration for the silicon detectors were done %by several means: %\begin{enumerate} %\item An $^{241}\textrm{Am}$ alpha source: the main alpha %particles have energies of 5.484~\mega\electronvolt\ (85.2\%) and %5.442~\mega\electronvolt\ (12.5\%). The source emits 79.5 %$\alpha\per\second$ in 2$\pi$~\steradian. %\item Test pulse with a fixed amplitude: the preamplifiers used for the %silicon detectors are come with the manufacturer's specification on the %response, namely a 66 \milli\volt\ fed into the test input will produce an %output equivalent to that of a 1 \mega\electronvolt\ energy deposition. %\item Minimum ionisation particles %(MIPs): electrons in the beam are MIPs with a nominal deposit energy of %388~\electronvolt\per\micro\meter\ Si. This is only applicable for thick %silicon detectors because the energy deposit is large enough and the energy %resolution is good enough. During the run, this peak was observed to make %sure the stability of the electronics. %\item Muons with different momenta: the thick silicon detectors were placed %at the target position during beam tuning period, allowed an accurate %momentum and range calibration. This also only works with thick silicon %detectors. %\end{enumerate} % subsection silicon_detectors (end) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Upstream counters} \label{sub:upstream_counters} The upstream detector consists of three counters: a 500~$\mu$m thick scintillator muon trigger counter ($\mu$SC); a muon anti-coincidence counter ($\mu$SCA) surrounding the trigger counter with a hole of 35 \milli\meter\ in diameter to define the beam radius; and a multi-wire proportional chamber ($\mu$PC) that uses 24 X wires and 24 Y wires at 2~\milli\meter~intervals. The upstream detectors provide signal of an incoming muon as coincident hits on the muon trigger and the wire chamber in anti-coincident with the muon anti-coincidence counter. This set of detectors along with their read-out system belong to the MuSun experiment, which operated at the same beam line just before our run. Thanks to the MuSun group, the detectors were well-tuned and ready to be used in our run without any modification. % subsection upstream_counters (end) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Germanium detector} %\begin{figure}[htbp] %\centering %\includegraphics[width=0.9\textwidth]{figs/neutron.png} %\caption{Setup of two %liquid scintillators outside the vacuum envelope for neutron detection.} %\label{fig:neutron} %\end{figure} We used a germanium detector to normalise the number of stopped muons by measuring characteristics muon X-rays from the target material. The primary X-rays of interest are the 346.828~keV line for aluminium targets, and the 400.177 line for silicon targets. The energies and intensities of the X-rays listed in Table~\ref{tab:xray_ref} follow measurement results from Measday and colleagues~\cite{MeasdayStocki.etal.2007}. \begin{table}[htb] \begin{center} \begin{tabular}{c l l l l } \toprule \textbf{Elements} & \textbf{Transition} & \textbf{Energy} & \textbf{Intensity}\\ \midrule $^{27}\textrm{Al}$ & $2p-1s$ & $346.828 \pm 0.002$ & $79.8\pm 0.8$\\ & $3p-1s$ & $412.87 \pm 0.05$ & $7.62\pm 0.15$\\ \midrule $^{28}\textrm{Si}$ & $2p-1s$ & $400.177 \pm 0.005$ & $80.3\pm 0.8$\\ & $3p-1s$ & $476.80 \pm 0.05$ & $7.40 \pm 0.20$\\ \bottomrule \end{tabular} \end{center} \caption{Reference values of major muonic X-rays from aluminium and silicon.} \label{tab:xray_ref} \end{table} The germanium detector is a 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-rays measurement with an ultra-thin entrance window of 0.5-mm-thick beryllium and a 0.3-\micron-thick ion implanted contact (Figure~\ref{fig:ge_det_dimensions}). This detector is equipped with a transistor reset preamplifier which, according to the producer, enables it to work in an ultra-high rate environment up to $10^6$ counts\per\second~ at 1~\mega\electronvolt. \begin{figure}[htb] \centering \includegraphics[width=0.9\textwidth]{figs/ge_det_dimensions} \caption{Dimensions of the germanium detector} \label{fig:ge_det_dimensions} \end{figure} The detector was installed outside of the vacuum chamber at 32 cm from the target, seeing the target through a 10-mm-thick aluminium window, behind a plastic scintillator counter used to veto electrons. Liquid nitrogen necessary for the operation of the detector had to be refilled every 8 hours. A timer was set up in the data acquisition system to remind this. \subsection{Plastic and liquid scintillators} \label{sub:plastic_scintillators} Apart from the scintillators in the upstream group, there were four other plastic scintillators used as veto counters for: \begin{itemize} \item punch-through-the-target muons, ScVe \item electrons and other high energy charged particles for germanium detector (ScGe) and silicon detectors (ScL and ScR) \end{itemize} The ScL, ScR and ScVe were installed inside the vacuum vessel and were optically connected to external PMTs by light-guides at the bottom flange. We also set up two liquid scintillation counters for neutron measurements in preparation for the next beam time where the neutron measurements will be carried out. % subsection plastic_scintillators (end) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Front-end electronics and data acquisition system} The front-end electronics of the AlCap experiment was simple since we employed a trigger-less read out system with waveform digitisers and flash ADCs (FADCs). As shown in Figure~\ref{fig:alcapdaq_scheme}, all plastic scintillators signals were amplified by PMTs, then fed into the digitisers. The signals from silicon and germanium detectors were preamplified, and subsequently shaped by spectroscopy amplifiers and timing filter amplifiers (TFAs) to provide energy and timing information. \begin{figure}[htbp] \centering \includegraphics[width=0.99\textwidth]{figs/alcapdaq_scheme} \caption{Schematic diagram of the electronics and DAQ used in the Run 2013} \label{fig:alcapdaq_scheme} \end{figure} The germanium detector has its own transistor reset preamplifier installed very close to the germanium crystal. Two ORTEC Model 142 preamplifiers were used for the thick silicon detectors. The timing outputs of the preamplifiers were fed into three ORTEC Model 579 TFAs. We used an ORTEC Model 673 to shape the germanium signal with 6~\micro\second shaping time. A more modern-style electronics was used for thin silicon detectors where the preamplifier, shaping and timing amplifiers were implemented on one compact package, namely a Mesytec MSI-8 box. This box has 8 channels, each channel consists of one preamplifier board and one shaper-and-timing filter board which can be fine-tuned independently. The shaping time was set to 1~\micro\second\ for all channels. The detector system produced signals that differs significantly in time scale, ranging from very fast (about 40~\nano\second\ from scintillators) to very slow (several \micro\second\ from shaping outputs of semiconductor detectors). This lead to the use of several sampling frequencies from 17~\mega\hertz\ to 250~\mega\hertz, and three types of digitisers were employed: \begin{itemize} \item custom-built 12-bit 170-MHz FADCs which was designed for the MuCap experiment. Each FADC board has dimensions the same as those of a single-width 6U VME module, but is hosted in a custom built crate due to its different power supply mechanical structure. The FADC communicates with a host computer through a 100-Mb/s Ethernet interface using a simple Ethernet-level protocol. The protocol only allows detecting incomplete data transfers but no retransmitting is possible due to the limited size of the module's output buffer. The FADCs accept clock signal at the frequency of 50~\mega\hertz\ then multiply that internally up to 170~\mega\hertz. Each channel on one board can run at different sampling frequency not dependent on other channels. The FADC has 8 single-ended LEMO inputs with 1~\volt pp dynamic range. \item a 14-bit 100-MS/s CAEN VME FADC waveform digitiser model V1724. The module houses 8 channels with 2.25~Vpp dynamic range on single-ended MCX coaxial inputs. The digitiser features an optical link for transmission of data to its host computer. All of 8 channels run at the same sampling frequency and have one common trigger. \item a 12-bit 250-MS/s CAEN desktop waveform digitizer model DT5720. This digitiser is similar to the V1724, except for its form factor and maximum sampling frequency. Although there is an optical link available, the module is connected to its host computer through a USB 2.0 interface where data transfer rate of 30 MB/s was determined to be good enough in our run (actual data rate from this digitiser was typically about 5 MB/s during the run). Communication with both CAEN digitisers was based on CAEN's proprietary binary drivers and libraries. \end{itemize} All digitisers were driven by external clocks which were derived from the same 500-\mega\hertz\ master clock, a high precision RF signal generator Model SG382 of Stanford Research System. The silicon detectors were read out by FADC boards feature network-based data readout interface. To maximize the data throughput, each of the four FADC boards was read out through separate network adapter. The CAEN digitisers were used to read out the germanium detector (timing and energy, slow signals) or scintillator detectors (fast signals). For redundancy, all beam monitors ($\mu$SC, $\mu$SCA and $\mu$PC) were also read out by a CAEN time-to-digital converter (TDC) model V767 which was kindly provided by the MuSun experiment. The Data Acquisition System (DAQ) of the AlCap experiment, so-called AlCapDAQ, provided the readout of front-end electronics, event assembling, data logging, hardware monitoring and control, and the run database of the experiment (Figure~\ref{fig:alcapdaq_pcs}). It was based on MIDAS framework~\footnote{ MIDAS is a general purpose DAQ software system developed at PSI and TRIUMF:\\ \url{http://midas.triumf.ca}} and consisted of two circuits, {\em i}) a detector circuit for synchronous data readout from the front-end electronics instrumenting detectors, and {\em ii}) a slow control circuit for asynchronous periodic hardware monitoring (vacuum, liquid nitrogen filling). The detector circuit consisted of three computers, two front-end computers and one computer serving both as a front-end and as a back-end processor. The slow circuit consisted of one computer. All computers were running Linux operating system and connected into a private subnetwork. %\hl{TODO: storage and shift monitor} \begin{figure}[htb] \centering \includegraphics[width=0.95\textwidth]{figs/alcapdaq_pcs} \caption{AlCapDAQ in the Run 2013. The {\ttfamily fe6} front-end is a VME single board computer belongs to the MuSun group, reads out the upstream detectors.} \label{fig:alcapdaq_pcs} \end{figure} The data were collected as dead-time-free time segments of 110~ms, called ``block'', followed by about 10-ms-long time intervals used to complete data readout and synchronize the DAQ. Such data collection approach was chosen to maximize the data readout efficiency. During each 110-ms-long period, signals from each detector were digitized independently by threshold crossing. The data segment of each detector data were first written into on-board memories of front-end electronics and either read out in a loop (CAEN TDCs and CAEN digitizers) or streamed (FADCs) into the computer memories. The thresholds were adjusted as low as possible and individually for each detector. The time 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 compares to the time scale of several \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 170~\mega\hertz\ could handle up to about 1.5 \second\ with its 28-bit time counter). To ease the task of handling data, the data collecting period was divided into short runs, each run stopped when the logger had recorded 2 GB of data. The data size effectively made each run last for about 5 minutes. The DAQ automatically starts a new run with the same parameters after about 6 seconds. The short period of each run also allows the detection, and helps to reduce the influence of effects such as electronics drifting, temperature fluctuation. \section{Data sets and statistics} \label{sec:data_sets} The main goal of this Run 2013 was to measure the rates and energy spectra of protons following muon capture on aluminium. Also for normalisation and cross checking against the existing experimental data, two types of measurements with different targets were carried out for silicon targets: \begin{itemize} \item[(a)] an active, thick target similar to the set up used by Sobottka and Wills~\cite{SobottkaWills.1968}. This provides a cross-check against the existing experimental data. The silicon detector package at the right hand side was moved to the target position with the thick detector facing the muon beam in this set up. \item[(b)] a passive, thin target and heavy charged particles were observed by the two silicon packages. The measurement serves multiple purposes: confirmation that the particle identification by dE/dx actually works, separation of components of heavy charged particles emitted from the silicon target. \end{itemize} As the emitted protons deposit a significant amount of energy in the target material, thin targets and thus excellent momentum resolution of the low energy muon beam are critical. Aluminium targets of 50-\micro\meter\ and 100~\micron\ thick were used. Although a beam with low momentum spread of 1\% is preferable, it was used for only a small portion of the run due to the low beam rate (see Figure~\ref{fig:Rates}). The beam momentum for each target was chosen to maximise the number of stopped muons. The collected data sets are shown in Table~\ref{tb:stat}. \begin{table}[htb!] \begin{center} \vspace{0.15cm} \begin{tabular}{l c c c} \toprule \textbf{Target} &\textbf{Momentum} & \textbf{Run time} & \textbf{Number}\\ \textbf{and thickness}&\textbf{scaling factor} & \textbf{(h)} &\textbf{of muons}\\ \midrule Si 1500 \micro\meter& 1.32& 3.07& $2.78\times 10^7$\\ & 1.30& 12.04& $2.89 \times 10^8$\\ & 1.10& 9.36& $1.37 \times 10^8$ \\ \midrule Si 62 \micro\meter & 1.06& 10.29& $1.72 \times 10^8$\\ \midrule Al 100 \micro\meter& 1.09& 14.37&$2.94 \times 10^8$\\ & 1.07& 2.56& $4.99 \times 10^7$\\ \midrule Al 50 \micro\meter m & 1.07& 51.94& $8.81 \times 10^8$\\ \bottomrule \end{tabular} \end{center} \caption{Run statistics. Momentum scaling normalized to 28 MeV/c.} \label{tb:stat} \end{table} % section data_sets (end) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Analysis framework} \subsection{Concept} \label{sub:concept} Since the AlCapDAQ is a trigger-less system, it stored all waveforms of the hits occured in 100-ms-long blocks without considering their physics significance The analysis code therefore must be able to extract parameters of the waveforms, then organises the pulses into physics events correlated to stopped muons (Figure~\ref{fig:muon_event}). In addition, the analyser is intended to be usable as a real-time component of a MIDAS DAQ, where simple analysis could be done online for monitoring and diagnostic during the run. \begin{figure}[htb] \centering \includegraphics[width=0.9\textwidth]{figs/muon_event.pdf} \caption{Concept of the AlCap analysis code: pulses from individual detector in blocks of time are analysed, then sorted centred around stopped muons.} \label{fig:muon_event} \end{figure} The analysis framework of the AlCap consists of two separate programs. A MIDAS-based analyser framework, \alcapana{}, processes the raw data and passes its ROOT data output to a second stage, \rootana{}, where most of the physics analysis is performed. Both programs were designed to be modularised, which allowed us to develop lightweight analysis modules that were used online to generate plots quickly, while more sophisticated modules can be applied in offline analysis. The DAQ system generated MIDAS files which stores the data as a stream of MIDAS ``banks''. In the AlCapDAQ, each bank corresponds to a single channel on a digitizer and was named according to a predefined convention. The map between detector channels and MIDAS bank names was stored in the MIDAS online database (ODB), along with other settings such as sampling frequencies, timing offsets, thresholds and calibration coefficients of each channel. %These can then be %accessed by both \alcapana{} and \rootana{} for either online or offline %analysis. The first step of the analysis framework is to convert the raw MIDAS data into waveforms, series of digitised samples continuous in time corresponding to pulses from the detector. The waveform is called \tpulseisland{}s, which contain the bank name, the ADC values of the samples and the time stamp of the first sample. This conversion is performed in \alcapana{} and the resulting objects are stored in a ROOT output file as a {\ttfamily TTree}. The next step of the analysis is to obtain summary parameters of the pulses from the digitized samples. The parameters of primary interest are the amplitude and time of the peak and the integral of the pulse. This extraction of parameters is done by a \rootana{} module, and the objects produced by this stage are called \tanalysedpulse{}s. Currently, we have a usable and simple algorithm that takes the pulse parameters from the peak of the waveform. In parallel, a pulse finding and template fitting code is being developed because it would provide more accurate pulse information. The first iteration of this code has been completed and is being tested. \begin{figure}[htb] \centering \includegraphics[width=0.85\textwidth]{figs/analysis_scheme} \caption{Concept of the analysis framework in \rootana{}} \label{fig:rootana_scheme} \end{figure} After obtaining pulse parameters for individual channel, the pairing up of fast and slow pulses from the same physical detector needs to be done. This entails looping through all fast and slow pulses from each detector, checking for correlated pulses in time and amplitude, creating {\ttfamily TDetectorPulse}s. The {\ttfamily TDetectorPulse}s allow better understanding of the hits on the detector by combining timing information from the fast channel and amplitude information from the slow channel. It also helps reduce the impact of pile-up on the amplitude measurement, where the improved time resolution of the fast channels can be used to separate the overlapping amplitudes in the slow channels. The pulse pairing are applicable to the silicon and germanium channels only. The scintillator channels provide only fast timing signals which can be used as {\ttfamily TDetectorPulse}s directly. The detector pulses are subsequently used to identify particles that hit the detectors. These particle hits are still stored in the time-ordered tree corresponds to the 110 ms block length from the AlCapDAQ. By iterating through the tree to find stopped muons and taking any hits within a certain window around this muon from every detector, a stopped-muon-centred tree shown in Figure~\ref{fig:muon_event} can be produced. This will make it much easier to look for coincidences and apply cuts, thereby bringing the end goal of particle numbers and energy distributions. % subsection concept (end) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Online analyser} \label{sub:online_analyser} The online analyser was developed and proved to be very useful during the run. A few basic modules were used to produce plots for diagnostic purposes including: persistency view of waveforms, pulse height spectra, timing correlations with respect to the upstream counters. The modules and their purposes are listed in Table~\ref{tab:online_modules}. \begin{table}[htb] \begin{center} \begin{tabular}{l p{6cm}} \toprule \textbf{Module name} & \textbf{Functions}\\ \midrule common/MUnCompressRawData & decompress raw MIDAS data\\ \midrule FADC/MOctalFADCProcessRaw & \multirow{3}{6cm}{convert raw data to {\ttfamily TPulseIsland}s}\\ v1724/MV1724ProcessRaw& \\ dt5720/MDT5720ProcessRaw&\\ \midrule muSC\_muPC/MCaenCompProcessRaw& \multirow{4}{6cm}{decompress data from {\ttfamily fe6}, make coincidence in upstream counters} \\ muSC\_muPC/MMuPC1AnalysisC&\\ muSC\_muPC/MMuPC1AnalysisMQL&\\ muSC\_muPC/MMuSCAnalysisMQL&\\ \midrule diagnostics/MCommonOnlineDisplayPlots& produce plots of interest\\ \midrule FADC/MOctalFADCBufferOverflow& \multirow{2}{6cm}{diagnostics for FADCs}\\ FADC/MOctalFADCPacketLoss&\\ \midrule common/MExpectedIslands&\multirow{4}{6cm}{diagnostics in general}\\ common/MMuSCTimeDifferences&\\ common/MNumberIslands&\\ common/MPulseLengths&\\ \midrule common/MTreeOutput& save {\ttfamily TPulseIsland}s tree\\ \bottomrule \end{tabular} \end{center} \caption{Online analysis modules in the Run 2013.} \label{tab:online_modules} \end{table} The \alcapana{} served the plots on port 9090 of the {\ttfamily abner} via the ROOT socket protocol. We then used a ROOT-based program called {\ttfamily online-display} to display the plots on the shift terminal ({\ttfamily alcap}). The {\ttfamily online-display} simply executed ROOT macros which retrieved plots from the ROOT server, sorted then drew them in groups such as upstream counters, silicon arms. It could also periodically update the plots to reflect real-time status of the detector system. %Screen %shots of the {\ttfamily online-display} with several plots are shown in %Figure~\ref{fig:online_display}. %\hl{Screen shots} \subsection{Offline analyser} \label{sub:offline_analyser} Some offline analysis modules has been developed during the beam time and could provide quick feedback in confirming and guiding the decisions at the time. For example, the X-ray spectrum analysis was done to confirm that we could observe the muon capture process (Figure~\ref{fig:muX}), and to help in choosing optimal momenta which maximised the number of stopped muons. \begin{figure}[htbp] \centering \includegraphics[width=0.7\textwidth]{figs/muX.png} \caption{Germanium detector spectra in the range of 300 - 450 keV with different setups: no target, 62-\micron-thick silicon target, and 100-\micron-thick aluminium target. The ($2p-1s$) lines from aluminium (346.828 keV) and silicon (400.177 keV) are clearly visible, the double peaks at 431 and 438 keV are from the lead shield, the peak at 351~keV is a background gamma ray from $^{211}$Bi.} \label{fig:muX} \end{figure} Although the offline analyser is still not fully developed yet, several modules are ready. They are described in detailed in the next chapter. % subsection offline_analyser (end) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % section analysis_strategy (end) % chapter the_alcap_run_2013 (end)