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% ATLAS-SCT-Present.tex: construct/install/commiss part for SCT
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% RN: 14/01/13: New start for FB13
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\subsection{The Semiconductor Tracker SCT} 
\label{sec:atlassct}
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Because of the MPP experience with silicon micro-strip sensors and the unique
possibilities offered by the associated semiconductor laboratory (HLL =
Halb\-Lei\-ter\-La\-bor), MPP participated in the design and construction of the
SCT.

Mechanically the SCT consists of three main units, namely the barrel and two
identical endcaps. The barrel covers the region of central rapidity, $|\eta|\le
1$, and the detector modules, \ie the smallest mechanically sensitive units of
the SCT, are arranged on four cylinders around the beam axis. The endcaps extend
the acceptance up to $|\eta|\le 2.5$. Here the modules are arranged on nine
disks per endcap, and are oriented perpendicular to the beam axis.

Due to the radial arrangement the endcap modules use wedge shaped sensors glued
back to back in pairs.
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The sensor dimensions are about $6.4\mathrm{cm}\times 6.4\mathrm{cm}$ and the
strip pitch of the 768 strips per sensor is varying in the range
(64--90)\,\micron.
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Electrically, the two sensors of either pair are chained in series. One pair has
a radial strip orientation, while the second pair is rotated by 40\,mrad in
order to yield stereo information.
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The binary readout electronics is located on a double sided hybrid attached to
the end of the module.
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In the binary mode the only information retained per channel is whether the
observed signal was above or below a discriminating threshold. To equalize the
efficiency of all channels for detecting a passing particle, the discriminating
thresholds can be set individually for each channel by means of calibration
capacitors.
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On a fully populated disk, three different types of endcap modules, inner,
middle and outer modules, are arranged in three rings.

A major challenge for the ATLAS SCT is the large radiation level which leads to
severe radiation damage of the sensors and the readout electronics, calling for
cold operation (-10$^\circ$C) of the silicon sensors.
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The cold operation and the dissipation of the heat generated by the
electronics and the sensor leakage currents leads to mechanical stress. To
ensure stable positions of the modules during operation needed for high quality
tracking, the system requires a very high geometrical stability,
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\begin{figure}[phtb]
\begin{center}
\includegraphics[width=0.9\linewidth]{./ATLAS_SCT_DSC01700cut2}
\caption{View of a middle module constructed by MPP technicians.}
\label{fig:ATLAS_SCT_Module}
\end{center}
\end{figure}
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\subsubsection{MPP Engagements in the ID}
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In the years 1997--2010 the MPP engagement mainly concentrated on
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1) the development of a cost-effective design for radiation-tolerant silicon
strip sensors,
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2) the design work for the endcap modules concentrating on the mechanical and
thermal performance,
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3) the construction of 424 SCT endcap modules of the middle type,
Fig.~\ref{fig:ATLAS_SCT_Module}, which amount to about 30$\%$ of all endcap
modules,
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4) the integration, as well as performance measurements with cosmic ray muons,
of the combined SCT and TRT detectors at the surface before their installation
into the ATLAS cavern, 
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5) the alignment of the silicon part of the ID with particle tracks, and
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6) the commissioning of the SCT at LHC collider and data quality control.

Detailed reports of the work performed by the MPP group in the years 1997--2010
can be found in~\cite{atl_fb04} (p.63-67), \cite{atl_fb06} (p.77-85),
and~\cite{atl_fb10} (p.132-137).
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The properties of the sensors are described in~\cite{ATLAS_SCT_Ahmad_1},
details of the endcap modules and their production process are given
in~\cite{ATLAS_SCT_Abdesselam_1}.

In the years 2011--2013 for the present ATLAS detector the members of the MPP
group mainly worked on the SCT commissioning with emphasis on detector
monitoring.
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\subsubsection{SCT Commissioning} 
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The MPP is contributing to the SCT detector commissioning in the area
of online monitoring.

The SCT read-out-system (ROS) is structured in various layers.
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The ROS monitoring is used to continuously control the detector
operation, and thereby also gives valuable information on the LHC beam
condition that ATLAS is facing. It comprises error searches at chip-,
link-, or read-out-driver level, but also information on the actual
illumination of the SCT by means of strip occupancies.
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The ROS monitoring is mostly independent of the ATLAS trigger system,
and can give even faster feedback on much more data, than what is
eventually written to tape.
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The MPP group provides a ROS monitoring expert, whose task is to
constantly maintain and extend the ROS monitoring software and its
interface to the online monitoring of the ATLAS SCT as a whole.
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The fast feedback achieved is a valuable input to the detector
operation, and has already led to a detailed understanding of some LHC
conditions that otherwise would have been difficult to get.
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