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\begin{document}
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\begin{center}
\LARGE{ \bf Assembly station}


\hspace{1cm}

\large{ versie 2}
\end{center}

\tableofcontents

\section{Introduction}
The task of the ATLAS detector at the LHC
 is to discover the Higgs boson and to be prepared for the unexpected, e.g. new physics.
Both requirements are covered by a detector optimized for
 final state muons with high momentum. 

New physics can lead to events with muons that emerge with a transverse momentum of 1~TeV. To be able to detect these interactions an accuracy of the order $\frac{\sigma_{P_T}}{P_T}$=10\% is needed. This translates to 50~\um uncertainty on the sagitta measurement in the 2~Tm magnetic field, provided by an air core toroid. 
At these energies, the major contributions to this uncertainty originate from the detector alignment and the wire positions. Therefore, the
 accuracy of these contributions should not exceed the (about) 20~\um.
At lower energies, 100~GeV, the momentum resolution is a few percent, dominated
by energy loss fluctuations in the inner detector.

The barrel muon system measures three space points. Each point is measured by
 6 drift tubes with an single hit resolution of about 100~\um. In some parts of the end-cap muon system the muon momentum has to be extraced from the also
measured direction of the muon trajectory, leading to a slightly less accurate
measurement. 

In this report we describe the construction of the muon detector at NIKHEF, where the production of the outer layer of the barrel has commenced.

 


\section{Monitored Drift Tube Chambers}
Figure~\ref{bol} shows a 
 muon chambers consisting of two panels (multi-layers),
 each with three drift tube layers, 
separated by a  support structure, the spacer. The spacer
 consists of three beams, called crossplates, that run orthogonal to the
 tubes which connect the multilayers. The crossplates are connected by two beams, called longbeams.

The drift tubes are the basic detection elements of the MDTs. These tubes have a diameter of 29.970$^{+0.030}_{-0.000}$~mm and are closed by precise
 end-plug which center the 50~\um gold-plated tungsten wire. The wire is fixed
 under tension (about 280 gram) leading to a gravitational sag of about 400\um for a tube with a length of 5~m. The intension is to adjust the sag of the chambers, depending on their orientation in ATLAS,  such that the wires are centered with respect to the tubes with an RMS of 100~\um.

The end-plugs are manufactured with a precise outer ring that allows to position the tubes during the assemblyin jigs with high precision. A so called twister
centers the wire.

Physical deformations of the chambers during construction, transport and operation are monitored with the In-Plane alignment system. This system consist logically of four RASNIK systems as shown in Fig.~\ref{bol}. In a test setup at CERN~\cite{DATCHA} an alignment accuracy better than 10~\um (on the wire position) can be achieved with this system. In addition, 14 temperature sensors deliver aanvullende information.


\subsection{Assembly of BOL-chambers at NIKHEF}
The task of NIKHEF is to assemble 96 
 'Barrel Outer Large' (BOL) chambers, which have dimensions of approximatley 5~m$\times$2~m, with a mechanical precision of 20~\um on the wire positions.
Over the past two years the assembly station, based on a granite block,
 has been prepared. Figure~\ref{bolroom} shows the assembly station.
The jigs or 'combs' which position the tubes of a single tube-layer are visible.
The assembly procedure starts with the assembly of the bare spacer 
to wich the tubes, layer by layer, are attached using glue. To avoid large
 deformations of the spacer due to the weigth of tube-layers, the layers are glued
 one by one on each side of the spacer.
 This involves, several times, rotating the
spacer (and already attached tube-layers on it) during assembly, but avoids  a large assymetric weight distribution.

In this report we present the preparation of the assembly station, the assembly of the first ten BOL chambers and the test of three BOL chambers in the tomograph at CERN.

\section{Preparation of the Assembly Station}
 granite table with dimensions
 (l$\times$w$\times$h) 5.5~m$\times$2.5~m$\times$0.5~m forms the basis of the assembly station. A new tube-layer to be attached on the spacer (or on the spacer with some tube layers already attached to it) is position in jigs. The spacer
is positioned by six sphere holders, and, 
supported by a sag compensation system.
Figure~\ref{bolroomtower} displays one precision tower. The removable 'ears' on the spacer accomodate a sphere that is support by the sphere holders on the towers.
Figure~\ref{bolroomtower} displays one precision tower, supporting the ear of the bare spacer.
The towers can be adjusted in heigth by changing the 'stacking blocks' corresponding to thickness of  tube layers that have been attached to the spacer, during the gluing process.
Figure~\ref{bolroomtower} also shows the  alignment system, RASAS, that monitors the position of the ears.

\begin{figure}[b]
\setlength{\unitlength}{1cm}
\begin{picture}(14,10)
 \put(0,0){\mbox{\psfig{file=fmmdtch1.epsi,height=110pt,angle=-90}}}
\end{picture}
\label{bol}
\caption{\sl Schematic view of a muon chamber.}
\end{figure}

\begin{figure}[b]
\begin{center}
\setlength{\unitlength}{1cm}
\begin{picture}(14,10)
\put(-1,0){\mbox{\epsfysize=10cm\epsffile{Bol_room_febr2000.ps}}}
\put(14.5,5){\mbox{\epsfysize=2cm\epsffile{frame.eps}}}
\end{picture}
\end{center}
\label{bolroom}
\caption{\sl View of the assembly station. The bare spacer is carried by the six sphere towers. The combs, which will support the tubes are also visible. In 
the background, the tube production can be seen.
The coordinate frame is also displayed in this figure; on the wall the character
 'N' defines the 'North' direction, which coincides with the $X$ direction. }
\end{figure}

\begin{figure}[b]
\begin{center}
\setlength{\unitlength}{1cm}
\begin{picture}(14,10)
\put(0,0){\mbox{\epsfysize=10cm\epsffile{bolroom_1.ps}}}
\end{picture}
\end{center}
\label{bolroomtower}
\caption{\sl The picture shows one sphere tower that carries the spacer. The
'removable' ear on the spacer is monitored by the aligment system. }
\end{figure}


\subsection{The global and chamber coordinate system}
\label{global}
The global coordinate system (at NIKHEF) is defined such that it coincides with the local chamber system, when the chamber is located on the granite table in the 'up' position. The $Y$ coordinate points to the sky, the $X$ coordinate lays in the horizontal plane and runs along the tubes, the long dimension of the chamber. The $Z$ coordinate is the precision coordinate and runs perpendicular to the tubes.  During assembly we flip the chamber to the 'down' position, by effectively rotating it over its $Z$ axis.  

In the cleanroom, we defined North and South as the $+X$ and $-X$ side respectively. In the 'up' ('down') position the RO and HV (HV and RO) side coincides with South and North respectively. The $X$, $Y$ and $Z$ axis form a left-handed system.

The so called 'reference-side' of the chamber is choosen to be the negative (or smallest) $Z$ side and coincides with 'West'. 


We follow the specs given in the ATLAS MUON TDR (page 138). Combining the intrinsic
accuracy and the positioning accuracy we summarize:
\begin{itemize}
\item
The $Y$, $Z$  accuracy of Frascati-combs and towers has to be 10\um.
\item
The $X$ accuracy has a spec of 100\um.
\item  Angles $A_X$, $A_Y$ and $A_Z$ of the Frascatie combs have to be precise to 25~\urad, 0.5~mrad and 0.5~mrad respectively.
\item  The concentricity of the wire in the tubes has to be accurate to 100~\um RMS.
\end{itemize}

We aim to align the combs and towers such that the wire positions in the bending plane are controlled
within 20~\um. The wires are centered in the tubes over the full length of the
tubes within 100~\um. Finally, we mention the importance of the torque, i.e., the angular difference between the North and South Frascati combs. The torque should be know whithin 20~\urad. In Appendix~\ref{tools}, the tools used for the alignment of the assembly station are decribed in detail. Below we discuss
the various elements of the assembly station and in particular their accuracy.





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\section{Conclusions}
chambers are within specs.

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\end{document}


































































