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%%%           pdftitle={First Cosmic Ray Results of the ATLAS Barrel Muon
%%%             Spectrometer with Magnetic Field, Summary}
%%%           pdfauthor={The ATLAS Muon Collaboration},
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\begin{document}

\begin{center}
  \textbf{\Large Construction of a High-Resolution Muon Drift Tube
    Prototype\\[1mm] Chamber for
    LHC Upgrades}\\[2mm]

\vspace*{7mm}

B.~Bittner$^{\text{1}}$,
J.~Dubbert$^{\text{1}}$,
S.~Horvat$^{\text{1}}$,
O.~Kortner$^{\text{1}}$,
H.~Kroha$^{\text{1}}$,
R.~Richter$^{\text{1}}$,\\
S.~Adomeit$^{\text{2}}$,
O.~Biebel$^{\text{2}}$,
A.~Engl$^{\text{2}}$,
R.~Hertenberger$^{\text{2}}$,
F.~Legger$^{\text{2}}$,
F.~Rauscher$^{\text{2}}$,
A.~Zibell$^{\text{2}}$\\[3mm]

{\small $^{\text{1}}$Max-Planck-Institut f\"ur Physik, F\"ohringer Ring 6,
80805 M\"unchen, Germany\\
$^{\text{2}}$Ludwig-Maximilians-Universit\"at M\"unchen, Am Coulombwall 1, 85748
Garching, Germany\\[5mm]}

\end{center}

The muon spectrometer of the ATLAS experiment at LHC at CERN uses
Monitored Drift Tube (MDT) chambers as precision tracking detectors.
These chambers consist of 2~$\times$~3~(or~4) layers of densely packed
aluminum tubes of 30 mm diameter---each equipped with a single central
anode wire---mounted on a lightweight aluminum support frame.
Operated with an Ar/CO$_{\text{2}}$ gas mixture at an absolute
pressure of 3~bar, the MDT chambers reach a spatial resolution of
40~$\mu$m and a tracking efficiency of almost 100\% up to counting
rates of 0.5~kHz/cm$^{\text{2}}$. Possible upgrades of the muon
spectrometer in the very forward region at pseudo-rapidities of $\eta$
= 2.0--2.7 and luminosity upgrades of the LHC beyond its design
luminosity $\text{1} \times \text{10}^{\text{34}}$
$\text{cm}^{\text{-2}}\, \text{s}^{\text{-1}}$ require muon tracking
detectors with a rate capability of up to 10~kHz/cm$^{\text{2}}$ with
the same spatial resolution as the current MDT chambers. The different
upgrade scenarios require the construction of a large number of new
muon chambers.

The design of new drift tube chambers consisting of 15~mm diameter
tubes has been completed. These chambers are operated with the same
gas mixture as the original MDT chambers, allowing for the re-use of
existing services. The decreased tube diameter allows for higher
granularity of read-out channels in the same detector volume,
improving the tracking capabilities of the detectors. At the same
time, space charge effects are suppressed and the maximum drift time
is reduced from 700 ns to about 200 ns, reducing the background occupancy 
induced by photons and neutron conversions in the tube walls
by a factor of 7. First test beam measurements confirm
the required spatial resolution and high rate capability.

A full-scale chamber, consisting of 1152 tubes arranged in 2 $\times$
8 layers and covering an area of 1~m$^{\text{2}}$, is currently being
built (cp.\ fig.~1 for the layout). It uses 15~mm aluminum tubes with
a wall thickness of 0.4~mm. An anode wire of 50~$\mu$m diameter is
centered within each tube. To achieve the required spatial resolution,
the anode wire positions have to known to better than 20 $\mu$m within
a chamber. The high mechanical accuracy is reached by using precision
wire locators on the inside of the tube endplugs (see fig.~2) and
reference surfaces on their outsides in conjunction with precision
combs for positioning during the gluing of the tube layers. The
assembly setup consisting of a stack of jigs (see fig.~3) allows for
the gluing of up to 8~layers in one day. Measurements with cosmic rays
on a test bundle of 96~tubes yielded an accuracy of 20~$\mu$m on the
relative anode wire positions.

The high density of tubes necessitates the design of new on-cahmber
services as well. A modular gas system using injection molded plastic
(Pocan) pieces has been developed. Compact HV distribution and
front-end electronics boards have been made possible by integrating
passive components into extensions of the signal caps of the tubes
(see fig.~4).
%
The drift tubes are produced using a semi-automated assembly facility
inserting and tensioning the anode wires and crimping the tubes and
wires on the endplugs. Each tube had to pass wire tension, gas leak
rate, and HV stability tests before being used in the chamber
assembly. We will also report on these test results.

\pagebreak

\begin{minipage}[b]{0.475\linewidth}
    \includegraphics[width=1\textwidth]{./figures/prototype.eps}\\[-7mm]
    {\small \textbf{Fig.~1:} Schematic drawing of the prototype drift
      tube chamber. The design allows a piggyback trigger chamber. 
      The increased size of the Fa\-ra\-day cages is due to the re-use of
      existing read-out electronics.}\\[5mm]
\end{minipage}%
\hspace*{0.05\linewidth}
\begin{minipage}[b]{0.475\linewidth}
    \includegraphics[width=1\textwidth]{./figures/endplug_cut_2.eps}\\[2mm]
    \includegraphics[width=1\textwidth]{./figures/endplug_2.eps}\\
    {\small \textbf{Fig.~2:} Schematic drawing of the drift tube and
      endplug design. Photograph of a cut un\-machined endplug.}\\[5mm]
\end{minipage}

\vspace*{3mm}

\begin{minipage}[b]{0.475\linewidth}
    \includegraphics[width=1\textwidth]{./figures/assembly_5.eps}\\[2mm]
    \includegraphics[width=1\textwidth]{./figures/assembly.eps}\\
    {\small \textbf{Fig.~3:} Schematic drawing of the precision jigs 
      and photograph of the jigs mounted on the granite assembly table.}
\end{minipage}%
\hspace*{0.05\linewidth}
\begin{minipage}[b]{0.475\linewidth}
    \includegraphics[width=1\textwidth]{./figures/dummy.eps}\\
    {\small \textbf{Fig.~4:} Photograph of the read-out side of a 15~mm drift tube
    bundle with gas distribution, passive and active read-out electronics.}
\end{minipage}



\end{document}
