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%%%           pdftitle={First Cosmic Ray Results of the ATLAS Barrel Muon
%%%             Spectrometer with Magnetic Field, Summary}
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\textbf{\Large Performance of Fast High-Resolution Muon Drift Tube Chambers\\[1mm]
  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 spectrometers of the LHC experiments require high precision
tracking detectors covering large areas. The only viable and
affordable solution are gaseous detectors. The muon spectrometer of
the ATLAS experiment uses 1150 Monitored Drift Tube (MDT) chambers
built of 2~$\times$~3~(or 4) layers of densely packed aluminum tubes
of 30~mm diameter, each equipped with a single central anode wire,
resulting in 350~k read-out channels. Operated with an
Ar/CO$_{\text{2}}$ gas mixture at 3~bar absolute, the maximum drift
time of primary ionization electrons to the anode wires is about 700
ns. Up to background counting rates of about 0.5~kHz/cm$^{\text{2}}$,
the MDT chambers reach a spatial resolution of 40~$\mu$m and a
tracking efficiency of almost 100\%. Limiting factors at higher rates
are space-charge effects and the long drift times,
respectively. Future 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 of
$\text{1} \times \text{10}^{\text{34}}$ $\text{cm}^{\text{-2}}\,
\text{s}^{\text{-1}}$ will require a rate
capability of up to 10~kHz/cm$^{\text{2}}$. A cost-effective upgrade
option consists of substituting MDT chambers in the high rate regions
by drift tube chambers with a smaller tube diameter of 15~mm. Keeping the
operating parameters (gas mixture, pressure, and gain) the same
as for the MDT chambers allows for integration into the muon
spectrometer while re-using existing services. The smaller tube diameter
significantly improves the rate capability:
\begin{itemize}
\item The maximum drift time is reduced to 200 ns.
\item Space charge effects are suppressed by a factor of about
  $(\text{1/2})^{\text{3}}$, keeping the resolution almost constant up
  to high rates.
\item The increased number of tube layers with the same geometrical
  envelope improves the pattern recognition.
\end{itemize}

The performance of prototype drift tubes with 15~mm diameter was
studied at high $\gamma$~background rates at the Gamma Irradiation
Facility (GIF) at CERN using cosmic muons tracked in two reference
chambers. Figure~1 shows the setup.  An average spatial resolution of
112~$\mu$m was measured for the 15~mm diameter tubes, compatible with
expectations from the measured spatial resolution of 30~mm diameter
tubes as a function of the drift distance (see fig.~2). This leads to
a chamber resolution of 40~$\mu$m without background radiation and of
45~$\mu$m at a background counting rate of 1~kHz/cm$^{\text{2}}$, the
maximum rate achieved in the GIF. The efficiency of detecting a muon
hit with the correct drift radius with 3 times the tube resolution
(3$\sigma$ tube efficiency) was measured up to a maximum background
counting rate of 1.2~MHz per tube (see fig.~3 for the first results).
The 3$\sigma$ single tube efficiency is 95\% without background
irradiation ($<100\%$ because of $\delta$ electrons knocked off the
tube wall by impinging muons) and drops to 85\% at a counting rate of
340~kHz per tube, corresponding to more than twice the maximum
counting rate expected at the LHC design luminosity.  Rates of up
0.7~MHz per tube are expected for LHC luminosity upgrades. At those
rates the 3$\sigma$ single tube efficiency is expected to be $>75$\%
at an occupancy of 13\%. The drop of tube efficiency can be
compensated by redundant measurements in the up to twice as large
number of layers, allowing for a muon track reconstruction efficiency
$>99$\% up to the a counting rate of 1.75~MHz per tube.

In addition to the tests at high $\gamma$~background, measurements
with a full-scale prototype chamber with high momentum muons are
scheduled for July/August 2010 at the H8 beam line at CERN. This
chamber will consists of 1152 tubes arranged in 2 $\times$ 8
layers. It will allow for tracking and resolution studies and the
verification of the performance of the auto-calibration algorithm used
to determine the space-to-drift time relation in-situ.

A further important test, the response of the 15~mm diameter drift
tubes and their read-out electronics to highly ionizing particles will
be performed in summer 2010 at the Tandem accelerator of the
Meier-Leibnitz-Laboratory at Munich, Germany. Results of all
measurements will be reported.  \enlargethispage{50mm} \vspace*{5mm}

\begin{minipage}[m]{0.475\linewidth}
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    \includegraphics[width=1\textwidth]{./figures/gif_09.eps}\\
    {\small \textbf{Fig.~1:} Setup used for resolution and efficiency
      studies of 15 mm
      drift tubes at the Gamma Irradiation Facility at CERN.}
    %%%\label{fig:setup}
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  \includegraphics[width=1\textwidth]{./figures/resolution.eps}\\
  {\small \textbf{Fig.~2:} Comparison of the spatial resolution of a
    15~mm  and a 30~mm diameter drift tube as a
    function of the
    distance of the track from the anode wire.}\\[5mm]
  \includegraphics[width=1\textwidth]{./figures/efficiency.eps}\\
  {\small \textbf{Fig.~3:} Measured
    3$\sigma$ single tube efficiencies. The dashed line indicate the
    expected efficiency from 30~mm tube tests, taking into account
    multiple hits.}
    %%% \caption{Top: Spatial tube resolution of 15~mm diameter tube in
    %%%   comparision with 30~mm diameter tubes. Bottom: Measured
    %%%   3$\sigma$ single tube efficiencies. The dashed line indicate the
    %%%   expected efficiency.}
    %%% \label{fig:res_eff}
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%%%     \caption{Setup used for resolution and efficiency studies of 15 mm
%%%       drift tubes at the Gamma Irradiation Facility at CERN. The
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%%%       bottom of the setup the cosmic ray scintillation trigger is
%%%       visible, in the middle the two reference chambers used for
%%%       tracking and the tube prototypes.}
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