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

\begin{center}
\textbf{\Large Performance of Fast High-Resolution Muon Drift Chambers
  for\\[1mm]
  LHC Upgrades}\\[2mm]

\vspace*{7mm}

S.~Adomeit$^{\text{2}}$,
O.~Biebel$^{\text{2}}$,
B.~Bittner$^{\text{1}}$,
J.~Dubbert$^{\text{1}}$,
A.~Engl$^{\text{2}}$,
R.~Hertenberger$^{\text{2}}$,
S.~Horvat$^{\text{1}}$,
O.~Kortner$^{\text{1}}$,
H.~Kroha$^{\text{1}}$,
F.~Legger$^{\text{2}}$,
F.~Rauscher$^{\text{2}}$,
R.~Richter$^{\text{1}}$,
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~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 at 3~bar absolute with an
Ar/CO$_{\text{2}}$ gas mixture, the maximum drift time of primary
ionization electrons to the anode wires is about 750 ns. Up to
background counting rate of 1~kHz/cm$^{\text{-2}}$ the MDT chambers
reach a spatial resolution of 40~$\mu$m and have a tracking efficiency
of almost 100\%. Limiting factors at higher rates are space charge
effects and the long drift times. Future enhancements 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 $\text{1} \times \text{10}^{\text{34}}$
$\text{cm}^{\text{-2}} \text{s}^{\text{-1}}$ (the so-called Super LHC)
will require a rate capability of up to 10~kHz/cm$^{\text{-2}}$ for a
large number of muon chambers. A cost-effective upgrade option
consists of substituting drift tube chambers with a smaller tube
diameter of 15~mm. Keeping the operation parameters (gas mixture and
pressure, and gas gain) the same as for the MDT chambers allows the
seamless integration into the muon spectrometer and the existing
services. The smaller tube diameter improves the rate capability:
\begin{itemize}
\item The maximum drift time is reduced to 200 ns
\item Space charge effects are approximately suppressed with a factor
  $(\text{1/2})^{\text{3}}$, keeping the resolution constant even at
  high rates
\item The increased number of tube layers in 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 in fall 2009 using cosmic muons tracked with
reference chambers. Figure~1 shows a photograph of the
setup.  An average spatial resolution of 112~$\mu$m was measured for
the 15~mm tubes, compatible with the spatial resolution in 30~mm
diameter tubes (see fig.~2). This leads to a
chamber resolution of 42~$\mu$m without background radiation and of
45~$\mu$m at a background counting rate of 1~kHz/cm$^{\text{2}}$ the
maximum rate in the GIF. The efficiency of detecting a muon hit with
the correct drift radius, the so-called 3$\sigma$ single tube
efficiency, was measured up to the maximum background counting rate of
340~kHz per tube in the GIF (see fig.~3).  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 as expected. 340~kHz per tube correspond to more than
twice the maximum counting rate the tubes would experience in ATLAS at
the LHC.  Rates of up 1.5~MHz per tube are expected for the SLHC. At
those rates the 3$\sigma$ single tube efficiency is expected to be
$>50$\% at an occupancy of 30\%. The drop of single tube efficiency
can be compensated by redundant measurements in the 16~tube layers
which ensure a muon track reconstruction efficiency $>97$\% in this
chamber up to the a counting of 1.5~MHz per tube.

The measurements at the GIF are being continued in spring 2010 with
higher counting rates made possible by an improved setup situated
closer to the radioactive source. A prototype chamber segment of 96
tubes arranged in 8 layers is used for data taking. The data analysis
is currently ongoing. 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 and will cover an area of 1~m$^{\text{2}}$. It will
allow 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 tubes and their read-out electronics to highly
ionizing particles---protons and neutrons, the latter, together with
photons, being the main background radiation at LHC and Super
LHC---will be performed in summer 2010 at the Tandem accelerator of
the Meier-Leibnitz-Laoratory at Munich, Germany. Results of all
measurements will be reported.
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\begin{minipage}[b]{0.475\linewidth}
%\begin{figure}
%  \begin{center}
    \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. The
      radioactive source is situated at the left side, on top and
      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.}
    %%%\label{fig:setup}
%  \end{center}
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\begin{minipage}[b]{0.475\linewidth}
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    \includegraphics[width=1\textwidth]{./figures/resolution.eps}\\
    {\small \textbf{Fig.~2:}Spatial tube resolution of 15~mm diameter tube in
      comparision with 30~mm diameter tubes 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.}
    %%% \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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%\end{figure}  
\end{minipage}






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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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