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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
  LHC Upgrades}\\[3mm]

\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\\[1mm]
$^{\text{2}}$Ludwig-Maximilians-Universit\"at M\"nchen, Am Coulombwall 1, 85748
Garching, Germany\\[5mm]}

\end{center}

\begin{twocolumn}

The muon spectrometers of the LHC experiments require high precision
tracking detectors covering large areas. The only viable and
afforadable solution are gaseous detectors. The muon spectrometer of
the ATLAS experiment uses 1200 Monitored Drift Tube (MDT) chambers
built of 2 $\times$ 3 layers of densly 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
seemless integration into the muon spectrometer and the existing
services. The smaller tube diameter improves the rate capability:
\begin{itemize}
\item The maxmimum drift time is reduced to 200 ns
\item Space charge effects are approximately suppressed by 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}

A first, small prototype chamber with 8 layers of 1~m long 1.5~cm
diameters tubes was constructed. Its performance at high $\gamma$
background rates was studied at the Gamma Irradiation Facility (GIF)
at CERN in fall 2009 and spring 2010 (with higher counting rates due
to an improved setup closer to the radioactive source) using cosmic
muons tracked with reference chambers. Figure~1 shows a photograph of
the setup in 2009. Currently only the data collected in 2009 has been 
analysed, the analysis of the 2010 data is still ongoing. 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
Figure~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 couting rate of 340~kHz per tube in the GIF.  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.

In addition to the tests of


A prototype chamber with 8 layers of 1~m long 1.5~cm diameters tubes
was constructed with better than 16~$\mu$m anode wire positioning
accuracy confirmed by measurements in a cosmic ray measurement
facility. This accuracy was achieved by the high mechanical precision
of the wire locator of the tube's end-plug and a stack of precision
combs allowing the gluing of several tube layers in one go (see
Figures~1(a,b)). In order to make compact high-voltage and read-out
boards possible the coupling capacitor and the protection resistor has
been incorporated into the signal cap.


\end{twocolumn}

\end{document}

This options allows to re-use most of the existing 
services  


The increased number of tube layers in the same geometrical envelope 
improves the pattern recognition at high background rates as fake hits
 




with a
resolution of 40 $\mu$m, covering an active area of 5000
m$^{\text{2}}$. These chambers are build of 2 $\times$ 3 layers of
densly packed aluminum tubes of 30 mm diameter, each with a single
central anode wire.  Operated at 3 bar absolute with an
Ar/Co$_{\text{2}}$ gas mixture



Drift tube detectors provide
a robust and efficient solution even at high background counting rates
like in the example of the ATLAS muon spectrometer where they can
reach 1~kHz\,cm$^{-2}$ in the forward regions. For the high-luminosity
upgrades of the LHC, the background rates may increase by a factor 10
increasing the demand on the rate capability of the muon
detectors. Drift tube detectors with 15~mm diameter aluminium drift
tubes and 400~$\mu$m thick walls have been developed. Filled with
Ar:CO$_2$(93:7) at 3 bar absolute pressure and operated at 2760~V they
have a maximum drift time of 200~ns instead of 700~ns in 3~cm diameter
tubes.

A prototype chamber with 8 layers of 1~m long 1.5~cm diameters tubes
was constructed with better than 16~$\mu$m anode wire positioning
accuracy confirmed by measurements in a cosmic ray measurement
facility. This accuracy was achieved by the high mechanical precision
of the wire locator of the tube's end-plug and a stack of precision
combs allowing the gluing of several tube layers in one go (see
Figures~1(a,b)). In order to make compact high-voltage and read-out
boards possible the coupling capacitor and the protection resistor has
been incorporated into the signal cap.

The performance of the prototype chamber was studied in the Gamma
Irradiation Facility (GIF) at CERN with a cosmic ray telescope for
performance studies (see Fig.~3). The drift tubes show an average
spatial resolution of 112~$\mu$m compatible with the spatial
resolution in 30~cm diameter tubes (see Figure~4). 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$^{-2}$ 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 couting rate of
340~kHz per tube in the GIF.  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.  Because of the promising high-rate capability of the 1.5~cm
diameter drift-tube chambers trapezoidal chambers like sketched in
Figure~5 are considered for the upgrade of the ATLAS muon
spectrometer.



\end{document}