\documentclass[a4paper,11pt]{article}
\usepackage{graphicx}

\topmargin=-25mm
\oddsidemargin 0.0in 
%%this makes the odd side margin go to the default of 1inch
\textwidth=16.5cm
\textheight=26.5cm
%%sets the textwidth to 6.5, which leaves 1 for the remaining right margin with 8 1/2X11inch paper 
\begin{document}

\title{Precision Drift Tube Chambers for the ATLAS Muon Spectrometer at Super-LHC}

\author{O.~Kortner$^1$, H.~Kroha$^1$, F.~Legger$^1$, R.~Richter$^1$, A.~Engl$^2$, F.~Rauscher$^2$}

\date{}
\maketitle

\begin{center}
$^1$ Max-Planck-Institut f\"ur Physik, Munich, Germany

$^2$ Ludwig-Maximilian University, Munich, Germany
\end{center}



\noindent
The ATLAS detector is one of the two general purpose detectors at
the Large Hadron Collider (LHC) at CERN. One of the most challenging aspects 
of the ATLAS experiment is the measurement of muon momenta with very high precision. 
The ATLAS muon detector is equipped with three layers of tracking chambers in a
toroidal magnetic field of 3--6~Tm bending power generated by a
superconducting air-core magnet system. 
Over most of the pseudo-rapidity coverage of the experiment, measurements of the track
sagitta are provided by precision drift-tube detectors, the
Monitored Drift Tube (MDT) chambers\cite{MuonTDR}. 
The ATLAS muon spectrometer has to cope with high radiation
background rates mainly caused by low-energy neutrons leaking out of the
calorimeter and $\gamma$ rays emitted from nuclei excited by
the low energy neutrons. At the LHC design luminosity, Monte-Carlo simulations 
predict background counting rates of about 10~cm$^{-2}$s$^{-1}$ in the
barrel part of the muon spectrometer, and of up to 100~cm$^{-2}$s$^{-1}$ in the end-caps \cite{baranov}.
The muon chambers are designed to cope with up to five times higher background rates.
At the Super-LHC, the high luminosity upgrade of the LHC, ten
times higher counting rates may be encountered assuming that the background
rate scales with the accelerator luminosity. 

The ATLAS MDT chambers consist of two triple or quadruple layers of
aluminum drift tubes of 30~mm diameter and 0.4~mm wall thickness. The tubes are filled 
with a gas mixture of Argon and CO$_2$ in the ratio 93:7 at an absolute pressure of 3~bar. 
The drift tubes are operated at 3080~V corresponding to a gas gain of 2~x~10$^4$ chosen
to prevent aging. Without background radiation, the average spatial resolution of individual
drift tubes is about $80~\mu$m.

It is known that high background counting rates lead to a degradation 
of the drift-tube spatial resolution due to space-charge fluctuations and to a reduction
of the muon detection efficiency depending on the maximum drift time in the tubes\cite{aleksa, deile}. 
The efficiency loss can be significantly reduced
by reducing the diameter of the drift tubes from 30~mm to 15 mm because of the shorter maximum drift distance
and the higher drift velocity closer to the sense wire\cite{IEEE07}.
Furthermore, the background counting rate is reduced by an additional factor of two
because of the smaller tube wall surface in which neutron conversions take place.
A first detector prototype equipped with 15~mm diameter drift tubes 
was successfully tested in a cosmic-ray test stand\cite{IEEE07}.

The possibility of improving the muon detection efficiency at 
Super-LHC by reducing the diameter of the drift tubes to 15~mm
has now been investigated in a high-intensity radiation environment at the Gamma Irradiation Facility (GIF)
at CERN where a cosmic ray test stand has been setup. 
The GIF provides a 590~GBq $^{137}$Cs source emitting
photons of 662~keV energy which corresponds well with the ATLAS background photon energies.
The test setup consists of a layer of 6 prototype drift tubes with 15~mm
diameter and 1~m length in between two reference tracking chambers, each consisting of
two triple layers of 8 standard ATLAS drift tubes with
30~mm diameter and of 50~cm length separated by a 30~cm high space frame (see Fig.~\ref{fig1}).

All parameters of the prototype drift tubes except for the radius were the same as for the
ATLAS MDT chambers.
In order to maintain the same electric field strength as in the 30~mm diameter tubes for
drift distances to the wire below 7.5~mm, the operating voltage
has been set to 2760~V. The standard ATLAS MDT readout electronics can be used.
The trigger for cosmic muons was provided by two layers of scintillation counters separated
by an absorber.
  
\clearpage
\noindent
We report on the performance of the prototype drift-tube detector
with thin-walled aluminum tubes of 15~mm diameter at different counting rates up to 1.5 kHz/cm$^2$
and compare the results with simulations.

\begin{figure}[!t]
\centering
\includegraphics[width=\textwidth]{/.at/wwwatlas/ftp/outgoing/s-lhc/mdt/GIF_08_4.eps}
% where an .eps filename suffix will be assumed under latex,
% and a .pdf suffix will be assumed for pdflatex; or what has been declared
% via \DeclareGraphicsExtensions.
\vspace{-0.8cm}
\caption{The cosmic ray test setup at the Gamma Irradiation Facility at CERN. The 15~mm diameter
drift tubes are positioned between the two reference chambers with 30~mm diameter tubes 
with an accuracy of about $50~\mu$m in the direction perpendicular to the tubes.
One recognizes the Faraday cages with the readout electronics of the tube bundels.
The trigger counters are mounted in the box underneath the drift tubes detectors. 
The $^{137}$Cs source is located in front of the setup to the left.}
\label{fig1}
\end{figure}

\begin{thebibliography}{1}

%\bibitem{LHC} \emph{LHC public pages} Available: http://lhc.web.cern.ch/lhc/.
%-------------------------------------------------
%\bibitem{ATLASworkshop}
%ATLAS collaboration, \emph{Workshop on ATLAS Upgrades for High
%Luminosity}, CERN 13-14 February 2005. Available:
%http://agenda.cern.ch/age?a045387.
%-------------------------------------------------
%\bibitem{slhc}
%W. Scandale, F. Zimmermann, \emph{Scenarios for the sLHC and the
%VLHC}, talk presented at the Hadron Collider Physics Symposium
%(Elba, 25 May 2007). Available:
%https://indico.pi.infn.it/getFile.py/access?contribId=87\&amp;sessionId=29
%\&amp;resId=0\&amp;materialId=slides\&amp;confId=0.
% ---------------------------------------------------------------------------- %
%\bibitem{ATLAStp}
%ATLAS collaboration, \emph{ATLAS Technical Proposal}, CERN/LHCC
%94-43, December 1994.
%-------------------------------------------------
\bibitem{MuonTDR}
ATLAS collaboration, \emph{Technical Design Report for the ATLAS
Muon Spectrometer}, CERN/LHCC 97-22, May 1997.
% ---------------------------------------------------------------------------- %
\bibitem{baranov}
S. Baranov et al., \emph{Estimation of Radiation Background,
Impact on Detectors, Activation and Shielding Optimization in
ATLAS}, ATLAS internal note, ATL-GEN-2005-001, 2005.
% ---------------------------------------------------------------------------- %
\bibitem{aleksa}
M. Aleksa et al., \emph{Rate Effects in High-Resolution Drift
Chambers}, Nucl.~Instr.\ and Meth.~{\bf A446} (2000) 435.
% ---------------------------------------------------------------------------- %
\bibitem{deile}
M. Deile et al., 
\emph{Performance of the ATLAS Precision Muon
Chambers under LHC Operating Conditions}, Nucl.~Instr.\ and Meth.~{\bf A518} (2004) 65;\hfill\break
M. Deile et al., 
\emph{Resolution and Efficiency of the ATLAS Muon Drift-Tube Chambers at High Background
Rates}, Nucl.~Instr.\ and Meth.~{\bf A535} (2004) 212;\hfill\break
S.~Horvat et al.,
\emph{Operation of the ATLAS Muon Drift-Tube Chambers at High Background
Rates and in Magnetic Fields},
IEEE Transactions on Nuclear Science, vol.~53, no.~2, pp.~562-566, 2006.
% ---------------------------------------------------------------------------- %
\bibitem{IEEE07}
J.~Dubbert et al., 
\emph{Development of Precision Drift Tube Detectors for the Very High Background Rates at the Super-LHC},
proceedings of the 2007 IEEE Nuclear Science Symposium, Honolulu, Hawaii, USA, 28 Oct.--2 Nov.~2007,
Nuclear Science Symposium Conference Record 2007, IEEE, vol.~3, pp.~1822-1825, 2007,
MPI report, MPP-2007-172, November 2007.
% ---------------------------------------------------------------------------- %
%\bibitem{aleksa-phd}
%M. Aleksa, \emph{Performance of the ATLAS Muon Spectrometer}, Ph.D.
%Thesis,
%TU-Vienna/CERN, 1999.
% ---------------------------------------------------------------------------- %
%\bibitem{garfield}
%R. Veenhof, \emph{GARFIELD: Simulation of gaseous detectors, Version
%8.01, CERN}, write-up: http://wwwinfo.cern.ch/writeup/garfield.
% ---------------------------------------------------------------------------- %

\end{thebibliography}

\end{document}
