\setcounter{figure}{0}

\section{Small-Diameter Muon Drift Tube Detectors}

{\it Detail of detector technologies proposed for use in the small wheel 
detector, including the status of developments. Include a description
on the status and size of available prototype chambers.\\

This section should describe the technology up to the level of a
single chamber.
}


% ========================================================================================
\subsection{Tracking in a high--back\-ground environment}
% ========================================================================================
%

In order to study the performance of the MDT chambers at high background rates, a full-scale ATLAS
BOS chamber has been extensively tested in the CERN Gamma Irradiation Facility (GIF)~\cite{ref_deile,ref_horvat}.
The results showed that the drift-tube chambers provide efficient and high-resolution tracking 
in combination with fast trigger and second-coordinate chambers up to very high occupancies above 20~%~\cite{ref_MDTtracking}. 
which are expected to be reached in the ATLAS endcap muon spectrometer already at twice the LHC design luminosity. 

Reduction of the outer drift-tube diameter from 30~mm in the present ATLAS Monitored Drift Tube (MDT) chambers to 15~mm
while keeping the other parameters, including the gas gain of $2\cdot 10^4$, the sense wire diameter of $50~\mu$m and 
the tube wall thickness of 0.4~mm, unchanged leads to a significant improvement of the rate capability 
of the drift-tube chambers which is more than sufficient for their operation in the new Small Wheels up to the highest  
background rates expected at 5 times the LHC design luminosity.  It allows for maintaining the main 
advantages of the drift tube concept:

\begin{enumerate}

\item High pattern recognition and tracking efficiency up to high backround rates. 

\item High position and angular resolution independent of the angle of incidence onto the chamber 
plane (in contrast to drift chambers with rectangular drift geometry).

\item Operational independance of the drift tubes where any malfunction of a tube can only 
generate a negligible inefficiency.

\item Modularity and mechanical robustness of chamber construction.

\item Direct connection of the high intrinsic mechanical precision of the chambers to the global
optical alignment system.   

\end{enumerate}

For the standard MDT gas mixture of Ar:CO$_2$ (93:7) at 3 bar absolute pressure, the gas gain of
$2\cdot 10^4$ is achieved at an operating voltage of 2730~V for 15~mm diameter drift tubes. 
Under these conditions, the maximum drift time is reduced by
a factor of 3.5 from about 700~ns to 200~ns by shrinking the tube diameter by a factor of two
(see Fig.~\ref{fig_dtspectrum})~\cite{ref_bittner1}.
In addition, the background flux hitting a tube of given length is reduced by a factor of two. 
Altogether, the drift-tube occupancy is reduced by a factor of 7. 
At the maximum background rate of 9 kHz/cm$^2$ expected at the innermost radius of the Small Wheels 
at 5 times the LHC design luminosity, the highest occupancy, occuring in the shortest tubes of 0.56~m length 
at the innermost radius of the Small Wheels, is $15~\%$ corresponding to a counting rate of 750~kHz. 
2 x 6 drift-tube layers at radii $R<2$~m in the Small Wheels and 2 x 4 layers at larger radii provide very robust tracking
with track segment reconstruction efficiencies above $99~\%$ at the highest background rates 
(see Fig.~\ref{fig_segmenteff})~\cite{ref_bittner2}.


\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{drifttime2.eps}
%\vspace{-1cm}
\caption{Drift-time spectra of 30~mm and 15~mm diameter drift tubes operated with Ar:CO$_2$ (93:7) gas
mixture at 3 bars and a gas gain of $2\cdot 10^4$, normalized to the same area. The measurements 
with cosmic ray muons for the 15~mm diameter tubes are compared to the Garfield simulation~\cite{ref_Garfield}.}
\label{fig_dtspectrum}
\end{figure}


\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{segmenteff.eps}
%\vspace{-1cm}
\caption{Track segment reconstruction efficiencies as a function of the background counting rate for different numbers of 
tube layers calculated requiring at least 4 hits on the track and using the measured 15~mm diameter single-tube efficiencies 
as a function of the counting rate for finding a hit within $3\sigma_r$ distance from the track ($3\sigma$ efficiency) where
$\sigma_r$ is the drift tube spatial resolution.} 
\label{fig_segmenteff}
\end{figure}


\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{dtresol30mm.eps}
%\vspace{-1cm}
\caption{Spatial resolution of 30~mm diameter drift tubes as a function of the drift radius r measured for different 
background rates at the Gamma Irradiation Facility at CERN~\cite{ref_horvat}.}
\label{fig_dtresol30mm}
\end{figure}

\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{rtrelation.eps}
%\vspace{-1cm}
\caption{Space-to-drifttime (r-t) relationship of circular drift tubes operated with Ar:CO$_2$ (93:7) gas mixtures at 3 bar.}
\label{fig_rtrelation}
\end{figure}

\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{dtresolrate.eps}
%\vspace{-1cm}
\caption{Dependence of the average spatial resolution of 30~mm and 15~mm diameter drift tubes on the background flux registered by the
tubes. The results are derived from Fig.~\ref{fig_dtresol30mm} by averaging the resolutions for different background rates 
over the radial ranges of the 30~mm and 15~mm diameter tubes, respectively. The reduction of the gain drop in 15 ~mm diameter tubes
compared to 30~mm diameter tubes is taken into account by multiplying the corresponding background rates by the relative signal 
suppression factor of 8 (see text).}
\label{fig_dtresolrate}
\end{figure}


The space-charge distribution caused by the ion clouds drifting towards the tube wall during a particle crossing modifies the local
electric field, influencing both the drift velocity and the gas gain. By lowering the effective potential experienced 
by the electrons drifting towards the wire, high background flux leads to decreasing gas gain. The resulting signal loss
grows with the inner tube radius $r_2$ proportional to $r_2^3\cdot \ln (r_2/r_1)$~\cite(ref_riegler}, where $r_1=25~\mu$m
is the wire radius, and therefore is about 8 times smaller in 15 mm compared to 30 mm diameter drift tubes.
Fluctuations of the space charge and of the electric field lead to variations of the drift velocity
causing a deterioration of the spatial resolution in non-linear drift gases like Ar:CO$_2$ (93:7)  
where the drift velocity depends on the electric field.
The latter effect increases strongly with drift distance above a value of about 7.5~mm while the gain drop effect
on the spatial resolution dominates for distances close to the sense wire 
(see Fig.~\ref{fig_dtresol30mm})\cite{ref_aleksa,ref_deile,ref_horvat}. 
For drift radii below 7.5~mm, i.e.\ for the 15~mm diameter tubes, the space-to-drift time relationship is more linear leading to a reduced sensitivity to
environmental parameters such as gas composition and density, magnetic field and irradiation rate (see Fig.~\ref{fig_rtrelation}).

Since the spatial resolution of the drift tubes improves with increasing drift radius, the average single-tube
resolution, after correction for time slewing effects, is degraded from $80~\mu$m for 30~mm diameter tubes to about $110~\mu$m
for 15~mm diameter tubes at low background rates. 
The average 30~mm diameter tube resolution deteriorates linearly with the counting rate to about $115~\mu$m at 0.5~kHz/cm$^2$,
the maximum background flux expected in the ATLAS MDT chambers at the LHC design luminosity 
(see Fig.~\ref{fig_dtresolrate})~\cite{ref_horvat}. 
Fig.~\ref{fig_dtresolrate} also shows the background rate dependence of the average 15~mm diameter tube resolution as expected from the 
measured 30~mm diameter tube resolutions given in Fig.~\ref{fig_dtresol30mm}.
The linear deterioration rate of the average 15~mm diameter tube resolution with increasing background flux is about 
10 times smaller than for 30~mm diameter tubes resulting in a resolution degradation by only $20~\mu$m at the maximum
background rate of 9~kHz/cm$^2$. 

At the highest background rates at 5 times the design luminosity, the point and angular resolutions of the drift tube chambers
in the new Small Wheels with 2 x 4 and 2 x 6 tube layers, depending on the radial position, will be uniformly $45~\mu$m and 0.5~mrad,
respectively, compared to $40~\mu$m and 0.45~mrad without background irradiation.    


%% -------------------------------------------------------------------------------------

% ========================================================================================
\subsection{Technical Implementation}
% ========================================================================================
%


% --- Fig.  4 ----------------------------------------------------------------
\begin{figure}[h]
\begin{center}
\includegraphics[width=0.9\linewidth,clip]{Figures/Sm_tbs_chamber_glueing}
\label{Sm_tbs_chamber_glueing}
%
\caption
{The small tube prototype chamber after assembly,
consisting of two modules of 8 tube layers, 72 tubes per layer and
1152 tubes in total.}
%
\end{center}
\end{figure}
% ----------------------------------------------------------------------------

The design of the muon drift-tube chambers with 15~mm diameter tubes for the new Small Wheels follows as much as possible 
the current ATLAS MDT chamber design in the endcap region of the muon spectrometer (see Fig.~\ref{fig_smdt_chamber design}). 
The chambers consist of two multilayers
of four or six layers of drift tubes each in densest package.
The aluminum tubes of 15~mm outer diameter and 0.4~mm wall thickness are produced according to industry standard
and are chromatized for cleaning and reliable electric contact. The tubes are separated by 0.1 mm wide glue gaps during chamber assembly.
The two multilayers are separated by a spacer frame carrying the supports of the integrated drift-tube and trigger chambers on the
Small Wheel structure and the in-plane optical alignment system monitoring the planarity of the chambers. The aluminum
spacer frame is of very similar design as for the present Small Wheel MDT chambers.   
The main changes in the chamber design are due to the four times denser packaging of the drift tubes and their gas and electrical 
connections.

\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{smdt_chamber_design} 
%
\caption{Small drift-tube chamber design for the new Small Wheels. The integration with the trigger
chambers is also shown.}
\label{fig_smdt_chamber_design}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{smdt_tube_design} 
%
\caption{Exploded view of a drift tube with gas connection, signal and ground connection  
and encapsulated high-voltage decoupling capacitor.}
\label{fig_smdt_tube_design}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{gasmanifold_design3D}
%
\caption{}
\label{fig_gasmanifold_design3D}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{gasmanifold_design1D}
%
\caption{}
\label{fig_gasmanifold_design1D}
% 
\end{center}
\end{figure}

\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{gasmanifold_photo}
%
\caption{}
\label{fig_gasmanifold_photo}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{signal_hedgehog}
%
\caption{}
\label{fig_signal_hedgehog}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{hv_hedgehog}
%
\caption{}
\label{fig_hv_hedgehog}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{groundpin}
%
\caption{}
\label{fig_groundpin}
% 
\end{center}
\end{figure}


Central to the chamber design is the design of the drift tubes and of their endplugs (see Fig.~\ref{fig_smdt_tube_design}). 
The aluminum tubes of 15~mm outer diameter and 0.4~mm wall thickness
are produced according to industry standards (DIN) with tolerances of $\pm 0.1$~mm on diameter, roundness
and concentricity of inner and outer circumference and of $\pm 0.5$~mm on straightness
and chromatized for cleaning and reliable electric contact.
The endplugs insulate the sense wire from the tube wall, center the wire in the tubes, position it with respect to the 
external reference surface on the endplug 
with an accuracy of better than $10~\mu$m and provide high-voltage-safe connections to the gas distribution manifold 
(see Figs.~\ref{fig_gasmanifold_design3D}, \ref{fig_gasmanifold1D} and \ref{fig_gasmanifold_photo}) and to the 24-channel readout and high-voltage interface boards 
(called hedgehog cards; see Figs.~\ref{fig_signal_hedgehog}, \ref{fig_hv_hedgehog}). 

The wire is fixed at both tube ends in copper crimping tubes inserted into the 
central brass inserts of the endplugs and connected to the signal and high-voltage hedgehog cards via brass signal
caps which are screwed onto the brass inserts sealing the tubes with respect to the gas manifold with rubber O-rings.       
The brass insert of the endplug holds the spiral-shaped wire locator (called twister) in a central bore on the inside of the tube and 
transfers the wire position to the precisely machined reference surface at the outside of the tube 
which is used to accurately position the drift tubes during chamber assembly. The wire position information is transferred solely
via the precision machined brass insert; precision machining of the complete endplugs is not necessary anymore improving the accuracy
of the chamber construction. The two most critical problems for the ATLAS MDT drift tube production, cracking of the Noryl$^\textregistered$
plastic insulator and precision machining of the injection moulded endplugs, have been eliminated by the new design.      

The insulators of the endplugs consist of Crastin$^\textregistered$, a PBT based thermoplastic with $30~\%$ glass fiber admixture to improve mechanical stability 
which shows little shrinkage and outgassing and (unlike the Noryl$^\textregistered$ material in the present MDT endplugs) high stability against cracking under stress.
The material is of similar chemical composition as Pocan$^\textregistered$ which is used for the injection moulded components of the gas manifolds of the present and the new drift tube chambers. 
The endplugs are fabricated by injection moulding and sealed in the aluminum tubes with O-rings by circular mechanical crimping 
of the tube walls.   

The modular gas distribution system consists of injection moulded plastic tubes made of Pocan$^\textregistered$ without glass fiber which interconnect 
adjacent tubes in the direction perpedicular to the multilayer and connect them to an aluminum gas distribution bar on the readout and the high-voltage
end of the multilayer (see Figs.~\ref{fig_gasmanifold_design3D}, \ref{fig_gasmanifold1D} and \ref{fig_gasmanifold_photo}) 

The readout and high-voltage supply scheme follows closely the one of the present MDT chambers using the same parameters and specifications.
The high-voltage decoupling capacitors on the readout side and
the terminating resistors on the high-voltage supply end are individually encapsulated in plastic containers for high-voltage protection.
The plastic containers are integrated with the hedgehog cards plugged onto the gold-plated pins of the signal caps
and ground connectors. The ground pins are screwed into the gaps formed by three adjacent tubes (see Fig.~\ref{fig_groundpin}) during chamber assembly 
to connect the tube walls to the ground of the hedgehog cards.
This scheme allows for the high-voltage-stable design of the readout hedgehog cards. Each readout hedgehog card carries a mezzanine card with 
the new radiation hard active readout electronics (see below). The mezzanine cards follow the shape of the hedgehog cards and are currently 
under development. The electronics boards are enclosed in aluminum Faraday cages which also shield the readout hedgehog cards against the mezzanine cards.  


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{jigging}
%
\caption{}
\label{fig_jigging}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{jigging_photo}
%
\caption{}
\label{fig_jigging_photo}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{assembly}
%
\caption{}
\label{fig_assembly}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{assembly_photo}
%
\caption{}
\label{fig_assembly_photo}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{prototype_assembly}
%
\caption{}
\label{fig_prototype_assembly}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{wirepos}
%
\caption{}
\label{fig_wirepos}
% 
\end{center}
\end{figure}


\subsection{Chamber Assembly}

{\it
  Describe the assembly procedure. Include details on
  \begin{enumerate}
  \item Achieved mechanical precision of signal generating parts
    (i.e. strips or wires)
  \item Speed of assembly
  \item Splitting of work and logistics between production site
  \end{enumerate}
}

The drift tubes are fabricated and tested in a climatized clean room of class 10000 using a semi-automated assembly station 
(see Fig.~\ref{fig_wiringline}). The wires are fed through the tube by air flow without touching them by hand over their whole length.
Wire tensioning and crimping is performed fully automatically. After fabrication, the tubes are immediately tested for wire tension,
gas tightness and leakage currents under high voltage (see below).

The individual tubes are assembled to chambers using precision jigs on a flat granite table in a climatized clean room as
shown in Figs.~\ref{fig_jigging} and \ref{fig_jigging_photo}. The jigs are machined with an accuracy of $10~\mu$m.
The two multilayers are assembled and glued together within one working day using two identical jigs. The assembly
procedure is illustrated in Figs.~\ref{fig_assembly} and \ref{fig_assembly_photo}. 
The jig elements positioning the endplugs and sense wires of each layer in the transverse plane are stacked layer-by-layer forming a 
precise two-dimensional grid. 
Bearings for mounting of the trigger chambers and, for the first multilayer, platforms holding the external alignment sensors 
are glued to the bottom tube layers positioning them precisely with respect to the assembly jigs and, therefore, to the wires.
On the second day, after letting the glue cure over night, the spacer frame is glued to the top of the first multilayer in its jig
and the in-plane alignment system is calibrated.  
The second multilayer is removed from the jig and glued to the top of the spacer aligning it precisely with respect to the
assembly jig and, thus, to the first multilayer. Monitoring of the tube and layer positions during chamber construction is not 
necessary with this assembly scheme.    

After the assembly of the drift tubes, the mounting of the gas manifolds and testing for leak tightness requires about one week.
Another two weeks are foreseen for mounting of the hedgehog cards, installation of the Faraday cages and cabling of the chamber.
Mounting of the trigger chambers on the drift tube chambers and combined test will be performed at CERN.  

A full-scale prototype of a 15~mm diameter drift tube chamber for the innermost region of the Small Wheels presently covered by CSC chambers
has been constructed in 2010 (see Figs.~\ref{fig_prototype_assembly} and tested in the CERN Gamma Irradiation Facility GIF 
as well as in the H8 testbeam in 2010 and 2011. The chamber consists of 2 x 8 layers of drift tubes and is readout with the existing MDT
mezzanine cards and chamber service module CSM. With this prototype chamber all aspects of the chamber design and performance (see below) 
as well as the construction and test procedures have been verified and the required time and manpower for chamber production and test has been evaluated
in detail. With the semiautomated drift tube assembly facility, 100 drift tube can be routinely produced per day at each construction site. 
The tubes assembled every day will be tested for gas tightness and leakage currents under high voltage in automated test stands over night.
The wire positions in the prototype chamber have been measured in the cosmic ray test stand at LMU Munich with an accuracy of few microns.
The measured wire grid agrees with the expected grid with better than $20~\mu$m rms (see Fig.~\ref{fig_wirepos}). 
   
The construction of the 96 drift tube chambers with approximately 70000 drift tubes of lengths between 0.5~m and 2~m for the new Small Wheels 
can be performed at two construction sites, the MPI Munich and the University of Freiburg, in the years 2013 to 2016. The intergration
of the drift tube and trigger chambers at CERN will proceed in parallel.
LMU Munich and the University of W\"urzburg will contribute to the testing of the drift tubes and of the completed chambers before and after
shipment to CERN and to the chamber integration and commissioning of the Small Wheels at CERN. The relative wire positions of all chambers
will be measured with an accuracy of few microns in the cosmic ray test stand at LMU Munich like for the BOS and BOF MDT chambers in ATLAS. 
With additional institutions joining the project, the chamber construction can proceed faster. 2-3 FTE manpower per construction site
are planned for the chamber construction. 

The participating core institutes have extensive experience with the construction of the ATLAS MDT chambers and the associated quality assurance
procedures. The necessary clean rooms and test equipment and the design of the tooling for chamber construction and test already exist. 
The University of Freiburg and MPI Munich will also be involved in the
design, fabrication and measurement of the alignment reference bars for the new Small Wheels in collaboration with Brandeis University,


\subsection{Quality Assurance and Commissioning}

{\it

  \begin{enumerate}
  \item Quality assurance of all individual chamber parts, include
    necessary manpower and rate of tests
  \item Quality assurance and commissioning scheme of full chambers,
    include necessary manpower and rate of tests
  \item How can/is data of the commissioning used later (e.g. strip or
    wire positions)?
  \end{enumerate}

  Note: A common commissioning strategy during and after assembly of
  the wheels will be given in a previous chapter

}

The quality assurance and commissioning procedures follow closely the specifications
for the MDT chamber production for the ATLAS muon spectrometer~\cite{ref_qadoc}.
Each tubes will be tested for wire tension, gas tightness and leakage currents under high voltage
before installation in a chamber.
Wire position measurement for individual drift tubes is not necessary anymore 
due to the new endplug design. Monitoring of the tube positioning in the chamber during assembly
has also become unnecessary. 

The hedgehog cards and the mezzanine readout boards with the new radiation hard ASD and HPTDC chips
as well as the new chamber service modules CSM (see below) will be desiged, produced and tested by MPI Munich.   
New improved noise filters for the high-voltage supply lines for each chamber layer have been designed and
will be produced at MPI Munich.

The completed chambers will be tested for broken wires, gas tightness, 
leakage currents and noise rate before and after delivery to CERN. In addition, all chambers will be tested
in the cosmic ray test stand at LMU Munich and their realtive wire positions will be measured with a few micron
accuracy like for the ATLAS BOS/F chambers.
Measurement of the relative positions of strip and wires in a fraction of the integrated trigger 
and drift-tube chambers is also foreseen.  
The full quality control procedures have already been excercised during the
prototype chamber construction. 

2-3 FTE manpower at each of the 4 core production sites are planned for chamber testing and commissioning.   


\subsection{Operation, Maintenance and Safety}

{\it

  Describe in detail the impact of failures of single components (gas
  leak, HV breakdown, front-end electronics) on the 
  detector operation. Comment on possible built-in redundancy.\\

  Describe in detail the possible scenarios for maintenance on the
  detector and the exchange or repair of detector parts.\\

  Describe the risks associated with the technology during operation
  and maintenance (HV, maximum currents, flammable gas etc.)

}

Due to their robust design and the high quality assurance standards during construction and commissioning, 
the ATLAS MDT chambers operate with very high reliability and data taking efficiency. The same design, fabrication and 
testing rules will be followed for the small diameter drift tube chambers. The maintenance and repair procedures are
essentially the same as for the existing MDT chamber system.
The foreseen numbers of drift tube layers provide sufficient redundancy.
The drift tubes are supplied with high-voltage separately layer-by-layer and separately for each radial segment. If a wire should break,
which is a very rare event, only one tube layer in one segment has to be disconnected from the high-voltage. During shutdown and access to the
Small Wheel, the affected tube can be disconnected from the hedgehog cards. 

Gas leaks in installed MDT chambers are due to cracked Noryl$^\textregistered$ endplugs and gas connections and perforated aluminum tubes.
The tubes are protected from the outside by the monted trigger chambers. To avoid gas leaks due to cracks in endplug insulators and gas connectors
thermoplastics with much higher robustness against cracking have been chosen. The smaller volume endplugs of the 15~mm diameter tubes are less prone  
to mechanical stress and cracks introduced by the injection moulding than the 30~mm diameter tube endplugs.

In the chamber layout and the design of the Small Wheel structure, the possiblity for opening and access of the Faraday cages of the drift tube chambers 
for the exchange of electronics boards has been taken into account. The chamber service modules (CSM) of all chambers will be installed in accessible
and radiation-safe regions at the outer radius of the Small Wheels. 


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{prototype_H8} 
%
\caption{}
\label{fig_prototype_H8}
% 
\end{center}
\end{figure}


\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{prototype_GIF11} 
%
\caption{}
\label{fig_prototype_GIF}
% 
\end{center}
\end{figure}
 

\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{efficiency_GIF11} 
%
\caption{}
\label{fig_efficiency_GIF11}
% 
\end{center}
\end{figure}
 

\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{efficiency_H8_rdep} 
%
\caption{}
\label{fig_efficiency_H8_rdep}
% 
\end{center}
\end{figure}
 

\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{resolution_H8_rdep} 
%
\caption{}
\label{fig_resolution_H8_rdep}
% 
\end{center}
\end{figure}
 

\subsection{Performance}

{\it
  Describe in detail the performance of the detector, including
  results from test beam studies. Include studies with background
  radiation ($\gamma$, n, p)

  \begin{enumerate}
  \item Detector occupancy as function of rate
  \item Spatial and angular resolution as functions of rate and angle
    of incidence for single point measurements and segments
  \item Time resolution
  \item Efficiency (single measurement and segment)
  \item Double track resolution
  \item Rejection of fake and background tracks
  \item Sensitivity to background radiation
  \item Performance limits (maximum rate)
  \end{enumerate}

}

To verify the performance of the 15 mm diameter (''small'') drift tube chambers, extensive tests of the  
full-scale prototype chamber with 15~mm diameter drift tubes have been performed 
in the H8 high-energy muon beam in 2010 and 2011 together with integrated TGC chambers (see Fig.~\ref{fig_prototype_H8}) and, 
using cosmic ray muon tracks, in the CERN Gamma Irradiation Facility (GIF) with a 500~GBq $^137$Cs source
(see Fig.~\ref{fig_prototype_GIF11}). The standard ATLAS MDT readout electronics has been used with the adjustable
deadtime set to the minimum value corresponding to an overall effective deadtime of 200~ns. 
The spatial resolution and efficiency of the individual drift tubes and of the whole chamber have been measured
for different $\gamma$ irradiation rates where the individual drift tubes have been illuminated uniformly over the whole length. 

As a measure relevant for evaluating the tracking efficiency, the
so called $3\sigma$ efficiency of individual drift tubes is determined which is the probability of detecting 
a hit within 3 times the drift tube resolution $\sigma$ of the muon track measured by the other tubes 
on the track extrapolated to the active volume of the tube. Similarly, the drift tube resolution as a function of the drift 
radius is determined by an iterative method excluding the evaluated tube from the track reconstruction 
in the chamber~\cite{ref_resolution_measurement}. 

Fig.~\ref{fig_efficiency_GIF11} shows the average #3\sigma$ efficiency of 15~mm diameter drift tubes
and the track segment reconstruction efficiency of the prototype chamber as a function of the
$\gamma$ background flux in comparison with the results for the 30~mm diameter tubes from previous
tests of a BOS MDT chamber in GIF~\cite{ref_deile}, \cite{ref_horvat}. At low background rate, the $3\sigma$ tube 
efficiencies of 30 and 15~mm diameter tubes are $94~\%$ and $96~\%$, respectively. 
The efficiency loss is due to $\delta$-rays created 
by interaction of the muons with the tube walls which produce hits earlier than the muons.
The effect increases with the distance of the muon track from the wire (see Fig.~\ref{fig_efficiency_H8_rdep}) and, therefore,
is smaller for 15~mm than for 30~mm diameter tubes.
The degradation of the efficiency with increasing rate follows the predicted behaviour. 
The redundancy of track measurements in the different drift tube layers is sufficient to achieve almost $100~\%$
track segment reconstruction efficiency up to the highest background rates.  

In Fig.~\ref{fig_resolution_H8_rdep} the spatial resolutions of 15~mm and 30~mm diameter drift tubes 
measured without background irradiation in the high-energy muon beams H8 and X5, respectively, at CERN
as a function of the drift radius $r$. The latter is determined using an external reference, the other drift tube layers of the
prototype chamber in the H8 beam vs.\ a silicon strip detector beam telescope in the X5 beam.   
The radial dependence of the 15~mm diamter tube resolution agrees with the 30~mm diameter tube resolution
measurement for $r<7.5$~mm as expected. The average resolutions including time-slewing corrections 
of 15 and 30~mm diameter tubes at low background rates are $110\pm 10~\mu$m and $80\pm 5~\mu$m, respectively. 


\subsection{Aging Tests}

{\it

  Describe in detail results from aging tests for all components, the
  detector itself including the on-chamber gas distribution, read-out
  and/or trigger, alignment, and detector control electronics.

}

No aging is expected to occur in aluminum drift tubes with clean Ar:CO$_2$ gas. 
No aging has been observed in test of the MDT tubes up to a collected charge 
on the wire of 0.6~C/cm. Low-outgassing endplug and gas system materials evaluated previously
and clean components of drift tubes and gas system are used to prevent contamination
of the drift gas. 

Aging tests of the new drift tubes up to 2-3 C/cm collected charge including irradiation
of the endplugs are on the way with strong radioactive sources at the accelerator laboratory   
at LMU Munich.


\subsection{Cost, Funding, and Manpower}

{\it

  \begin{enumerate}
  \item Table of cost of each component, full chambers and total
  \item Additional costs (e.g. integrating two technologies)
  \item Arrangements for funding (if any so far)  
  \item List of manpower needs, especially include statements on the
    the current and future availability of experts (for at least the
    full construction and commissioning period, i.e. 2012 -- 2019,
    better also for the operation)
  \end{enumerate}
    
}






\begin{thebibliography}{1}
% ---------------------------------------------------------------------------- %
\bibitem{ref_deile}
M.~Deile~{\it et al.}, {\sl Performance of the ATLAS Precision Muon
Chambers under LHC Operating Conditions}, Nucl. Instr.\ and Meth.~{\bf A 518} (2004) 65;
M.~Deile~{\it et al.}, {\sl Resolution and Efficiency of the ATLAS Muon Drift-Tube 
Chambers at High Background Rates}, Nucl.~Instr.\ and Meth.~{\bf A535} (2004) 212.
% ---------------------------------------------------------------------------- %
\bibitem{ref_horvat}
S.~Horvat~{\it et al.}, {\sl Operation of the ATLAS Precision Muon
Drift-Tube Chambers at High Background Rates and in Magnetic Fields},
IEEE trans. on Nucl. Science Instr. Vol. 53, 2 (2006) 562.
% ---------------------------------------------------------------------------- %
\bibitem{ref_MDTtracking}
O.~Kortner~{\it at al.},
{\sl Alignment of the ATLAS Muon Spectrometer with Tracks and 
Muon Identification at High Background Rates},
Nucl.~Instr.\ and Meth.~{\bf A581} (2007) 545-548.
%----------------------------------------------------------------------------- %
\bibitem{ref_bittner1}
B.~Bittner~{\it et al.}, {\sl Development of Muon Drift-Tube Detectors for High-Luminosity
Upgrades of the Large Hadron Collider},  
Nucl.~Instr.\ and Meth.~{\bf A617} (2010) 169.
%----------------------------------------------------------------------------- %
\bibitem{ref_garfield}
R.~Veenhof, {\sl GARFIELD: Simulation of Gaseous Detectors, Version 8.01, CERN},
write-up: http://wwwinfo.cern.ch/writeup/garfield.
% ---------------------------------------------------------------------------- %
\bibitem{ref_riegler}
W.~Riegler, {\sl High Accuracy Wire Chambers}, Nucl. Instr.\ and Meth.~{\bf A 494} (2002) 173.
% ---------------------------------------------------------------------------- %
\bibitem{ref_bittner2}
B.~Bittner~{\it et al.},
{\sl Development of Fast High-Resolution Muon Drift-Tube Detectors for High Counting Rates},
Nucl.~Instr.\ and Meth.~{\bf A 628} (2011) 154.   
%----------------------------------------------------------------------------- %
\bibitem{ref_aleksa}
M.~Aleksa~{\it et al.}, {\sl Rate Effects in High-Resolution Drift Chambers}, 
Nucl. Instr.\ and Meth.~{\bf A 446} (2000) 435.
%----------------------------------------------------------------------------- %
\bibitem{ref_qadoc}
H.~Kroha, {\sl Quality Assurance and Quality Control Reference Document 
for ATLAS MDT Chamber Construction},
ATLAS Note, ATL-MUON-2000-008, May 1999.
%----------------------------------------------------------------------------- %
\bibitem{ref_resolution_measurement}
S.~Horvat, O.~Kortner, H.~Kroha,
{\sl Determination of the Spatial Drift-Tube Resolution Using Muon Tracks},\newl
ATLAS Note, ATL-MUON-PUB-2006-008, July 2006.
%----------------------------------------------------------------------------- %

\end{thebibliography}
\end{document}
