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\begin{document}
%
\section{The sMDT Chambers}
%
\begin{figure}[tb]
\centering
\includegraphics[width=0.9\columnwidth]{tubes}
\caption{Picture of MDT (left) and sMDT (right) tubes.}
\label{fig:tubes}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.9\columnwidth]{spectrum}
\caption{Drift time spectra of MDT (green) and sMDT tubes (grey) together with the prediction for sMDT tubes from the GARFIELD simulation (red line)~\cite{sMDTperf1}.}
\label{fig:spectrum}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.9\columnwidth]{gasgain}
\caption{Measurements of the relative gas gain of MDT and sMDT as a function of the background $\gamma$ flux with respect to a nominal gas gain of 20000 in each case compared to 
predictions based on the Diethorn formula~\cite{sMDTperf3}.}
\label{fig:gasgain}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.9\columnwidth]{rtrelation}
\caption{Space-to-drift time relationship of MDT drift tubes. The part for drift radii r below 7.5~\mm{}, relevant for the sMDT tubes, is linear to good approximation.}
\label{fig:rtrelation}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.9\columnwidth]{res_with_TS_new.pdf}
\caption{Spatial resolution without irradiation of MDT and sMDT drift tubes measured in the H8 muon beam with time-slewing corrections (TSC) as a function of the drift radius~\cite{sMDTperf2,sMDTperf3}.
As expected, there is good agreement between 15 and 30~\mm{} diameter tubes for drift radii below 7.5~\mm{}.}
\label{fig:resolr}
\end{figure}

The ATLAS Monitored drift tube (MDT) chambers provide reliable muon tracking with excellent spatial resolution and high tracking efficiency independent of the track incident angle.
Small-diameter muon drift tube (sMDT) chambers~\cite{sMDTperf1} with a tube diameter of 15~\mm{}, i.e.\ half of the tube diameter of the MDT chambers, 
have been developed since 2008 to cope with the higher background irradiation rates at HL-LHC and to allow for more tube layers or additional trigger chambers
under the tight space constraints in the muon spectrometer. Standard aluminium tubes with 0.4~\mm{} wall thickness like for the MDT chambers are used.
Figure~\ref{fig:tubes} shows a comparison of the MDT and the sMDT drift tubes. The drift time spectra are shown in Figure~\ref{fig:spectrum}. The maximum drift time
of the sMDT tubes is only 175~\ns{} compared to about 700~\ns{} of the MDT chambers leading to much lower occupancy and a linear space-to-drift time relationship
in the standard MDT drift gas Ar:CO$_2$ (93:7) at 3 bar pressure.

A full-scale sMDT prototype chamber of trapezoidal shape has been constructed and tested in the H8 muon beam and in the Gamma Irradiation Facility (GIF) 
at CERN in 2010~\cite{sMDTprototype}.
The chamber has been operated in the ATLAS cavern in 2012. In 2014, two BME sMDT chambers~\cite{BMEproposal} have been installed in the elevator regions of the barrel muon spectrometer,
together with two new RPC chambers, and are in operation since the start of LHC Run~2~\cite{BMEop}. In the winter shutdown 2016/17, 12 BMG sMDT chambers, which are currently under 
construction~\cite{BMGconstr}, will be installed in the detector feet in the barrel sectors 12 and 14~\cite{BMEop,BMGproposal}. 
In the long shutdown 2019-2021, 16 BIS~$7\& 8$ sMDT chambers with integrated thin-gap RPC chambers will be installed at the ends of the BIS layers in the barrel
in order to improve the trigger efficiency and the rate capability in the transition region~\cite{BIS78proposal}.
This upgrade serves as pilot project for the complete replacement of the BIS layer with very similar chamber packages as discussed in this document. 


\subsection{Performance of sMDT chambers}
%
\begin{figure}[tb]
\centering
\includegraphics[width=0.72\columnwidth]{resolution}
\caption{Average spatial resolution of MDT and sMDT drift tubes as a function of the $\gamma$ background flux measured at the Gamma Irradiation Facility at CERN.}
The measurement results for standard MDT readout electronics agree well with the expectations from detailed simulations of detector and electronics response and are compared to
predictions for electronics employing additional active baseline restoration.}
\label{fig:resolution}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.9\columnwidth]{efficiency}
\caption{Muon detection efficiencies of  MDT and sMDT drift tubes with corresponding electronics deadtime settings (see text) as a function of the $\gamma$ background counting rate per tube.  
The efficiency is defined as the probablity to find a hit on the extrapolated track within $3\sigma$ of the tube spatial resolution ($3\sigma$ efficiency). 
The measurement results for standard MDT readout electronics agree well with the expectations from detailed simulations of detector and electronics response and are compared to
predictions for electronics employing additional active baseline restoration.}
\label{fig:efficiency}
\end{figure}

The performance of MDT and sMDT chambers has been extensively studied at the Gamma Irradiation Facility at CERN using the existing ATLAS MDT readout electronics with 
bipolar shaping~\cite{MDTperf, sMDTperf1, sMDTperf2}.
The MDT chambers can be operated up to background rates of 500~kHz/$\cm^2$ and 300~kHz per tube.  At background fluxes above 500~Hz/$\cm ^2$, the gas gain drops by more than 20\%  
(see Figure~\ref{fig:gasgain}) leading together with the effect of space charge fluctuations to rapid deterioration of the spatial resolution with increasing background flux. 
The limitations of the MDT chambers are overcome by using drift tubes with half the diameter of the ATLAS MDT tubes while leaving the operating parameters, 
Ar:CO$_2$ (93:7) gas mixture at 3 bar pressure and nominal gas gain of 20000 (for a wire potential of 2730~V with respect to the tube wall), unchanged~\cite{sMDTperf1}. 

As the space charge density inside the drift tubes is proportional to the third power of the tube radius, 15~mm diameter drift tubes show a significant gain drop only at 8 times higher 
background rates compared to 30~mm diameter drift tubes (see Figure~\ref{fig:gasgain}). 
At the same time, the deteriorating effect of space charge fluctuations on the spatial resolution is eliminated because the drift gas is linear to good approximation for drift radii below 7.5~mm 
(see Figure~\ref{fig:rtrelation}). Without irradiation and associated space charge effects, the average sMDT drift tube resolution is $106\pm 2~\mum$ (see Figure~\ref{fig:resolr})
compared to $83\pm 2~\mum$ for the MDTs~\cite{sMDTperf3}.
The dependence of the average spatial resolution of MDT and sMDT drift tubes on the $\gamma$ background flux is shown in Figure~\ref{fig:resolution}. The spatial resolution deteriorates quickly
with increasing background flux for the MDTs while it is almost unaffected by space charge effects up to very high irradiation rates for the sMDTs. 

For the same background flux, the small-diameter drift tubes have 8 times lower occupancy than the 30~mm diameter tubes because of the 4 times shorter maximum drift time (see Figure~\ref{fig:spectrum})
and the twice smaller tube cross section exposed to the radiation. Because of the much shorter maximum drift time, the dead time of the MDT readout electronics 
(which for the MDTs is set to a nominal value of 820~ns, slightly above the maximum drift time, to prevent the detection of secondary ionization clusters) 
can be reduced to the minimum adjustable value of 220~ns, just above the maximum drift time of the sMDT tubes. 
In this way, the masking of muon hits by preceding background pulses is strongly reduced increasing the muon detection efficiency defined as the probability to find a hit on the extrapolated 
muon track within 3 times the drift tube resolution ($3\sigma$ efficiency). Figure~\ref{fig:efficiency} shows the improvement of the $3\sigma$ efficiency of sMDT tubes at high 
background counting rates compared to the MDT tubes. 
Muon detection efficiencies of almost $100\%$ and spatial resolutions of better than $30\mu$m are achieved with 8-layer sMDT chambers up to background rates well beyond the maximum rates
expected at HL-LHC with standard MDT readout electronics.

For the reduced electronics dead time, a significant part of the recovered
muon signals overlaps with the long undershoots of the bipolar shaped pulses of preceding background hits leading to reduced signal amplitudes and additional jitter in the threshold crossing time and, therefore, to
a degradation of both the resolution and the efficiency of these muon hits (so called signal pile-up effects). Simulations of the drift tube and electronics response show that the spatial resolution 
and muon efficiency of the sMDT tubes at high background rates is limited by signal pile-up effects and the rate capability of the readout electronics~\cite{blr}.  
The simulations and first measurements with new electronics also show that both resolution and efficiency of the sMDT tubes 
can be further improved by employing active baseline restoration (BLR) in the readout electronics and by further reducing the electronics dead time~\cite{blr} 
(see Figures~\ref{fig:resolution} and \ref{fig:efficiency}). Improvement of the performance of the readout electronics is, however, not needed for the Phase 2 upgrades of the ATLAS
muon spectrometer.


\subsection{Drift Tube Design and Production}
%
The sMDT chamber design and construction procedures have been optimized for mass production while they provide highest mechanical accuracy in the sense wire positioning.
Standard industrial aluminium tubes with 15~\mm{} outer diameter and a wall thickness of 0.4~\mm{} are used. The tubes are chromatised on the in- and outside for the cleaning purposes 
and reliable electrical ground contact. The ground pins are screwed into the holes between adjacent tube triplets during the glueing of the tube layers. 

The drift tubes are assembled using a semi-automated wiring station in a temperature-controlled clean room (see Figure~\ref{fig:wiring}). 
The sense wires are fed through the tubes and the endplugs (see Fig.~\ref{fig:endplug1} and \ref{fig:endplug2}) by means of air flow without manual contact and fixed in 
copper crimp tubelets after tensioning to $350\pm 15$~g corresponding to a gravitational sag of only $17\pm 1~\mum$ (absolute tolerances).
The wires are positioned at each tube end with a few micron precision with respect to a cylindrical external reference surface
on the central brass insert of the endplug which also holds the spiral shaped wire locator on the inside of the tube. 
The drift tubes are sealed with the endplugs using two O-rings per endplug and mechanical swaging of the tube walls. For the injection molded endplug insulators 
and gas connectors for the individual tubes, plastic materials with minimum outgassing have been selected which are also immune against cracking. 
 
Only materials already certified for the ATLAS MDT chambers are used for sMDT drift tubes and their gas connections in order to prevent ageing.
The sMDT tubes, including the plastic material of the endplugs, have been irradiated with a 200 MBq ${}^{90}$Sr 
source over a period of 4 months with a total charge accumulation on the sense wire of 9~C/\cm{} without any sign of ageing~\cite{sMDTref3,aging}.

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{wiring_station}
	\caption{Semi-automated drift-tube assembly station in a climatised clean room.}
	\label{fig:wiring}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{sMDT_endplug_exploded.pdf}
	\caption{Exploded view of an sMDT endplug with interfaces for precise wire positioning and measurement, for gas and high-voltage supplies and for readout 
electronics~\cite{sMDTprototype}.}
	\label{fig:endplug1}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[angle=90,width=0.7\textwidth]{sMDT_endplug_cross_section_labels_2.pdf}
	\caption{Cross section of an sMDT endplug with internal wire locator and external reference surface for tube and wire positioning during construction and for 
	wire position measurement~\cite{BMGconstr}.}
	\label{fig:endplug2}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{assembly_station}
	\caption{Assembly jigging of sMDT chambers for the example of BMG chamber construction. The wire grid is defined at each chamber end by the grids of holes in the jigs 
         into which the cylindrical endplug reference surfaces are inserted. The tube and jig layers are stacked one after the other including the spacer in between the two multilayers.}  
	\label{fig:assembly}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{glueing}
	\caption{Glue distribution between adjacent tubes of a tube layer during chamber assembly using an automated glue dispenser.} 
	\label{fig:glueing}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{CMM_measurement}
	\caption{Assembly jigging of sMDT chambers for the example of BMG chamber construction. The wire grid is defined at each chamber end by the grids of holes in the jigs 
into which the cylindrical endplug reference surfaces are inserted. The tube and jig layers are stacked one after the other including the spacer in between the two multilayers.}  
	\label{fig:CMM}
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=1.14\textwidth]{BMG3_residuals_r.pdf}
	\caption{Residuals of the sense wire positions measured at both ends of a BMG sMDT chamber with 356 tubes with respect to the nominal wire grid. 
	The width of the distribution includes the accuracy of the coordinate measureing machine of about 2~\mum{}~\cite{BMGconstr}.}  	
	\label{fig:residuals}
\end{figure}

A typical production rate of 100 tubes per day has been achieved with a failure rate of about $4\%$ due to occasional mechanical failures of the assembly tooling. 
During the production of the 4300 BMG drift tubes, the failure rate of the standard drift tube quality tests of wire tension ($350\pm 15$~g), gas leak rate ($<10^{-8}$~bar~l/s) and leakage current ($< 2$~nA/m) at the nominal operating voltage of 2730~V was only $0.5\%$~\cite{vci2016}. 


\subsection{Chamber Construction and Test}
%
\begin{figure}[th]
\centering
	\includegraphics[width=1.14\textwidth]{gassystem}
	\caption{BMG chamber with mounted gas distribution system.}
	\label{fig:gassystem}
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=1.14\textwidth]{stacked_mezzanine_card}
	\caption{sMDT signal distribution board with stacked mezzanine card carrying three ASD chips (middle layer) and the HPTDC with programming FPGA (top layer).}	
	\label{fig:stacked_mezzanine_card}
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=1.14\textwidth]{HV_side}
	\caption{HV distribution boards~\cite{sMDTprototype} mounted inside the Faraday cage of a BMG chamber.}
	\label{fig:HV_side}
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=1.14\textwidth]{RO_side}
	\caption{Signal distribution and readout electronics boards mounted inside the Faraday cage of a BMG chamber.}
	\label{fig:RO_side}
\end{figure}

After passing the quality assurance tests, the drift tubes are assembled to chambers in a climatised clean room by inserting the endplug reference surfaces into a grid of fitting bores in the assembly jigs 
at each chamber end which define the wire positions with an accuracy of better than 5 micron (see Figure~\ref{fig:assembly}) and glueing them together and to the spacer and support frame 
using an automated glue dispenser (see Figure~\ref{fig:glueing}). A chamber can be assembled within one working day. 
The precise mounting of the alignment sensor platforms with respect to the wires may require an additional day depending on their location. 
  
After the glueing of the tube layers, the positions of the individual endplug reference surfaces and, thus, of the sense wires are measured at the two chamber ends 
with an automated coordinate measuring machine with a precision of about 2~$\mu$m (see Figure~\ref{fig:CMM}). The measurement can be performed within a few hours for every chamber or only used 
as a spot check. In particular, the positions of the alignment sensor platforms with respect to the wire grid can be measured in this way with a few micron accurracy.
Sense wire positioning accuracies of better than 10~$\mu$m (rms) have been routinely achieved during BME and BMG chamber construction~\cite{BMEop,BMGconstr}. 
An ultimate wire positioning accuracy of 5~$\mu$m (rms) has been achieved in BMG chamber construction which reaches the precision of the 
assembly jigs (see Fig.~\ref{fig:residuals}). After the measurement, the individual wire positions are known with 2~$\mu$m accuracy.
 
After the wire position measurement, the parallel gas distribution system consisting of modular injection molded gas connectors (see Figure~\ref{fig:endplug1}) 
is mounted (see Figure~\ref{fig:gassystem}. Gas leak rates at 3 bar pressure of better than the required 0.2 mbar/h have been achieved for all BMG chambers~\cite{BMGconstr}.


\subsection{sMDT Chamber Readout Electronics}
%
After the installation of the gas distribution system, the Faraday cages and the high-voltage distribution boards and the signal-distribution and front-end electronics 
baords (see Figure~\ref{fig:stacked_mezzanine_card}) with 6 x 4 readout channels matching the transverse cross section of the multilayers are mounted are mounted on the HV and RO side, respectively 
(see Figures~\ref{fig:HV_side} and \ref{fig:RO_side}). The decoupling capacitors on the RO side and the termination resistors on the HV side are enclosed in plastic
containers in order to guarantee HV stability. The custom-designed two-layer readout boards stacked on top of the signal distribution boards contain three 8-channel MDT ASD chips 
as well as a CERN HPTDC chip~\cite{HPTDC_manual} and a FPGA for the JTAG programming, respectively (see Figure~\ref{fig:RO_side}).  

\section{Chamber Layout and Design}
%
\begin{figure}[th]
\centering
	\includegraphics[width=\textwidth]{BIS78_3D.pdf}
	\caption{Drawings and 3D view of a BIS78 sMDT chamber with integrated triple thin-gap RPC for the muon spectrometer upgrade in 2019-20.}
	\label{fig:BIS78_3D} 
\end{figure}

The design and construction of new BIS78 sMDT chambers with 2 x 4 tube layers and integrated thin-gap RPCs with three gas gaps for replacement of the current BIS7 and BIS8 MDT chambers 
in the 2019-20 LHC shutdown is a pilot project for the complete replacement of the BIS MDT chambers by sMDT + RPC chambers for HL-LHC. The design of the remaining new BIS chambers will 
be very similar to the new BIS78 sMDT + RPC chambers. The common challenge for the mechanical design is the small available space and the control of gravitational deformations 
with a minimum  of support structure. The preliminary design drawings of a BIS78 chamber are shown in Figures~\ref{fig:BIS78_3D}.


\begin{table}

\begin{tabular}{|l||c|c|c|c|c|c|c|c|c|c|}

\begin{small}

\hline
Type & BIS78-A2 & BIS78-A4/6 & BIS78-A8/10 & BIS78-A12 & BIS78-A14 & BIS78-A16 & BIS78-C2 & BIS78-C4/6 & BIS78-C8/10 & BIS78-C12 & BIS78-C14 & BIS78-C16 \\
\hline\hline
Radial distance from beam (mm)    & 4592 & 4550 & 4550 & 4550 & 4550 & 4635 & 4635 & 4550 & 4550 & 4550 & 4550 & 4635 \\
Chamber width in z (mm)           & 1655 & 1655 & 1655 & 1655 & 1655 & 1655 & 1655 & 1655 & 1655 & 1655 & 1655 & 1655 \\
Tubes width in z (mm)             & 1638 & 1638 & 1638 & 1638 & 1638 & 1638 & 1638 & 1638 & 1638 & 1638 & 1638 & 1638 \\
Chamber length in x (mm), long    & 1839 & 1839 & 1839 & 1839 & 1839 & 1839 & 1839 & 1839 & 1839 & 1839 & 1839 & 1839 \\ 
Chamber length in x (mm), cutout  & 1709 & 1709 & 1709 & 1709 & 1709 & 1709 & 1709 & 1709 & 1709 & 1709 & 1709 & 1709 \\ 
Chamber length in x (mm), short   & 1179 & 1179 & 1179 & 1179 & 1179 & 1179 & 1179 & 1179 & 1179 & 1179 & 1179 & 1179 \\ 
Al tube length (mm), long         & 1660 & 1660 & 1660 & 1630 & 1630 & 1660 & 1660 & 1660 & 1660 & 1630 & 1630 & 1660 \\
Assembled tube length (mm), long  & 1669 & 1669 & 1669 & 1639 & 1639 & 1669 & 1669 & 1669 & 1669 & 1639 & 1639 & 1669 \\
Al tube length (mm), cutout       & 1530 &      &      & 1530 & 1530 & 1530 & 1530 &      &      & 1530 & 1530 & 1530 \\
Assembled tube length (mm), cutout& 1539 &      &      & 1539 & 1539 & 1539 & 1539 &      &      & 1539 & 1539 & 1539 \\
Al tube length (mm), short        & 1000 & 1000 & 1000 & 1000 & 1000 & 1000 & 1000 & 1000 & 1000 & 1000 & 1000 & 1000 \\
Assembled tube length (mm), short & 1009 & 1009 & 1009 & 1009 & 1009 & 1009 & 1009 & 1009 & 1009 & 1009 & 1009 & 1009 \\
\hline
Tube layers, long/cutout  & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 & 2 x 4 \\
Tube layers, short/cutout & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 \\
Tubes/layer, long, ML1/2  & 78/66 & 78/96 & 66/84 & 66/54 & 66/54 & 66/54 & 78/66 & 78/96 & 66/84 & 66/54 & 66/54 & 66/54 \\
Tubes/layer, cutout ML2   & 30    &       &       & 30    & 30    & 30    & 30    &       &       & 30    & 30    & 30    \\
Tubes/layer, short ML2    & 12    & 12    & 12    & 12    & 12    & 12    & 12    & 12    & 12    & 12    & 12    & 12    \\
Tubes/layer, ML1          & 78    & 78    & 65    & 66    & 66    & 66    & 78    & 78    & 66    & 66    & 66    & 66    \\
Tubes/layer, ML2          & 108   & 108   & 95    & 96    & 96    & 96    & 108   & 108   & 96    & 96    & 96    & 96    \\
Tubes/chamber, long       & 576 & 696 & 600 & 480 & 480 & 480 & 576 & 696 & 600 & 480 & 480 & 480 \\
Tubes/chamber, cutout     & 120 &     &     & 120 & 120 & 120 & 120 &     &     & 120 & 120 & 120 \\
Tubes/chamber, short      &  48 &  48 &  48 &  48 &  48 &  48 &  48 &  48 &  48 &  48 &  48 &  48 \\
Tubes/chamber             & 744 & 744 & 648 & 648 & 648 & 648 & 744 & 744 & 648 & 648 & 648 & 648 \\  
Spacer height (mm), long  & 45.6 & 45.6 & 45.6 & 45.6 & 45.6 & & 45.6 & 45.6 & 45.6 & 45.6 & 45.6 & 45.6 & 45.6 \\
Tubes height (mm)         & 139 & 139 & 139 & 139 & 139 & 139 & 139 & 139 & 139 & 139 & 139 & 139 \\
Chamber height (mm)       & 249 & 249 & 249 & 249 & 249 & 249 & 249 & 249 & 249 & 249 & 249 & 249 \\
1\hline
Gas volume/chamber (l)    & 188.1 & 190.6 & 165.3 & 160.6 & 160.6 & 162.9 & 188.1 & 190.6 & 165.3 & 160.6 & 160.6 & 162.9 \\
Chamber weight (kg)       & 170   & 170   & 150   &  150  &  150  &  150  &  170  &  170  &  150  &  150  &  150  &  150  \\ 
\hline
Mezz.~cards (24 ch.)/chamber & 31 & 31 & 27 & 27 & 27 & 27 & 31 & 31 & 27 & 27 & 27 & 27 \\ 
T-sensors/chamber & 16 & 16 & 16 & 16 & 16 & 16 & 16 & 16 & 16 & 16 & 16 & 16 \\ 
Praxial alignment platforms & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 \\
Survey targets & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 & 4 \\
\hline

\end{small}
  
\end{tabular}

\end{table}



\begin{thebibliography}{1}

\bibitem{sMDTperf1}
B.~Bittner et al., 
{\sl Development of Muon Drift-Tube Detectors for High-Luminosity Upgrades of the Large Hadron Collider},
arXiv:1603.09504, June 2009, 
Nucl.~Instrum.\ and Meth.~{\bf A617} (2010) 169;

\bibitem{sMDTprototype}
H.~Kroha et al., 
{\sl Construction and test of a full-scale prototype drift-tube chamber for the upgrade of the ATLAS muon spectrometer at high LHC luminosities}, 
Nucl. Instrum. and Meth. A718 (2013) 427. 

\bibitem{BMEproposal}
G.~Aielli et al.,
{\sl Proposal for the Upgrade of the Elevator Regions in the ATLAS Barrel Muon Spectrometer},
ATL-MUON-INT-2014-001, November 2013.

\bibitem{BMEop}
C.~Ferretti, H.~Kroha (on behalf of the ATLAS muon collaboration), 
{\sl Upgrades of the ATLAS Muon Spectrometer With sMDT Chambers},
proceedings of the $13^{\mathrm th}$ Pisa Meeting on Advanced Detectors, La Biodola, Isola d'Elba, Italy,
24--30 May 2015, arXiv:1603.09544, June 2015,
Nucl.~Instrum.~and Meth.\ {\bf A 824} (2016) 538.

\bibitem{BMGconstr}
H.~Kroha et al., 
{\sl Construction and Test of New Precision Drift Tube Chambers for the ATLAS Muon Spectrometer},
arXiv:1603.08760, March 2016, 
Nucl.~Instrum.~and Meth.~A, doi:10.1016/j.nima.2016.05.091.

\bibitem{BMGproposal}
H.~Kroha,
{\sl Proposal for the Improvement of the ATLAS Muon Spectrometer Momentum Resolution in Barrel Sectors 12 and 14},
ATL-MUON-INT-2015-001, February 2015.

\bibitem{BIS78proposal}
G.~Aielli et al.,
{\sl The BIS78 Project}, 
ATLAS Internal Note, ATL-COM-MUON-2016-018, ATL-MUON-INT-2015-xxx, February 2015.

\bibitem{MDTperf}
M.~Deile et al.,
{\sl Resolution and Efficiency of the ATLAS Muon Drift-Tube Chambers at High Background Rates},
Nucl.~Instrum.\ and Meth.~{\bf A535} (2004) 212;
S.~Horvat et al.,
{\sl Operation of the ATLAS Muon Drift-Tube Chambers at High Background Rates and in Magnetic Fields},
IEEE Trans.~Nucl.~Sci., Vol.~53, No.~2 (2006) 562.

\bibitem{sMDTperf2}
B.~Bittner, 
{\sl Development and Characterisation of New High-Rate Muon Drift Tube Detectors},
PhD thesis, Technical University Munich, MPI Report, MPP-2012-130, CERN-THESIS-2012-122, Juli 2012;
P.~Schwegler, 
{\sl High-Rate Performance of Muon Drift Tube Detectors}, CERN-THESIS-2014-091.
PhD thesis, Technical University Munich, MPI Report, MPP-2014-302, CERN-THESIS-2014-091, Juli 2014.

\bibitem{sMDTperf3}
B.~Bittner et al., 
{\sl Performance of drift-tube detectors at high counting rates for high-luminosity LHC upgrades}, 
Nucl.~Instrum.~Meth.~{\bf A732} (2013) 250.

\bibitem{aging}
O.~Kortner et al.,
{\sl Precision Muon Tracking Detectors and Read-Out Electronics for Operation at Very High Background Rates at Future Colliders},
Nucl.~Instr.~and Meth.\ {\bf A824} (2016) 556.

\bibitem{blr} 
P.~Schwegler et al., 
{\sl Optimization of the Front-End Electronics of Drift Tube Chambers for High-Rate Operation}, proc.~of the 2014 IEEE NSS conference, Seattle, Washington, USA, November 2014,
MPI report, MPP-2014-556, November 2014;
S.~Nowak et al., 
{\sl Optimisation of the Read-out Electronics of Muon Drift-Tube Chambers for Very High Background Rates at HL-LHC and Future Colliders}, 
proceedings of the 2015 IEEE NSS conference, San Diego, California, USA, November 2015, MPI report, MPP-2015-480, arXiv:1603.08841, November 2015.

\bibitem{HPTDC_manual}
J.~Christiansen et al.,
{\sl HPTDC--High Performance Time to Digital Converter, Version 1.3},
CERN/EP-MIC, March 2004, https://cds.cern.ch/record/1067476/files/cer-002723234.pdf.



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\end{document}
