%%%%%  The following statement gives section #5 to the MDT-TGC,
%%%%%  independent of the number of previous sections
%
\setcounter{section}{4}
%
\newpage


%%% ==== 5   ============================================================
%%%
\section{Detector concept 1 :  MDT + TGC  (working title)}
%%%
%%% =====================================================================

\if{0}

{\it Introduction to this concept:
This section should describe the technology concept, details 
up to the level of a single chamber should be given in the appropriate
appendix.}

\fi

%%% ==== 5.1 ============================================================
%%%
\subsection{Introduction}
\label{Introduction}
%%%
%%% =====================================================================

\if{0}

%% ----- 5.1.1-----------------------------------------------------------
%%
\subsubsection{Basic Layout}
\label{Basic_Layout}
%%
%% ----------------------------------------------------------------------

\fi

The dominant motivation behind the present proposal is to  considerably increase the
performance and rate capability of the triggering and precision tracking system in the
Small Wheel in order to match the higher background levels in the phase II upgrade
scenario of the LHC.

For this purpose, we propose the following layout for the upgrade of the Small Wheel. 

\begin{enumerate}

\item[(a)] a new type of TGC trigger chamber, providing considerably improved spatial resolution
in the bending plane but preserving the basic rugged structure together with the operating
parameters of the proven TGC system, as presently installed in the muon end-cap.

\item[(b)] a new type of MDT chamber, made from tubes of half the diameter of the presently 
used tubes, otherwise operating under the same conditions with respect to gas gain, gas 
composition and pressure. The reduction of the tube diameter from 30 to 15 mm leads to an 
increase of the rate capability of the tubes by a factor of 7, sufficient to cope with the 
level of background hits expected at the SLHC.

\end{enumerate}

%%%%%%%%%%%%%%

\begin{figure}[!htb]

   \centering

\includegraphics[width=0.5\linewidth,clip]{Figures/TGC_MDT_package_BB}

\caption{Cross section through the proposed arrangement of trigger TGCs and precision MDTs. The
MDT, made out of two multilayers (with 4 or 6 tube layers) is sandwiched between two packages of
4-layer TGCs. The distance of 300 mm between the TGC packages is instrumental for the angular
resolution of the triggering muon track.}

\label{TGC_MDT_package}

\end{figure}

%%%%%%%%%%%%%%%

With this upgrade of the existing chamber technology towards higher performance the uniformity 
of the present muon spectrometer (MDT as tracking detectors, RPC as trigger detectors in the 
barrel and TGCs in the end-cap) is preserved while profiting from the existing technical 
infrastructure and the experience to construct such an assembly at the relevant institutions.

For the trigger chambers we propose to maintain the basic operating concept of TGC chambers, 
complementing them with a new, high-performance readout system.  The pulseheight distribution  of
signals induced into the strips of the TGC cathodes (measuring $\eta$) are convoluted to  give the
precision coordinate with an accuracy of $ < $100 $\mu m$. To distinguish the new TGCs, proposed
for the NSW from the existing ones in the Big Wheel, we will refer to them as to ''sTGCs''
throughout this article.  

In the proposed detector layout, a package of 4 sTGC chambers is placed in front and behind the 
MDT chambers, each. A sTGC package will contain 4 wire layers (wires running radially, 
perpendicular to the strips), 4 cathode layers for centroide finding and another 4 cathode  layers
segmented into pads, for fast selection of a Region of Interest (Fig.~\ref{TGC_MDT_package}). In
the offline  analysis the pads will be an important tool for resolving ambiguities in the x-y
pairing of  coordinates, when more than one track is present in a chamber.

One important limitation of the present L1-trigger in the end-cap is due to the fact that a 
majority of tracks crossing the Small Wheel do not originate from the primary vertex and should 
therefore not be considered candidates for a high-p$_T$ muon by the triggering system. The 
present trigger, however, can not determine the slope of the candidate track in the r-z plane 
and for this reason is unable to discard tracks not coming from the primary vertex. With a 
distance of about 300 mm between the sTGC packages in front and behind the MDT, an angular 
resolution of $< 0,3$ mrad can be achieved, which allows to only consider tracks pointing to 
the interaction as high-p$_T$ candidates for the L1 trigger. The slope of the track in the 
$\phi$ direction will also be determined, though with less accuracy, to further reduce the 
number of false triggers. A detailed presentation of the method is given in 
section~\ref{TGC_technology}.

The limitations of the MDT precision chambers at high luminosity are mainly caused by
isolated hits in the tubes (''fake hits''), coming from neutron and gamma conversions in
nearby support structures, tube walls, chamber gas and tungsten wires. The presently
used 30 mm diameter tubes (''Large tubes'') start to lose efficiency if hit rates go
beyond about 300 kHz per tube and, therefore, a new MDT chamber type with half the tube 
diameter is proposed, leading to 7 times higher rate capability. 

Both new chamber systems, the sTGCs and the MDTs, can be operated with the same chamber gas
as in the present system. Power requirements for the electronics of both systems will be
higher due to higher channel count. Whether this requires active cooling, as is now used
for the electronics of the CSC chambers in the region $\eta$ $>$ 2 is under study. Aim is
to have services arranged in such a way as to be able to use the existing service
infrastructure like e.g. the cable Schlepps.

%% ----- 5.1.1-----------------------------------------------------------
%%
\subsubsection{Upgrade of the TGC System}
\label{New_TGC}
%%
%% ----------------------------------------------------------------------

The basis of the proposed concept is to have as many as possible accurate measurements in
the trigger layers, to allow to discard a high percentage of measurements where $\delta$-rays,  
additional $\gamma'$s or neutrons are present and still obtain a position
measurement from each of the two packages with a precision of 100$\mu$m.  To obtain such a
precision using the 3.5mm pitch strips present in each gas-gap, while keeping simple
electronics, a charge extrapolation method using Time-Over-Threshold as an approximated
charge measurement has been developed. The achieved performance using this method for
large size sTGCs is discussed in detail in~\ref{Detector_technology}. 

Achieving this high accuracy for the bending coordinate in each of the gas gaps allows the TGCs  to
provide a very good complement to the MDT measurements (of similar precision) and reduce the 
number of tubes. In particular, in the high background rate region of the Small Wheel, having a 
total of 16 potential measurement points, of which up to 30\% may have to be discarded, ensure 
that an accurate particle vector will be provided in front of the End-Cap Toroidal field, even  if
some of the layers become non-operational during the exploitation of ATLAS at SLHC. The  overall
signal processing to produce the trigger is schematically  shown in figure~\ref{appendix:TGC1} of
the appendix (\ref{appendix:Latency}).  A demonstrator to study the signal path, logics and latency
in detail has recently been completed.

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
\vspace{-2 pt}

%% \includegraphics[width=16cm]{Figures/TGC1}

\includegraphics[width=0.65\linewidth,clip]{Figures/TGC1}

\caption{Schematics of the internal layout of a 4-layer TGC package, 
showing the pad-strip and wire readout for each gas-gap.}
\label{TGC1}
\end{figure}
%%%%%%%%%%%%%%%

By making a coincidence among the pads in successive layers (within a 1 $\deg$ aperture), one
is able to reduce substantially the number of channels that have to be readout for a fast
trigger decision. Furthermore, the pad-coincidence provide a unique 3-D address, where a
corresponding angle and displacement comparison can be made with the trigger input of the
normal muon end-cap trigger originated in the ATLAS big wheels. 

To simplify the logic of what channels to read for trigger purposes, one might loose some of 
the potential muon candidates, which can be recuperated by the measurements of the MDT (smaller 
number of channels). The MDTs provide, furthermore an accurate alignment that can be used to 
align with tracks the exact TGC positions.

 %% ----- 5.1.2-----------------------------------------------------------
%%
\subsubsection{Upgrade of the MDT System}
\label{New_MDT}
%%
%% ----------------------------------------------------------------------

As mentioned above, the large majority of hits in the Monitored Drift Tubes (MDT) are caused by 
Compton scattering of $\gamma'$s which, in turn, come from neutron capture in the material of  the
chambers and adjacent support material. The resulting hit densities strongly increase  towards the
inside of the Small Whell, while, on the other hand, the length of the tubes in the  trapezoidal
geometry of the SW decreases proportional to the distance from the beam line (r).  As hit densities,
however, are increasing considerably faster than with 1/r, the highest tube hit  rates are at the
innermost radii of the chambers. 

% --- Fig.  5 ---------------------------------------------------------------------
%
\begin{figure}[!htb]
\begin{center}
\includegraphics[width=0.35\linewidth,clip]{Figures/Large_small_tubes}

\caption
{The tracking quality in 30 mm and 15 mm drift tubes in a region of high n/$\gamma$
background. The occupancies from background hits (red dots) are 50\% in
the 30 mm tubes but only 7\% in the 15 mm tubes due to shorter drift time and smaller 
area.
}

\label{Large_small_tubes}

\end{center}
\end{figure}


Table~\ref{NSW_efficiencies} presents tube hit rates, occupancies and efficiencies for the 
innermost tubes of the 4 chambers in a Large Sector of the NSW. Chamber EIL0 covers the area 
now taken by the CSC. EIL3 is the small rectangular extension at the outside of the sector. 
The numbers in col. 4 are based on hit rate measurements at a luminosity of $9\cdot10^{32}$ 
cm$^{-2}$s$^{-1}$, scaled up linearly to $5\cdot10^{34}$ cm$^{-2}$s$^{-1}$. Rates in the CSC 
region (1 m$<$r$<$2 m) are extrapolated from larger radii, using an exponential 
law~\cite{ref_Conti}. In addition, a factor of 1,7 has been applied to increase rates to the 9 
kHz/cm$^2$ specification of this Technical Proposal. This could also be considered an 
additional safety factor in case the real rates will increase faster than predicted by the 
extrapolation. The efficiencies are calculated from exp(-$\tau\cdot$f) $\approx$ 
1-$\tau\cdot$f, where $\tau$ is the dead time after a hit (we use the maximum drift time of 200 
ns) and f the hit rate per tube.

{
\begin{table}[ht!]   %%% Table:  1
\centering
\begin{tabular}{|l|c||c|c|c|c|c|c|c|} \hline
%% \small  -- doesn't work
  Chamber& Tube  & Location & Tube    & Expected      & Count rate & Occup-   & Single-tube  & Track segment \\
%\hline
  type   & layers& in r    & length  & hit rate      & per tube      & ancy     & efficiency & efficiency \\
\hline
         &       & {\it cm}& {\it cm}& {\it hHz/cm$^{2}$}& {\it kHz}& {\it \%} & {\it \%} & {\it \%} \\
\hline
  EIL0 &2$\times$6& 93  &   56 & 9000  &  756  & 15,1 & 86,0 & 99.9 \\
  EIL1 &2$\times$4&208  &  125 & 3309  &  620  & 12,4 & 88,3 & 99.9 \\
  EIL2 &2$\times$4&318  &  179 & 1271  &  341  &  6,8 & 93,4 & 99.9 \\ \hline
  \end{tabular}
\caption{\small Expected hit rates and efficiencies for 15 mm tubes in a large sector of the NSW.
         In col.~3 the radial location of the innermost tube of each chamber and in
         cols.~4 to 8 the tube lengths, the expected hit rates, the counting rates per tube, the occupancies and 
         the $3\sigma$ single-tube
         and the track segment reconstruction efficiencies of a chamber are given for the tubes with highest counting rate 
         at a luminosity of 5 $\times$ nominal (times a
         factor of 1,7, see text).}
  \label{NSW_efficiencies}
\end{table}
}


Reduction of the outer drift-tube diameter from 30~mm to 15~mm,  while keeping the other parameters
unchanged (gas composition and gain, sense wire  diameter and tube wall thickness, see
Table~\ref{Sml_MDT_parameters}) leads to a significant improvement of the rate capability of the
drift-tube chambers, 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. This improvement is
illustrated in Fig.~\ref{Large_small_tubes}. 

Being able to use MDT at very high rates allows to preserve the main advantages of the drift tube 
concept:

\begin{enumerate}

\item[(a)] High pattern recognition and tracking efficiency, even at high backround rates. 

\item[(b)] High position and angular resolution, independent of the angle of incidence onto the
chamber plane. This property of the {\it{circular}} drift geometry of chambers made from tubes is a
decisive advantage over drift chambers with {\it{rectangular}} drift geometry.

\item[(c)] Operational independence of the drift tubes, where a malfunction of any individual tube
can only generate a negligible inefficiency.

\item[(d)] Modularity and mechanical robustness of chamber construction.

\item[(e)] Direct connection of the high intrinsic mechanical precision of the chambers 
to the global optical alignment system.   

\end{enumerate}

% --- Fig.  1 ----------------------------------------------------------------
%
\begin{figure}[!htb] 
\centering 
\includegraphics[width=0.65\textwidth]{Figures/segmenteff} 
%\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}

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
(Table~\ref{Sml_MDT_parameters}). Under these operating conditions, the maximum drift time is
reduced by a factor of 3.5 from about 700~ns to 200~ns~\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 thus reduced by a factor of 7. At a background rate of 9 kHz/cm$^2$, the
expected maximum at the innermost radius of the Small Wheels at 5 times the LHC design luminosity,
the highest occupancy is 15~\%, corresponding to a counting rate of 750~kHz (cf.
Table~\ref{NSW_efficiencies}). The 2$\times$6 drift-tube layers at radii $R<2$~m in the Small Wheels
and 2$\times$4 layers at larger radii provide very robust tracking with track segment
reconstruction efficiencies above 99~\% up to the  highest background rates
(Fig.~\ref{fig_segmenteff})~\cite{ref_bittner2}. In this configuration, the point and angular
resolution of the drift tube chambers will be uniformly $45~\mu$m and 0.5~mrad, respectively,
compared to $40~\mu$m and 0.45~mrad without background irradiation.

%   Table 1b  --- Small tubes
%
\begin{table}[!htb]

  \begin{center}
%   
    \begin{tabular}{|l|c|} \hline %{@{}llc@{}}

     \textbf{Parameter}        &      \textbf{Design value}   \\  \hline

    Tube material              &      Aluminum                \\ 
    Outer tube diameter        &      15.000~mm               \\
    Tube wall thickness        &      0.4 mm                  \\ 
    Wire material              &      Gold-plated W/Re (97/3) \\
    Wire diameter              &      50~$\mu$m               \\
    Wire tension               &      $300\pm 15$~g           \\ 
    Gas mixture                &      Ar/CO$_2$/H$_2$O (93/7/$\leq 1000$~ppm) \\
    Gas pressure               &      3 bar (absolute)        \\
    Gas gain                   &      2 x $10^4$              \\
    Wire potential~~~~~~~~~~~~~&      2730 V                  \\
    Maximum drift time         &      $\sim$ 200 ns           \\
    Average resolution per tube&      $\sim$ 100 $\mu$m       \\
    \hline

    \end{tabular}
   \caption{The operating parameters for small-diameter MDTs.\label{Sml_MDT_parameters}}
  \end{center}
%
\end{table}

The design of the mechanical structures and of the alignment system follows as closely as possible the one for
the present Small Wheel architecture. Like in the case of the TGCs, faster readout electronics 
will have to be installed (see section~\ref{chapter:MDT_electronics}).

%% =======================  section 5.2  ==============================
%%
% ==== 5.2 ============================================================
%%%
\subsection{Detector technology and layout}
\label{Detector_technology}
%%%
%%% =====================================================================

\if{0}

{\it
    Description of detector concept (a detailed description of the
    working principle of each detector should be given as an
    appendix).\\
    If more than 1 technology is used motivate the decision and detail
    how they work together (e.g.\ can services be shared, is the
    information of the trigger chambers used in the precision
    chambers, how are combined chambers assembled etc.)
	
    List of all operating parameters\\
    Detailed layout: acceptance, description of chamber overlap
    and dead areas, drawings\\ 
    Tables with chambers sizes, number of channels\\
    Internal alignment scheme (the overall common endcap alignment scheme
    is described in the previous chapter)\\
    Calculations about mechanical stability and expected
    deformations due to gravity, temperature changes, (magnetic field) etc.\\
    Requirements for mount points\\
    Details of service points and other positions where access is needed\\
    Concept for chamber replacement (what needs to be dismounted etc.)
}

\fi


%% ----- 5.2.1-----------------------------------------------------------
%%
\subsubsection{Detector technology and layout of the sTGC chambers}
\label{TGC_technology}
%%
%% ----------------------------------------------------------------------

%%%%%%%%%%%%%%

\begin{figure}[!htb]

   \centering
%% \vspace{-2 pt}

\includegraphics[width=0.65\linewidth,clip]{Figures/TGC_details_BB}

\caption{Construction of a TGC chamber.}

\label{TGC_details}

\end{figure}

%%%%%%%%%%%%%%%

The structure of the new sTGC chambers is given in Fig.~\ref{TGC_details}, while the  schematic
layout of the proposed detection system is shown in Fig.~\ref{TGC3}. It should be consider as one
of the possible arrangements of identical small-large sectors. The 8-layer small tube MDTs are
sandwiched between two sTGC packages, each 5~cm thick, containing 4 gas-gap each. Each gas-gap
provides pad-strip and wire readout. Since the trigger chambers, unlike the MDT, do not need an
overlap region for relative alignment, that part of the MDT surface is used to implement the
alignment connection (purple boxes) to the projective alignment bars. The internal structure of
each sTGC package is the same as that shown in Fig. ~\ref{TGC_MDT_package}. The innermost radial
sTGC package are subdivided into two stair-like packages (to provide full coverage in 3 out of 4
layers), in order to keep short wires in the high background region. The readout of the wires is
done as an OR of the segments, flagged by the corresponding pad coincidence. The preliminary
dimensions of each sTGC package can be seen in  Fig.~\ref{TGC3}. This corresponds to a total 192
packages, each of them containing 4 gas-gaps, corresponding to a total of 768 gas-gaps.

The corresponding number of electronics channels is given in Table~\ref{tab:TGC1},
where the pad surfaces increases from 7.5$\times$7.5cm$^2$ to 20$\times$20cm$^2$ 
at the outermost
radial layers.  The surface of the pads are determined by the corresponding
background rates. The pads would be shifted every two layers (Fig.~\ref{TGC4})
to reduce the size of the corresponding Region-of-Interest (ROI), from which the
strips (wires) will be digitized and used for trigger.

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
%%  \vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC3.pdf}
\includegraphics[width=0.65\linewidth,clip]{Figures/TGC3}

\caption{Schematic of the detector layout for the case of equal  small-large sectors.
Insert A shows the stairs-type arrangement for the innermost radial sTGC package. The
purple boxes represent the alignment elements for the MDT detectors.}

\label{TGC3}
\end{figure}
%%%%%%%%%%%%%%%

The internal alignment achieved by the production of the strip patterns in the sTGC
gas gap is precise to within less than 50~$\mu$m, however the transfer of this
precision from gap-to-gap has only been achieved to within better than 150$~\mu$m. For
this reason, the final precision alignment between gaps will be done using tracks
from the sandwiched MDT. This requires very few tracks, since the internal strip
pattern within a gap is extremely precise.

The mechanical stability of a TGC package is very good for vertically mounted
detectors, based on the ATLAS experience. The use of composite materials allows for
very small sensitivity to temperatures (in particular within the range of the
temperatures in the small wheel), as well as no sensitivity to the magnetic field in
the Small Wheel region. To be able to achieve this lack of sensitivity, the mounting
needs to be kinematic (one fix point, one sliding point and one plane), of the same
type as presently used in ATLAS.

The processing of the sTGC signals is considerably more complex than for the existing TGCs. An
accurate determination of the track position in the two chamber packages has to be done, and the
resulting vector has to be extrapolated to the corresponding trigger sector in the Big Wheel. In
this process the overall latency must stay below the maximum allowed in the present ATLAS readout
scenarion and in phase I. Details of the proposed readout scheme are given in~\ref{chapter:TGC}.

%%%%%%%%%%%%%%

\begin{figure}[!htb]

   \centering
%% \vspace{-2 pt}

\includegraphics[width=0.55\linewidth,clip]{Figures/TGC4}

\caption{Schematics of Pad and strips arrangement (only 2 layers shown).}

\label{TGC4}

\end{figure}

%%%%%%%%%%%%%%%

Each package will be connected to 4 HV lines, 3 LV lines and the corresponding
optical fibers to transmit the trigger/data information. The HV lines will be
connected to one corner of the chamber, while the LV and fibers will be connected to
a trigger box, where the logic will be installed. In case of a major failure, the
full TGC-MDT-TGC package will be replaced. This means that 2 such spare packages
for each of the station types need to be constructed.

 \begin{table}[h]
    \centering
    \begin{tabular}{lr}
      Component & Number of channels\\
      \hline
      Strips / side & 135,000 \\
      Pads / side & 30,000 \\
      Wires / side & 27,500 \\
      For two sides detector & 385,000 \\
      \hline
    \end{tabular}

	\caption{Channel distribution for the 3 readouts for the 192 packages containing
     768 gas gaps.}

    \label{tab:TGC1}
  \end{table}

\newpage

%% ----- 5.2.2-----------------------------------------------------------
%%
\subsubsection{Detector technology and layout of the sMDT chambers}
\label{MDT_technology}
%%
%% ----------------------------------------------------------------------


\begin{figure}[!htb]

   \centering
%% \vspace{-2 pt}

\includegraphics[width=0.65\linewidth,clip]{}

\caption{Structure of the NSW for the sMDT-sTGC system.}

\label{fig_layout_structure}

\end{figure}


\begin{figure}[!htb]

   \centering
%% \vspace{-2 pt}

\includegraphics[width=1.0\linewidth,clip]}

\caption{Layout of the sMDT and sTGC chambers in the large sectors of the NSW.}

\label{fig_layout_large}

\end{figure}


\begin{figure}[!htb]

   \centering
%% \vspace{-2 pt}

\includegraphics[width=1.0\linewidth,clip]{}

\caption{Layout of the sMDT and sTGC chambers in the small sectors of the NSW.}

\label{TGC_details}

\end{figure}


The layout of the sMDT chambers for the NSWs follows exactly the present layout 
of the MDT chambers in the Small Wheels with fully projective small and large chamber towers 
in combination with the EM and EO layers (see Figs.~\ref{fig_layout_structure}-\ref{fig_layout_small}). 
The chamber parameters are summarized in Table~\ref{sMDT_layout_parameters}. 
There are only three radial chamber segments in each sector. 
The sTGC chambers are precisely positioned and kinematically mounted on the outer tube layers of the sMDT chambers on
both sides. The integrated sMDT-sTGC chamber packages are connected by light rays to the global optical alignment system
and with the neighbouring chambers via sensors mounted precisely on the outer tube layers of the inner multilayers of the 
sMDT chambers while the in-plane alignment system is integrated into the sMDT spacer structure which also carries
the common chamber supports. The well understood alignment system implementation guarantees precise absolute chamber 
alignment from the beginning which is essential for the performance of the muon spectrometer. 

{
\begin{table}[ht!]   %%% Table:  3
\centering
\begin{tabular}{|l|c||c|c|} \hline

sMDT chambers                & EIL0     & EIL1     & EIL2     & EIS0    & EIS1    & EIS2    \\
Number of chambers           & 2 x 8    & 2 x 8    & 2 x 8    & 2 x 8   & 2 x 8   & 2 x 8   \\  
Radial extension (mm)        & 1095     & 1095     & 1457     & 1215    & 1276    & 1095    \\
Minimum chamber width (mm)   & 740      & 1428     & 1968     & 678     & 1056    & 1434    \\     
Maximum chamber width (mm)   & 1380     & 1878     & 2418     & 1002    & 1380    & 1704    \\
Minimum tube length (mm)     & 560      & 1248     & 1788     & 498     & 876     & 1254    \\
Maximum tube length (mm)     & 1200     & 1698     & 2238     & 822     & 1200    & 1524    \\
Spacer height (mm)           & 68       & 120      & 120      & 68      & 120     & 120     \\
Thickness in z, tubes (mm)   & 228      & 228      & 228      & 228     & 228     & 228     \\ 
Thickness in z, chamber (mm) & 300      & 300      & 300      & 300     & 300     & 300     \\
Weight/chamber (kg)          & 150      & 130      & 180      & 140     & 120     & 130     \\ 
Number of tube layers        & 2 x 6    & 2 x 4    & 2 x 4    & 2 x 6   & 2 x 4   & 2 x 4   \\ 
Number of tubes/layer        & 72       & 72       & 96       & 80      & 84      & 72      \\
Number of tubes/chamber      & 864      & 576      & 768      & 960     & 672     & 576     \\
Total number of tubes        & 13824    & 9216     & 12288    & 15360   & 10752   & 9216    \\ 
Gas volume/chamber (l)       & 122      & 162      & 219      & 102     & 110     & 127     \\     

\end{tabular}
\caption{\small Parameters of the sMDT chambers in the NSWs. The total number of drift tubes is 70656
and the total gas volume in each NSW 6750 l.}  

  \label{sMDT_layout_parameters}
\end{table}
}

The design of the muon drift-tube chambers with 15~mm diameter tubes for the new Small Wheels
follows as closely as possible the current ATLAS MDT chamber design in the endcap region of the muon
spectrometer (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 
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.

% --- Fig.  5 ----------------------------------------------------------------
%
\begin{figure}[!htb] 
\begin{center}
\includegraphics[width=1.0\linewidth,clip]{Figures/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}

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_design}) and to the  24-channel readout and high-voltage interface boards 
(''hedgehog cards'', Figs.~\ref{fig_hedgehogs}). 

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 O-rings. The brass insert of the endplug holds the spiral-shaped wire locator (''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,
facilitating the production of the tubes and improving the accuracy of chamber construction. The
two most critical problems for the ATLAS MDT drift tube production, cracking of the
Noryl\textsuperscript{\textregistered} plastic insulator and precision machining of the injection
moulded endplugs, have been eliminated by the new design.      

% --- Fig.  6 ----------------------------------------------------------------
%
\begin{figure}[!htb] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{Figures/smdt_tube_design_3D} 
%
\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}


The insulators of the endplugs consist of Crastin\textsuperscript{\textregistered}, a PBT based
thermoplastic with 30~\% glass fiber admixture to improve mechanical stability  which shows little
shrinkage and outgassing and, unlike Noryl\textsuperscript{\textregistered} (material in the
present MDT endplugs), high stability against cracking under stress. The material is of similar
chemical composition as Pocan\textsuperscript{\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\textsuperscript{\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.

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

%%   Table 4
%
\begin{table}[!htb]

 \begin{center}
 
    \begin{tabular}{|c|c|c|c|c|c|c|c|} \hline %{@{}llc@{}}
Chamb.& Tubes$\times$      & Tubes/    & Tubes & Mezz.cards/ & Mezz.cards & CSMs/    & CSMs  \\ 
type  & layers$\times$MLs  & chamber   & total & chamber     & total      & chamber  & total \\   
\hline 
\hline
  EIL0 &  $72\times6\times 2$ &  864 & 13824 &  36  &  576 &  2  & 32  \\
  EIL1 &  $72\times4\times 2$ &  576 &  9216 &  24  &  384 &  1  & 16  \\
  EIL2 &  $96\times4\times 2$ &  768 & 12288 &  32  &  512 &  2  & 32  \\
\hline
  EIS0 &  $80\times6\times 2$ &  960 & 15360 &  40  &  640 &  2  & 32  \\
  EIS1 &  $84\times4\times 2$ &  672 & 10752 &  28  &  448 &  2  & 32  \\
  EIS2 &  $72\times4\times 2$ &  576 &  9216 &  24  &  384 &  1  & 16  \\

\hline \hline

	  \multicolumn{3}{|l|}{Total side A + C:}
	 &\multicolumn{1}{c|}{70656}&\multicolumn{2}{l|}{}
	 &\multicolumn{1}{c|}{2944} &\multicolumn{1}{l|}{}
	 &\multicolumn{1}{c|}{160}\\
\hline

  \end{tabular}
 \end{center}
 
\caption{Electronics Channels (drift tubes) and readout boards in the NSW. 
Each mezzanine card serves 24 drift tubes, matching the modularity of the chambers with 
6 and with 4 tube layers per multilayer, and each CSM 24 mezzanine cards. MODIFIED-HK \label{NSW_MDT_modularity}} 

\end{table} 


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 
%%(Fig.~\ref{fig_groundpin})  DID'T find this FIGURE !!
during chamber assembly  to connect the tube walls to the ground of the hedgehog cards. This scheme
allows for the high-voltage-free 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.  
The modularity of the sMDT chambers in the NSW is given in Table~\ref{NSW_MDT_modularity}. 




% --- Fig.  7 ---------------------------------------------------------------- %

\begin{figure}[!htb]  \begin{center}

\includegraphics[width=0.45\linewidth,clip]{Figures/gasmanifold_photo}
\hspace{5mm}
\includegraphics[width=0.25\linewidth,clip]{Figures/Small_tubes_w_elx} 
 
\caption{Left: The gas manifold for the Small tube chambers. The gas distribution is integrated
into the endplug design. No gas bar with separate connections to each tube is needed. Right: The 
HV-coupling capacitors, housed in small plastic cylinders, are directly
plugged onto the signal pins. In the next layer: signal distribution (''hedgehog'') and frontend
electronics (''mezzanine card'').}

\label{fig_gasmanifold_design}   

\end{center} 

\end{figure}

\if{0}

%% Fig. 5  -------------------------------------------------------------------------

\begin{figure}[!htb] 

\begin{center}

\includegraphics[width=0.35\linewidth,clip]{Figures/Small_tubes_w_elx}

\caption{ Space requirements at the tube ends on the Small tube chambers necessitate a separation
of the HV-coupling capacitors from the signal routing card (''hedgehog card''). The HV capacitor is
mounted in a small plastic cylinder.}


\label{Small_tubes_w_elx}
\end{center}
\end{figure}

\fi

The modular gas distribution system consists of injection moulded plastic tubes made of
Pocan\textsuperscript{\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_design}) 




%% =======================  section 5.3  ==============================

% ==== 5.3 ============================================================
%%%
\subsection{Performance}
\label{Performance}
%%%
%%% ===================================================================



{\it
  Summary of chamber performance, details in appendix of technology. 

  Spatial and angular resolution as functions of rate and angle of incidence\\
  Time resolution\\
  Efficiency (single measurement and segment)\\
  Double track resolution\\
  Rejection of fake and background tracks\\
}

\fi

%% ----- 5.3.1-----------------------------------------------------------
%%
\subsubsection{Performance of the sTGC trigger chambers}
\label{TGC_performance}
%%
%% ----------------------------------------------------------------------

\if{0}

The most important issue for a detector that needs to operate at high rate during a period of  more
than 10 years, is its aging characteristics. For this reason, a series of small TGCs (10 $\times$
10~cm$^2$) detectors were constructed  and exposed to $\gamma$ irradiation for various periods of
time. No deterioration was observed in any of these prototypes. The one with the longest exposure
was opened after a total irradiation of 6~Cb/cm of wire (i.e. 33~Cb/cm$^2$).  Fig.~\ref{TGC5} shows
the rate and current behavior of the detector during the irradiation. No noticeable degradation is
observed after the MIP dose equivalent to  2,000 days of operation at the hottest location in the
ATLAS Small Wheel.

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
%%\vspace{-2 pt}
%%%   \includegraphics[width=8cm]{Figures/TGC5.pdf}
%%%   \includegraphics[width=8cm]{Figures/TGC5b.pdf}
\includegraphics[width=7cm]{Figures/TGC5}
\hspace{5mm}
\includegraphics[width=7cm]{Figures/TGC5b}
\caption{Counting rate and current as a function of the integrated charged in the detector.}
\label{TGC5}
\end{figure}
%%%%%%%%%%%%%%%

The irradiated detector was opened to see the effect of the irradiation. Fig.~\ref{TGC6} shows a
microscope picture of a wire in the irradiated region, as well as in the non-irradiated region. The
deposits in the irradiated region were analyzed, the deposits being mainly Carbon and Oxygen, as
expected from the gas composition.

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
\vspace{-2 pt}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC6a}
\hspace{5mm}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC6b}
%\includegraphics[width=4cm]{Figures/TGC6c}
\caption{Deposits observed in the irradiated (left) and non-irradiated region of a wire.}
\label{TGC6}
\end{figure}
%%%%%%%%%%%%%%%

In order to perform the R\&D needed for the use of TGC\'s in the ATLAS Small Wheels, six large
(ranging from 120$\times$100~cm$^2$ to 70$\times$40cm$^2$) packages of TGCs, each one containing
four gas-gaps, each gap with pads-strips and wire readout, have been constructed. These detectors
were irradiated in various facilities to evaluate their behavior and position resolution. The time,
position and angular resolutions were determined from measurements performed in the H8 facility at
CERN, while the efficiency for MIP\'s under a high full area irradiation with $\gamma$\'~s was
performed at the SOREQ Nuclear Center (Israel) using Co(60) source, while triggering on cosmic
muons. 

The sensitivity to neutrons was measured at the Demokritos Nuclear Center (Greece).  To simulate
the present proposal, the trigger used at the H8 test-beam was the expected 3-out-of-4 coincidence
of pads. The time resolution of this trigger with respect to a beam-defined scintillator is shown
in units of 0.8~ns in Fig.~\ref{TGC7} (left). It can be seen that the time resolution of the system
is able to provide the bunch crossing identification with high efficiency.

Fig.~\ref{TGC8} shows the individual efficiencies of each of the layers for Pads,
Strips and wires of one such package, using the Time-over-Threshold method. It can be
see that they are all fully efficient at an operating voltage of 2.9~kV, which was
used for the rest of the tests.

The position resolution of each individual gap was measured by using clean muon
tracks in the small tube MDT test detector that was part of the test beam set-up. The
strip multiplicity distribution that provides the charge centroid is shown in
Fig.~\ref{TGC7} (right). 

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
%%\vspace{-2 pt}
%%%   \includegraphics[width=12cm]{Figures/TGC7.pdf}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC7}
\hspace{5mm}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC9_BB}

\caption{Left: Time difference between 3-out-of-4 pad trigger with respect to the beam
scintillator in units of 0.8~ns. The FWHM is 11~ns. 99\% of the distribution is within 25~ns.
Right: Strip multiplicity for a MIP on a typical gas gap.}

\label{TGC7}
\end{figure}
%%%%%%%%%%%%%%%

\vspace{5mm}

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering

%%% \vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC8.pdf}
\includegraphics[width=0.45\linewidth,clip]{Figures/TGC8_BB}
\caption{Plateau curve for each of the gaps used in the test, using the T-O-T method.}
\label{TGC8}
\end{figure}
%%%%%%%%%%%%%%%

The TGC position resolution was calculated from a fit to the difference between the measured
position in a given layer and the position extrapolated from a fit to the other three layers, as
shown in Table~\ref{tab:resolution} (before and after the correction for inter-gap misalignment). A
typical distribution for one layer is shown in Fig.~\ref{TGC10}. It can be concluded that the
position resolution achieved with the TGC layers fulfills the requirements for the NSW and provides
a valuable complement to the position measurements of the MDT.

The position resolution as a function of incident angle has been measured in the past using a
regular ADC to determine the charge in each individual strip. This measurement will be repeated in
the August 2011 test beam using Time-over-Threshold. The dependence of the single
layer resolution as a function of angle is shown in Fig.~\ref{TGC11}.

%%%%%%%%%%%%%%%%
 \begin{table}[!htb]
    \centering
    \begin{tabular}{crcc}
     Layer & $\sigma_{corr}(\sigma_{nocorr})\pm\Delta\sigma$[$\mu$m] \\
     \hline
     1  & 66.2 (120.9)  $\pm$ 1.2 \\
     2  & 66.7 (79.8)   $\pm$ 1.1 \\
     3  & 63.6 (76.0)   $\pm$ 1.0 \\
     4  & 63.8 (116.49) $\pm$ 1.0 \\

      \hline
    \end{tabular}

	\caption{Position resolution in the 4 layers after (before) correcting
             for individual layer position.}

    \label{tab:resolution}
  \end{table}
%%%%%%%%%%%%%%%%%%%%%%%%

The angular resolution of a single sTGC package was obtained by comparing the angle
between one package of sTGC\'s (4 gaps) with the one measured with the Small Tube MDT test
chamber. The difference has a resolution of 3~mrad. Having such a resolution with a
single sTGC package (5~cm thick) implies that with two packages separated by 300~mm, the
angular resolution of the system would be better than 0.3~mrad at the Level-1 trigger.

The measured single layer efficiencies for MIP\'s in a $\gamma$ background has
been measured by triggering with cosmics under a high irradiation flux. The results
are shown in Fig.~\ref{TGC12}, left. Similarly, this was done with 5.5~MeV neutrons. The
corresponding single layer efficiency is shown in Fig.~\ref{TGC12}, right.

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
%% \vspace{-2 pt}
%%%   \includegraphics[width=12cm]{Figures/TGC10.pdf}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC10}
\caption{Typical residual for one of the layers with respect to the fitted line.}
\label{TGC10}
\end{figure}
%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
%%% \vspace{-2 pt}
%%%   \includegraphics[width=12cm]{Figures/TGC11.pdf}
\includegraphics[width=0.5\linewidth,clip]{Figures/TGC11_CC}
\caption{Resolution vs. incident angle as measured with muons in H8.}
\label{TGC11}
\end{figure}
%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
%%% \vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC12.pdf}
\includegraphics[width=0.45\linewidth,clip]{Figures/TGC12}
\hspace{5mm}
\includegraphics[width=0.45\linewidth,clip]{Figures/TGC13}
\caption{Left: Single layer efficiency in kHz/cm$^2$ on a large 
chamber under uniform $\gamma$-irradiation (strip length= 1.2~m).
         Right: Single layer efficiency for MIPs under 5.5~MeV neutron irradiation.}
\label{TGC12}
\end{figure}
%%%%%%%%%%%%%%%

Finally, by using the Time-over-Threshold method, one can reject single gas-gaps measurements,
polluted by a $\delta$-ray, a $\gamma$ or a neutron-conversion, by demanding less than six
strips in a cluster and a cut on the ToT. This is illustrated in Fig.~\ref{TGC14}, where one can
see the passage of a muon through the four gas-gaps, (a) without contamination, (b) with a
$\delta$-ray in layer one and (c) with a neutron impinging in layer three.

%%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
\includegraphics[width=4cm]{Figures/TGC14a}
\hspace{3mm}
\includegraphics[width=4cm]{Figures/TGC14b}
\hspace{3mm}
\includegraphics[width=4cm]{Figures/TGC14c}

\caption{Event display of a) a clean muon track, b) a muon track  with a $\delta$-ray in the first
gas-gaps and c) a neutron interacting in the third gas gap. The unit of the horizontal axis is
25~ns/division.}

\label{TGC14}
\end{figure}
%%%%%%%%%%%%%%%


%% ----- 5.3.2-----------------------------------------------------------
%%
\subsubsection{Performance of the sMDT tracking chambers}
\label{MDT_performance}
%%
%% ----------------------------------------------------------------------

\if{0}

The outer region of the ATLAS detector, where the muon chambers are located, receives 
high rates of low-energy neutrons, mainly due to shower leakage from calorimeters and 
shielding structures in the high-$\eta$ region.  At the nominal luminosity, gammas from 
neutron capture and related conversion electrons are expected to generate hit rates in 
the range 50--300 kHz in each MDT tube. A conversion electron may create an inefficiency 
if the signal arrives {\it before} the muon signal. The muon detection efficiency thus 
becomes exp(-$\tau\times$f) $\approx$ 1-$\tau\times$f, where $\tau$ is the average drift 
time in the MDT tubes and f the hit rate due to gamma conversions.

At high rates of n/$\gamma$ background, the efficiency may be further reduced by a 
decrease of the gas amplification due to space charge from slowly drifting positive ions 
in the tubes, while the {\it fluctuations} of the space charge tend to degrade the 
spatial resolution by up to about 20 \% at the highest rates.

The effects of gamma conversions in the MDT tubes have been studied in detail using a 
muon beam in the presence of intense $\gamma$-irradiation of up to about 1~kHz/cm$^2$ 
500 kHz/tube delivered by the Gamma Irradiation Facility at CERN 
(GIF)~\cite{mdt_highrates}. The results 
allow to define the baseline of a chamber design with much 
improved tracking capability: MDT drift tubes with only half the tube diameter offer a 
reduction of the drift time by a factor 3.5, due to the non-linear relation between track 
distance from the central wire and drift time (r-t relation) and in addition by a factor 
2 from the exposed area, thus yielding a factor 7 in the reduction of the hit rate due to 
n/$\gamma$ background. Moreover, up to two times more tube layers can in principle be 
accommodated in the available space, leading to improved track finding efficiency and 
position resolution (see Fig.~\ref{Large_small_tubes}). 

In order to verify the performance of the 15 mm diameter (''small'') drift tube chambers, 
tests of the full-scale prototype chamber with 2 x 8 layers of 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_test}, left) and, using cosmic ray muon tracks, at the
GIF in the presence of adjustable $\gamma$ background rates of up to 8.5~kHz/cm$^2$ and 
1200 kHz/tube in 1~m long tubes from a 500~GBq $^137$Cs source
(see Fig.~\ref{fig_prototype_test}, right)~\cite{mdt_slhc}. 
The maximum $\gamma$ flux corresponds to a background hit rate of 17 kHz/cm$^2$
in 0.5~m long tubes as they are in the highest background region at the inner radius of the NSWs.   
The standard ATLAS MDT readout electronics was used
with the adjustable deadtime set to the minimum value of 200~ns.  

A tube along the track is called "efficient" if the hit is detected 
inside a 3$\sigma$ road of the track reconstructed in the other tube layers of the chamber
serving as reference. The drift tube resolution is determined by an iterative method 
described in~\cite{ref_resolution_measurement}.  

Fig.~\ref{Large_small_efficiency} shows the 
efficiency of small and large tubes vs. hit rate from gamma conversions. As expected, 
small tubes provide a much better performance at high background rates.  The efficiency 
at rate zero deviates from 100 \% due to tracks passing across or close to the tube walls 
and due to $\delta$-electrons shifting the hit position outside the 3$\sigma$ acceptance 
road. The average position resolution in the small tubes was about 120 $\mu$m which, due to 
strong suppression of space charge effects, shows little dependence on the 
background rate.


% --- Fig. 14 ----------------------------------------------------------------
%
\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.3\linewidth,clip]{Figures/H8_setup} 
\hspace{5mm}
\includegraphics[width=0.4\linewidth,clip]{Figures/GIF_drawing} 
%
\caption{The test setup in the H8 test beam (left) and in the GIF (right).}
\label{fig_prototype_test}
% 
\end{center}
\end{figure}

As a measure for evaluating the tracking efficiency, we define the ''3$\sigma$-efficiency'' as the
probability of detecting a tube hit within a distance of 3 times the drift tube resolution $\sigma$
from the muon track, the muon track being defined 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. 15 ----------------------------------------------------------------
%
\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.5\linewidth,clip]{Figures/efficiency_vs_rate} 
%
\caption{Efficiency versus background hit rate as measured at the GIF.}
\label{fig_efficiency_GIF11}
% 
\end{center}
\end{figure}

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 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.  

% --- Fig. 16 ----------------------------------------------------------------
%
\begin{figure}[ht] 
\begin{center}
\includegraphics[width=0.5\linewidth,clip]{Figures/resolution_measured_vs_simulated} 
%
\caption{Spatial resolution of large and small tubes in the H8 test beam together with simulation
results.}
\label{fig_resolution_H8_rdep}
% 
\end{center}
\end{figure}
%

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. 



%% =======================  section 5.4  ==============================

% ==== 5.4 ============================================================
%%%
\subsection{L1 trigger and electronics}
\label{chapter:TGC}
%%%
%%% ===================================================================

\if{0}

{\it
  How the L1 signal are produced, starting from the detector signal
  till the formation of SL input.\\
  Latency (calculation, measurement with demonstrator)\\
  Compatibility with Phase II upgrade    RR to supply text !!! \\
}

{\it Comment: Here we will show a diagram showing the two packages with the MDT in
between. }

\fi


The strip signals in the sTGC of the NSW are used to extrapolate tracks to the Big Wheel where the
Sector Logic corroborates it with the existing 2-out-of-3 and 3-out-of-4 coincidences in the Big Wheel
chambers. The fine granularity of the extrapolation results in high efficiency for trigger rejection.
The logic added to accomplish this must not cause the latency to the Sector Logic to exceed 1.05 $\mu$s.
A detailed breakdown of the sTGC trigger latency is given in the appendix (\ref{appendix:Latency}).

The sub-sector of the extrapolated muon is found as follows: In the two sTGC packages of the NSW, a
local trigger is made from two sets of 3-out-of-4 coincidences, corresponding to a tower of two times 4
pads. Pads in alternating layers are shifted by half a pad in both dimensions to give a 4 times higher
granularity of logical towers. The triggering tower selects the strips in its region for transmission to
8 centroid finders, one for each layer. This selection leads to a 20-fold reduction in the number of
strips to be processed as well as in the number of centroid finders. The four centroids in each sTGC
package are averaged to give two points in 3-D space, defining a vector which points to a sub-sector in
$\eta$,~$\phi$ of the Big Wheel. The \~60~$\mu m$ together with their distance of 250~mm provides an
angular accuracy of better than 0.3~mrad. A block diagram of the sTGC trigger logic is shown in the
appendix (\ref{appendix:Latency}). A demonstrator has been implemented from commercial components to
verify the simulated behaviour, in particular the resulting latency of the trigger logic. Each pair of
sTGC chamber packages has its dedicated trigger logic. The extrapolation of the track vector to the Big
Wheel can be very fast, using a Look-up-Table (LUT). 

Some details about the centroid finding strategy:

\begin{itemize}

  \item The centroid finder currently calculates the centroid from the charge on five
strips.

  \item The charge on each strip is approximated by the Time-over-Threshold (ToT), which is currently
measured in steps of 1~nsec and with a 8-bit dynamic range. Optimized parameters will depend on the final
ASD characteristics.

  \item Centroids are included in the average only if the number of strips above threshold is below a
  certain maximum value in order to exclude $\delta$-rays and conversions (see~\ref{TGC14}).

  \item To compensate for time differences in signal routing and particle time-of-flight, strip signals
can be delayed in steps of 1 nsec, up to 25 nsec, with groups of 16 strips sharing a delay parameter.

\end{itemize}

\paragraph{{ASD\\}\\}

The baseline is to use BNL\'s VMM Front End chip, now under development, including
both an ASD and readout logic. The planned parameters of this chip (see table) match
the needs of the sTGC detectors. The first prototype chips of the Front End will be
available at the end of 2011. The sTGC version of this ASD has LVDS outputs suitable
for the ToT trigger logic.

\paragraph{{Division into On-chamber and Off-chamber Logic\\}\\}


We seek to minimize the amount of on-chamber electronics, because access and power is limited,
and on-chamber electronics must be radiation tolerant. Therefore, ASIC\'s in radiation
tolerant technology may be preferable over reprogrammable FPGAs. 

The row selector, driven by the pad tower coincidence, reduces the number of channels to be
read out to the off-chamber electronics by a factor of about 20. This selector and the pad
coincidence must clearly be on chamber. The channel delays depend on the channel, so they
must be before the selector. If the Time-over-Threshold (ToT) logic is done on-chamber, the
bandwidth needed between on and off chamber electronics is much reduced, since ~100 bits of
1~ns signal samples are reduced to an 8-bit ToT. To do this reduction, however, the ASIC\'s
must be more complex. On the other hand, if placed off-chamber, more bandwidth, meaning more
links, fibers and probably more serializer power, is needed, while the radiation tolerant
electronics is simpler. The final decision on this optimization will be taken when details
of radiation tolerant ASICs and of serializer speeds and power are better known. The
centroid finding through calculation of the Big Wheel sub-sector will be done in off-chamber
FPGAs.

\if{0}

  %%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
\vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC15.pdf}
\includegraphics[width=0.6\linewidth,clip]{Figures/TGC15}
\caption{The trigger demonstrator board based on a commercial evaluation board.}
\label{TGC15}
\end{figure}
%%%%%%%%%%%%%%%

\fi

 
    
\paragraph{{Latency (calculation, measurement with demonstrator)\\}\\}    

A detailed discussion of the latency expected for the sTGC readout is presented in the appendix
(\ref{appendix:Latency}).

  
\paragraph{{Compatibility with Phase II upgrade\\}\\}
  
 The pipelines and derandomizers will be designed to handle a 500 kHz Level-1 trigger
rate and to accommodate up to 10$\mu$s  Level-1 latency.


\paragraph{{Time schedule for the sTGC electronics development\\}\\}

The schedule for prototyping, development and production of the sTGC 
trigger electronics is given in the appendix~\ref{appendix:TGC_project}, Table~\ref{tab:TGC2}.



%% =======================  section 5.5  ==============================

% ==== 5.5 ============================================================
%%%
\subsection{Readout Electronics of the MDT and integration in DAQ}
\label{chapter:MDT_electronics}
%%%
%%% ===================================================================

\if{0}

{\it
  Detailed description of electronics chain.-- Integration to DAQ (Mention the GBT?).--
  Readout related parameters, e.g. bandwidth requirements, number and granularity of 
  read-out links.-- Compatibility with Phase II upgrade.
}
\fi

The readout of the Small MDT tubes will follow the proven architecture of the present MDT
system. A number of additions and modifications, however, will be necessary in order to
adapt the rate capability of the readout chain to the requirements of the SLHC. For the
mechanical integration of the readout electronics with the Small tube chambers, the
layout of the electronics will have to be adapted to the 4 times higher tube density at
the ends of the Small tube chambers. Finally, the radiation tolerance of all components
will have to comply with the high radiation levels, in particular at the inner border of
the Small Wheel.


%% ----- 5.5.1-----------------------------------------------------------
%%
\subsubsection{Architecture of the present MDT readout}
\label{MDT_architecture}
%%
%% ----------------------------------------------------------------------

In the present readout scheme tube signals are routed via a PCB ("hedgehog card") to a
piggy-back card ("mezzanine card"), containing an Amplifier and Shaper, followed by a
Discriminator with adjustable threshold, all three functions being integrated in a
radiation tolerant ASIC (ASD). The discriminator outputs, in turn, are routed to a TDC,
where each leading and trailing edge signal receives a high-precision time stamp, which
is retained, together with the corresponding channel number, in a large internal buffer
of the TDC ("Level-1 buffer").

When the TDC receives a Level-1 trigger, a subset of the recorded hits, corresponding to
a pre-defined time window, are retained for readout and are forwarded to the data
concentrator of this chamber, the "Chamber Service Module" (CSM). From there, data are
sent to the off-chamber electronics in USA15, the "Readout Driver" (ROD). A CSM can
serve up to 18 mezzanine cards. The operation parameters of the analog frontend (ASD)
and the TDC are controlled by a JTAG string, which is distributed by the Detector
Control System (DCS) to the CSM, which sends it individually to each mezzanine card.
Fig.~\ref{readout_chain} gives the layout of the present system. A detailed presentation
of the MDT readout electronics is given in ~\cite{MDT_ELX_paper}. The ASD and TDC ASICs
are described in ~\cite{ASD_manual} and ~\cite{TDC_manual}.

%% Fig. 3 ---------------------------------------------------------------------------

\begin{figure}[!htb]
%
\begin{center}
%
\includegraphics[width=0.45\linewidth,clip]{Figures/MDT_electronics_scheme_old}
\hspace{5mm}
\includegraphics[width=0.39\linewidth,clip]{Figures/Elx_diagram_sm_tubes}

\caption{Schematic diagram of the present (left) and of the future MDT readout chain (right). The
Chamber Service Module (CSM) collects all data of a given chamber and sends them to the ROD via an
optical link. Up to 18 TDCs can be serviced by a CSM. The 40 MHz clock and the Level-1 trigger are
broadcast by the TTC system to the TDCs.}

\label{readout_chain}

\end{center}

\end{figure}


%% ----- 5.5.2-----------------------------------------------------------
%%
\subsubsection{Architecture of the readout for MDTs with Small tubes}
\label{MDT_architecture_Small_tubes}
%%

For the evolution of this architecture into matching the requirements of the Small tube
readout, a number of problems limiting the performance of the present scheme has to be
overcome. Given the high rate capability of the Small MDT tubes, the bandwidth of the
readout system has to be increased. Due to the limitations of the internal buffering
scheme and processing speed, the present TDC can only handle average tube rates up to
about 300 kHz per tube without losing data, while Small tubes would operate beyond 1 MHz
per tube. Therefore, an improved TDC is an essential requirement for a new readout
design. Another limitation of the readout bandwidth comes from the optical link,
connecting the CSM to the ROD, as the S-link protocol only supports a usable bandwidth
of 1,4 Gbit/s. Fig.~\ref{occ_and_link} shows occupancy and efficiency of Large and Small
tubes as a function of tube hit rate.


\if{0}

%% Fig. 4 ---------------------------------------------------------------------------
%
\begin{figure}[!htb]
%
\begin{center}
%
\includegraphics[width=0.50\linewidth,clip]{Figures/Elx_diagram_sm_tubes}

\caption{Schematic diagram of the chain for the readout of the Small tube chambers. The
higher tube density at the tube ends requires a higher modularity for the readout architecture.}

\label{Elx_diagram_sm_tubes}

\end{center}

\end{figure}

\fi

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

When upgrading the performance of ASICs (ASD, TDC), we have to take into account that
their technologies are no longer supported by industry. The following new components had
therefore to be introduced into the readout system.

%% Fig. 4  -------------------------------------------------------------------------

\begin{figure}[!htb] 

\begin{center}

\includegraphics[width=0.5\linewidth,clip]{Figures/print_summary_15_08_2011}

\caption
{Three performance parameters for the 15 mm MDTs as a function of the tube hit rate. Red
curve: efficiency, assuming a dead time of 200 ns (equal to the maximum drift time) after
each hit. Blue: Saturation of the data link from TDC to CSM. Purple: Saturation of the
link from CSM to ROD. The links have a useful bandwidth of 80 Mbit/s and 1,4 Gbit/s,
respectively. Both have a base load of about 15\% due to the overhead of the transfer
protocol.
}

\label{occ_and_link}
\end{center}
\end{figure}

\begin{enumerate}

  \item[(a)] The ASD is re-designed in the IBM 130 nm technology.
   Most analog parameters of the previous design are preserved. A 4-channel
   prototype, demonstrating the analog parameters, has already been produced and tested 
   and works correctly. The 4-channel version of the complete ASD chip has been delivered 
   in September 2011.   

  \item[(b)] The TDC will be replaced by the HPTDC, designed by the CERN-MIC group
  (~\cite{The_HPTDC}). This 32-channel device has an improved internal buffering
  scheme as well as higher transfer and processing speeds.

  \item[(c)] The CSM collects data from a MDT chamber formats the event and sends
  data, trigger-by-trigger, via an optical link to the ROD in USA15. All logical
  operations are performed by an FPGA, which will need to be upgraded to higher
  radiation tolerance. The interface to DCS, needed for controlling electrical and
  temperature parameters on the chamber will be done via the GBT (see below). The
  adapter box to the CANbus (ELMB) can therefore be dropped, reducing complexity of
  cabling.

  \item[(d)] The link connecting the CSM to the ROD will be replaced by a GigaBit
  Transmitter link (GBT), developed by CERN. This link provides a 3 times higher
  transfer rate (Gbit/s), compared to the S-link~\cite{GBT}.  

\end{enumerate}

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

%%%  (see Figs.~\ref{fig_gasmanifold_design}) 

% --- Fig.  8 ----------------------------------------------------------------
%
\begin{figure}[!htb] 
\begin{center}
\includegraphics[width=0.4\linewidth,clip]{Figures/hv_hedgehog}
\hspace{5mm}
\includegraphics[width=0.4\linewidth,clip]{Figures/signal_hedgehog}
%
\caption{High voltage (left) and signal distribution boards (right), 
the ''hedgehog cards'', to be mounted at the two ends of the tubes.}
\label{fig_hedgehogs}
% 
\end{center}
\end{figure}

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

Another stringent requirement for the realization of the new readout scheme is
the mechanical integration of the on-chamber readout electronics with the chamber
mechanics, as the density of channels at the tube ends is four times higher than in the
case of the Large tubes. The following design changes will therefore be implemented.

\begin{enumerate}

  \item[(a)]
  HV decoupling capacitors are located in 15 mm diameter cylinders at the tube ends, see
  Fig.~\ref{fig_hedgehogs}. The density of signal routing on hedgehog cards
  is thus no longer limited by HV insulation distances between HV 
  and signal traces on the PCB. 

  \item[(b)] To match the higher tube density, the space on the mezzanine card has to be used
  more efficiently. For this purpose, the passive protection circuitry of the ASD inputs
  will be moved to the hedgehog card. In addition, the modularity of the ADSs will be
  changed from 8 to 16 channels per chip, which goes well with the 32-channel modularity
  of the HPTDC.  

  \item[(c)] For the interconnection between mezzanine cards and New CSM 
  we foresee to use the same 40-wire cables as was used in the present system. 
  Its moderate diameter and high flexibility may be important for fitting the new MDTs
  into the limited space in the Small Wheel.
  
\end{enumerate}

\paragraph{{Power consumption of the electronics\\}\\}

While the power consumption of the on-chamber frontend electronics of the present system is about
30--40~W/chamber (\cite{MDT_ELX_paper}, table 3), the consumption will now be about 2.5 times as
higher due to tube density and increased  consumption of TDC, optical link and other component.

A detailed breakdown of the expected power consumption of the MDT chambers in the NSW is presented
in Table~\ref{NSW_Small_tube_elx}. The total power consumption of 8.8 kW is less than the
consumption of the presently installed CSC (11.5 kW, see~\cite{Aad:2008zzm}, Table 46).  Yet, the
removal of the heat may require flushing with dry air, as significant temperature gradients may
influence the spatial resolution of the MDT in an uncontrolled way and natural air convection is
reduced by the tight space constraints in the high-$\eta$ region of the Small Wheel.

%   Table 5
%
\begin{table}[!ht]

 \begin{center}
    \begin{tabular}{|c||c|c|c|c|c|c|c|c|c|} \hline %{@{}llc@{}}
	
Chamb.& curr.&mezz.s/&curr./& power&curr./&curr./&power  &regul.&power/ \\ 
type  & mezz.&chamb. & mezz.& mezz.& CSM  &chamb.&electr.&5-3.3V&chamb. \\ 
\hline 
      &\it{A}&       &\it{A}&\it{W}&\it{A}&\it{A}&\it{W} &\it{W}&\it{W} \\
\hline
  EIL0& 0.58 &   36  & 20.9 & 69.3 &  2.2 & 23.2 &   77  &  39  &  116  \\
  EIL1& 0.69 &   18  & 12.5 & 41.2 &  1.1 & 13.6 &   45  &  23  &   68  \\
EIL2+3& 0.69 &   24  & 16.6 & 54.8 &  2.2 & 18.8 &   62  &  32  &   94  \\
\hline
  EIS0& 0,58 &   36  & 20.9 & 69.3 &  2.2 & 23.2 &   77  &  39  &  116  \\
  EIS1& 0.69 &   22  & 15.2 & 50.2 &  2.2 & 17.4 &   58  &  30  &   88  \\
  EIS2& 0.69 &   18  & 12.5 & 41.2 &  1.1 & 13.6 &   45  &  23  &   68  \\

\hline \hline

	  \multicolumn{7}{|l}{Power per Large + Small sector}
	 &\multicolumn{1}{c|}{348}&\multicolumn{1}{c|}{179}&\multicolumn{1}{c|}{550}\\
\hline
	 
      \multicolumn{9}{|l}{Power per side}
	 &\multicolumn{1}{c|}{4400}\\

\hline

  \end{tabular}
 \end{center}
 
 \caption{Power requirements for the readout electronics of the NSW. 
		  Mezzanine cards and CSMs with 24 channels will be used. Voltage regulators
		  on CSM and mezzanine boards define the nominal 3.3 V from the incoming 5 V.
		  \label{NSW_Small_tube_elx}} 

\end{table} 


Now something about mechanics, real estate, component density, packaging etc.

Finally something about rates.


%% =======================  section 5.6  ==============================

% ==== 5.6 ============================================================
%%%
\subsection{Services, infrastructure, and DCS}
\label{chapter:Services}
%%%
%%% ===================================================================


{\it
  \begin{enumerate}
  \item Description of service scheme (including power system,
    read-out, trigger, alignment), cooling needs and other special
    requirements
  \item Table with number of services (number of cables, outer
    diameter, cross section of leads)
  \item Table with power consumption (per channel, chamber, total)
  \item Required rack space 
    \begin{enumerate}
    \item UX15 (include maximum allowed distance to detector if any) 
    \item US15 (power system)
    \item USA15 (DAQ)
    \end{enumerate}
  \item Gas system and distribution\\
    Details on number of gas manifolds per sector (include drawings)
    and connections to chambers (serial, parallel?)). Size of pipes\\
    Required nominal, minimum, and maximum flow\\
    Required precision of gas mixture\\
    Safety measures in case of inflammable gas\\
    Required rack space for gas system in SGX1, USA15, UX15
  \item Integration in DCS system, requirements for DCS !!!  Via GBT or ELMB++ ?? !!!
  \end{enumerate}
}

   
\paragraph{Gas system and distribution}

The gas system is a natural extension of the present TGC gas system,  which will be extended from
the present eight manifolds  (four for EIL4 TGCs and four for the SW TGC chambers)  to one
containing 20 manifolds (four for the EIL4 chambers  and 16 for the new Small Wheel chambers
(one/layer for the  upper and similarly for the lower chambers). This will be less  than the normal
manifolds in the Big-Wheels and can be easily  be added to the present SW racks. The manifolds will
be running at a flow rate of 40l/hr of the regular TGC gas mixture, which corresponds to a volume
exchange every four hrs. The extra flow corresponds to a 25\% increase to the present TGC gas flow,
which can be implemented in the present gas system, without any additional modifications, except
the extra flowmeters in the present SW racks. The safety system for the new detectors will be the
same as for the present one, that were approved by TIS.

\paragraph{Integration in DCS system, requirements for DCS}

   \begin{itemize}
 \item Chamber charge monitor
 \item HV control and monitoring, trip recovery
 \item LV power control and monitoring
 \item Temperature monitoring
 \item Link health statistics
 \item ASD configuration parameters (threshold, peaking time, etc.)
 \item Front End trigger and readout configuration parameters (delays, channel masks,
       pipeline depth, etc.) 

   \end{itemize}
 


  \begin{table}
    \centering
    \begin{tabular}{lccc}
      Cable & Number of cables & Outer cable & Cross section\\
      & (granularity) & diam. (mm) & of leads (mm$^{2}$))\\
      \hline
      HV & & & \\
      LV & & & \\
      Monitoring and control & & & \\
      Front-end links & & & \\
      Calibration & & & \\
      Alignment & & & \\
      Miscellaneous & & & \\
      \hline
    \end{tabular}
    \caption{Example table number of services per chamber}
    \label{tab:services}
  \end{table}


  \begin{table}
    \centering
    \begin{tabular}{lccc}
      Chamber & Number of channels & Power consumption per channel &
      Total power consumption\\
      \hline
      & & & \\
      \hline
    \end{tabular}
    \caption{Example table: Number and types of chambers per sector}
    \label{tab:sector_type}
  \end{table}

%%% Sample tables commented out on 22.07.11 RR

\if{0}

Now comes a first example of a table 

  \begin{table}
    \centering
    \begin{tabular}{lrr}
      Micromega/TGC/RPC/MDT & Type 1 & Type 2\\
      \hline
      Number of chambers & & \\
      Radial extension (mm)& & \\
      Minimum length in $\phi$ (mm)& & \\
      Maximum length in $\phi$ (mm)& & \\
      Thickness in z (mm)& & \\
      Mass (kg) & & \\
      Number of layers & & \\
      Number of channels & & \\
      \hline
    \end{tabular}
    \caption{Example table chamber types}
    \label{tab:chamber_type}
  \end{table}

Now comes a second example of a table 

  \begin{table}
    \centering
    \begin{tabular}{lrr}
      Chamber & Small sector & Large sector\\
      \hline
              &  put in      &  a number! \\
      \hline
    \end{tabular}
    \caption{Example table: Number and types of chambers per sector}
    \label{tab:sector_type}
  \end{table}

\fi

