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\begin{document}
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\begin{center}
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\LARGE{Letter of Intent for the Upgrade of the ATLAS Experiment\\}
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\end{center}
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%% 1.
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\section{Muon spectrometer}
\label{Muon_spectrometer}

%
%% 1.1
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%\if{0}
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\subsection {Introduction (TK) 1 page}
\label{Introduction}
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%\fi
%

%
%% 1.2
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\subsection {Present muon system (and its limitations) (TK) 1 page}
\label{Present}

%% 1.2.1                bbb

\subsubsection {MDT (Kroha, RR) 1.5 page}
\label{MDT_1_2}

% (This is a draft for the part referring to the present readout electronics.)

The MDT chambers~\cite{ATLAS_detector_paper} and its readout electronics~\cite{MDT_ELX_paper} have
been designed for the nominal luminosity of 10$^{34}$~cm$^{-2}$s$^{-1}$ of the LHC. The actual
particle rate seen by a given chamber type varies over a wide range, depending on the location
inside the Muon spectrometer. At nominal luminosity  hit rates are expected to range from about 50
Hz/cm$^2$ in barrel and outer wheel up to 500 Hz/cm$^2$ in the small and big wheel.\footnote{Hit
rates are derived from ref.~\cite{BG_simulation} and increased by a factor 5 (''safety factor'') to
account for uncertainties of the simulation.} The rapid rate increase with decreasing distance
from the beam pipe in the endcap is partly compensated by the decreasing tube lengths of the
trapezoidal chambers. At the inner boundary of the small wheel the tube length is about 150 cm,
resulting in a rate of about 230 kHz/tube, which can be considered as the maximum hit rate from a
single tube to be expected in the MDT.

%
% figure 1       ccc
% 

\begin{figure}[!htb] 

\begin{center}

\includegraphics[width=0.85\linewidth,clip]{figures/Summary_900_kHz}

\caption{
Left: efficiency as a function of tube hit rate for dead time 750 ns (blue) and 200 ns (red).
Right: saturation of the readout link between TDC and CSM at 100 kHz L1 trigger rate in edge mode
(solid line) and pair mode (dotted) for the same dead times as in the left diagram. The dotted
horizontal lines mark the minimum efficiency required for reliable tracking (left) and the maximum
saturation level recommended for the readout.}

\label{occ_and_link}
%
%   xxxxx yyyyy
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\end{center}

\end{figure}

%

The large majority of these ionizing hits is not caused by high energy particle tracks but by
converted photons, which in turn are generated by neutrons emerging from the calorimeters and the
forward beam pipe (''background hits''). Each hit leads to a certain dead time in the electronics,
as an additional hit cannot be recorded while the amplifier output is above the discriminator
threshold. The dead time caused by each hit is depending on the distance of a track from the wire
and typically varies between about 100 and 750 ns, the latter being the maximum drift time,
corresponding to a particle passing close to the wire. In the present mode of operation the
discriminator is intentionally disabled after each hit for the duration of the maximum drift time
in order to avoid multiple hits from a given track. With a particle rate $f$ and a dead time $T$
for each hit, the occupancy of the amplifier is $f\times T$ and the efficiency $\epsilon
=exp(-f\times T)$. At the maximum expected rate of 230 kHz/tube, occupancy and efficiency would
thus be 17~\% and 84~\%, respectively\footnote{The inefficiency of the drift tubes due to the
aluminium wall thickness of about 2,5\% is not included in this number.}. This value of the hit
efficiency is well above the 75~\% required for reliable track
reconstruction~\cite{GIF_and_dead_time_2}.

The dead time of the discriminator can be selected via the JTAG control of the frontend electronics
in the range 200 to 750 ns. With shorter dead times the probability of a background hit masking a
''real'' track hit is considerably reduced, leading to higher efficiency, while, on the other hand,
the overall rate of recorded hits is increased. Fig.~\ref{occ_and_link} (left) shows the computed
efficiencies, demonstating the advantage of operating with a short dead time. Measurements at the
GIF facility at CERN  have shown
(\cite{GIF_and_dead_time_2}~\cite{GIF_and_dead_time_1}~\cite{GIF_test_Deile}) that this leads to an
increase of the hit rate by about a factor of 1.5, leading to an effective dead time of about 300
ns. This correction is already included in the efficiency values of the figure.

%%% figure 2
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\begin{figure}[!htb]
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\begin{center}
%
\includegraphics[width=0.60\linewidth,clip]{figures/MDT_electronics_scheme_2}
%

\caption{
Schematic diagram of the MDT readout chain. The Chamber Service Module (CSM) collects the data on
each chamber and sends them to the ROD via an optical link. Up to 18 TDCs, serving in up to 432
tubes, can be serviced by a CSM.}

\label{readout_chain}

\end{center}

\end{figure}
%

A practical consequence of higher recorded hit rates due to short dead time is an increase of data
rates to be transferred by the DAQ. Fig.~\ref{readout_chain} shows the readout scheme of the MDT
tubes. In the present system the readout bandwidth of the link between TDC and CSM is 80 Mbit/s,
while the one between CSM and ROD is 1.4 Gbit/s (effective). In the standard readout mode the TDC
generates two 32-bit words for each L1-trigger, one for the leading and one for the trailing edge
(''edge mode''). In addition, the MDT electronics allows for a second, alternative readout mode,
where the two words are compounded into one word (''pair mode'')~\cite{AMT3}. In this mode
redundant information is suppressed, while the required readout bandwidth is reduced by about a
factor of two. As no relevant information is lost, this mode would obviously be the preferred
option for MDT readout. Because of a problem with short noise pulses, however, this mode did
not work reliably and a re-design of TDC and/or ASD chips would be required to bring it into
operation. For future upgrade of the MDT readout the correction of the pair mode will be at a high
priority.

Fig.~\ref{occ_and_link} (right) shows the load of the TDC-to-CSM link at 100 kHz L1 trigger rate
for long and short dead times in edge and pair mode (80 MHz = 100\%). In pair mode the short dead
time could be used up to about 700 kHz hit rate per tube with a saturation of 80\% of the readout
bandwidth, which is considered a safe limit. At this rate the efficiency would still be as high as 80\%
(left diagram).
\footnote
{It should be noted (right diagram) that the link saturation does not go to zero, even with no
data present. This is due to the readout protocol, creating three control words of 32 bits
for each level-1 trigger. At a L1 trigger rate of 100 kHz this results in a base load of about 10
Mbit/s.}

The efficiency calculations given above demonstrate that the rate capability of the MDT tubes reach
up to about 900 kHz per tube, which is nearly four times the value expected for the innermost tubes
of the small wheel. This rate capabiltity, however, is presently not matched by the readout
electronics. A presentation of upgrade options for the MDT electronics will be given in section
xxx.

\if{0}
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%	demo minipage
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\begin{figure}[!htb]
%-------------
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\hfill

\begin{minipage}[b]{0.45\linewidth}
%..................................
\begin{center}
%
%\vspace{-10mm}
%
\includegraphics[width=\linewidth,clip]{figures/print_summary_LARGE_C_1b}
%\vfill
\caption{Efficiency vs. tube hit rate for dead time 750 (blue) and 200 ns (red). The dotted line
marks the minimum efficiency requirement for reliable tracking.}
\label{Muon_cross-section_Barrel}
\end{center}
\end{minipage}\hfill
%.............
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%   figure x_b
%
\begin{minipage}[b]{0.45\linewidth}
%..................................
\begin{center}
\includegraphics[width=\linewidth,clip]{figures/print_summary_LARGE_C_2}

\caption{The saturation of the readout link between TDC and CSM in edge mode vs. tube hit rate for
dead time 750 (blue) and 200 ns (red). The dotted line marks the maximum load recommended for the
readout.}
\label{Muon_cross-section_Endcap}
\end{center}
\end{minipage}
%
\hfill

\end{figure}
%-----------
\fi
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% end demo minipage


\if{0}

% 1.2.2

\subsubsection {CSC (Polychronakos) 1.5 page}
\label{CSC_1_2}

% 1.2.3

\subsubsection {Alignment (Bensinger) 1 page}
\label{Ali_1_2}

% 1.2.4

\subsubsection {RPC (Santonico, di Ciaccio) 1 page}
\label{RPC_1_2}

% 1.2.5

\subsubsection {TGC (Mikenberg) 1 page}
\label{TGC_1_2}

% 1.2.6

\subsubsection {Level-1 trigger (TK/Nagano) 1.5 page}
\label{LV1_1_2}

% 1.2.7

\subsubsection {DAQ system (Pasqualucci) 1 page}
\label{TGC}

%
%% 1.3
%

\subsection {Upgrade Overview (??) 1 page}
\label{Upgrade}

%
%% 1.4
%

\subsection {Phase-1 upgrade (Palestini) 1 page}
\label{Phase-1}

%
%% 1.5
%

\subsection {Phase-2 upgrade (??) 0.5 page (Introduction)}
\label{Phase-2}

%
%% 1.6
%

\subsection {Muon R\&D's (TK) 0.5 page (Introduction)}
\label{Muon_R_and_D}

%% 1.6.1

\subsubsection {Thin tube MDT (Kroha) 1 page}
\label{Thin_tube_1_6}

% 1.6.2

\subsubsection {Micromegas (Wotschack) 1 page}
\label{Micromegas_1_6}

% 1.6.3

\subsubsection {TGC (Mikenberg) 1 page}
\label{TGC_1_6}

% 1.6.4

\subsubsection {Plasma Planel Sensors ??? (Chapman) 0.5 page}
\label{Plasma_1_6}

% 1.6.5

\subsubsection {microPIC micropattern detector (Ochi) 0.5 page}
\label{microPIC_1_6}

\subsubsection {MDT gas studies, if necessary --- 1.5.1}
\label{MDT_gas_1_6}

% 1.6.6

\subsubsection {RPC, if necessary --- 1.5.1}
\label{RPC_1_6}

% 1.6.7

\subsubsection {MDT readout electronics, if necessary --- 1.5.1 or 1.5.3 (??)}
\label{MDT_readout_1_6}

% 1.6.8

\subsection {Cost and responsibilities (Palestini / Pontecorvo) 1 page}
\label{Cost_1_6}

\subsection{Limitations of the readout system at phase I}
\label{phase_I}

 
\subsection{Limitations of the readout system at phase II}
\label{phase_II}


\section{Small drift tubes for regions of high background}
\label{small_tubes}
%
\fi

\newpage

%
\begin{thebibliography}{00}
%
%
\bibitem{ATLAS_detector_paper}
The ATLAS collaboration,
\emph{The ATLAS Experiment at the CERN Large Hadron Collider},
JINST {\textbf 3}  S08003 (2008)
%
\bibitem{MDT_ELX_paper}
Y. Arai et al.,
\emph{ATLAS Muon Drift Tube Electronics},
JINST {\textbf 3}  P09001 (2008)
%
\bibitem{GIF_and_dead_time_2}
S. Horvat et al.,
\emph{Operation of the ATLAS Muon Drift-Tube Chambers at High Background
Rates and in Magnetic Fields}, IEEE Trans. Nucl. Sci. 53, 562-566, 2006
%
\bibitem{GIF_and_dead_time_1}
O. Kortner,
\emph{Operation of the MDT electronics with Reduced Dead Time},
http://indico.cern.ch/conferenceDisplay.py?confId=a05878, (2004)
%
\bibitem{GIF_test_Deile}
M. Deile et al.,
\emph{Resolution and efficiency of the ATLAS muon drift-tube chambers 
at high background rates},
Nucl. Instrum. Meth. {\bf{A535}} (2004) 212--215
%
\bibitem{AMT3}
Y. Arai,
\emph{AMT-3 User Manual},
https://edms.cern.ch/file/897562/1/AMT3manual\_033.pdf,
http://atlas.kek.jp/tdc/amt3/AMT3manual\_033.pdf

\bibitem{BG_simulation}
M. Bosman et al.,
\emph{Estimation of Radiation Background, Impact on Detectors, 
Activation and Shielding Optimization in ATLAS}, 
ATLAS Note {\textbf ATL-GEN-2005-001} (2005)
%
\bibitem{tracking}
D. Primor et al.,
\emph{A novel approach to track finding in a drift tube chamber}, 
JINST {\textbf 2}  P01009 (2007)
%
%
\if{0}
%
\bibitem{BG_radiation_0}
I. Dawson et al.,
\emph{Predicting the radiation environment at high-luminosity 
hadron-collider experiments},
Nucl. Instrum. Meth. {\bf{A510}} (2003) 219--232
%
\bibitem{BG_radiation_1}
V. Hedberg, M. Shupe,
\emph{Radiation and induced activation at high luminosity}, 
ATLAS Technical Note {\textbf ATL-TECH-2004-002} (2004)
%
\bibitem{BG_radiation_2}
G. Darbo et al.,
\emph{Outline of R\&D activities for ATLAS at an upgradeed LHC}, 
ATLAS Communication Note {\textbf ATL-COM-GEN-2005-002} (2005)
%
\bibitem{Muon_TDR}
The ATLAS Muon collaboration,
\emph{ATLAS muon spectrometer: Technical Design Report},
CERN-LHCC-97-022, ATLAS-TDR-010 (1997)
%
\fi
%

\end{thebibliography}
%
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\end{document}
