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\section{The Facilities}

\subsection{The Hall A RICH Detector (Contributed by B.~Reitz)}

\subsubsection{Introduction}

JLab experiment E94-107, ``High Resolution Hypernuclear Spectroscopy'' \cite{e94107}
has been scheduled to run January until March 2004. 
Due to the high background from protons and pions in this experiment,
the present HRS PID detector package consisting of two aerogel 
threshold Cherenkov detectors and scintillators for time-of-flight
measurements is not adequate. Therefore, a proximity focusing 
RICH detector has been build. It uses liquid freon as radiator, 
a segmented CsI photocathode as photo converter and as cathode for 
a MWPC to detect the photo-electrons.
This RICH detector is conceptually identical
to one module of the ALICE-HMPID RICH \cite{alice}. A description 
can be found in the 2001 Hall A Report \cite{halla01}, the facilities
for the handling of the CsI photocathodes are described in the 
2002 Hall A Report \cite{halla02}.


\subsubsection{Cosmic Test and System Improvements}

The RICH detector was installed in the EEL building and 
regularly operated to perform test runs with cosmic radiation.
Therefore system improvements could be tested immediately 
and long term changes in the detector performance could 
be monitored. The cosmic test were performed in the same 
manner as described in previous Annual Hall A Reports 
\cite{halla01,halla02}. However, several improvements have been 
implemented. 

\begin{itemize}
\item The front-end electronics have been rearranged
in six groups instead of three, so that now groups of 
240 (instead of 480) pads are read out sequentially by one ADC,
reducing the associated deadtime by a factor of two.
A second VME crate with the now necessary additional 12 
CRAMS modules, a Sequencer and a Read-Out Controller has 
been set-up and tested. For the cosmic tests only one of the 
two VME-Crates (connected to one half of the electronics) was 
used at one time.

\item A tiny leak between the radiator and the MWPC was detected,
which decreased the performance of the detector. It was repaired
and the freon vessel was reinforced. Additionally a second 
radiator was built, which will be used as a spare.

\item The anode wire plane was re-strung with higher wire-tension,
eliminating problems with individual loose wires not being
able to hold high voltage. This problem was observed in 
earlier tests and resulted in the need to turn off those wires.
\end{itemize}

\begin{figure}[h]
\begin{center}
\setlength{\epsfxsize}{0.8\textwidth}
\leavevmode
\epsffile{rich_run_august.eps}
\caption{ \it Distribution of the number of photo-electrons and of the charge per cluster
for the cosmic tests in August 2003. The MWPC was operated at 2150~V with pure methane.
The average number of photo-electrons per ring was 10.6.} 
\label{fig:run_aug}
\end{center}
\end{figure}

Figure \ref{fig:run_aug} shows results from a cosmic run performed 
in August 2003. The RICH was operated with pure methane as counting gas
and a high voltage of 2150~V. At this high voltage the nominal gain
($\approx 8 \; 10^{4}$ was achieved. The number of photo-electrons
was 10.6 per ring, about 20\% smaller than expected. It was also
observed that the number of PE was slightly less in August as compared
to the first measurements shortly after evaporation. This may 
have been caused by contamination of the counting gas with oxygen
or moisture.

\begin{figure}[h]
\begin{center}
\setlength{\epsfxsize}{0.8\textwidth}
\leavevmode
\epsffile{rich_run_november.eps}
\caption{ \it Distribution of the number of photo-electrons and of the charge per cluster
for the cosmic tests in November 2003. The MWPC was operated at 2120~V with pure methane.
The average number of photo-electrons per ring was 13.4.} 
\label{fig:run_nov}
\end{center}
\end{figure}

In November three new photo-cathodes were evaporated and installed,
together with a new anode wire plane, the latter being necessary
to keep the gap between cathode and anode at its nominal value.
Due to mechanical intolerances the cathodes and wire planes have 
to be exchanged together. The new printed-circuit-boards used 
for this generation of photo-cathodes were manufactured with a new
cleaning procedure, resulting in a 10\% higher quantum efficiency
after evaporation.
Figure \ref{fig:run_nov} shows the results from cosmic tests 
with these cathodes, the number of photo-electrons increases to 13.4
per ring, at the operating voltage of 2120~V. Under these conditions
the charge per cluster is also larger than before.
Figure~\ref{fig:evdisp} shows the event display of a single cosmic
particle crossing the RICH detector, the cluster in the middle is the
signal from the particle itself (Minimum Ionizing Particle or MIP), 
the clusters around it are 
caused by Cherenkov light.
The previously used photocathodes have been re-evaporated,
are safely stored in a dry and oxygen-free environment, and can
be used as spares. 

\begin{figure}[h]
\begin{center}
\setlength{\epsfxsize}{0.5\textwidth}
\leavevmode
\epsffile{rich_ev8run1608.eps}
\caption{ \it The event display for a single cosmic trigger. The large cluster in the 
middle is the signal from the MIP, the clusters around it are from the Cherenkov light.} 
\label{fig:evdisp}
\end{center}
\end{figure}

\subsubsection{Conclusions}

The RICH detector for Hall A was successfully tested with 
cosmic radiation. The optimal operating parameters have 
been found, and it was shown that with cosmics the RICH
performance meets its design specifications. 
For all critical parts of the detector (photocathodes, radiator)
a full set of spares is available at JLab.
The RICH detector 
is now (December 2003) installed in the left HRS of Hall A,
and ready for the final in-beam commissioning and the use
in experiment E94-107.

\begin{thebibliography}{9}
\bibitem{e94107} JLab E94-107, Spokespersons, 
S.~Frullani, F.~Garibaldi, J.~LeRose, P.~Markowitz, T.~Saito.

\bibitem{alice} Alice~Collaboration, {\it Technical Design Report of the
High Momentum Particle Identification Detector}, CERN/LHCC98-19, Alice~TDR~1, 14~August~1998.

\bibitem{halla01} Hall A Report 2001.

\bibitem{halla02} Hall A Report 2002.

\end{thebibliography}

\end{document}
