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\section{Spectrometers}

\subsection{High Resolution Trigger Counter S2m}
{\it Contributed by R.~Feuerbach}

A need for an improved resolution timing was recognized after the initial
set of experiments in Hall A. To address this need, the high-resolution S2m
scintillator planes were designed to replace the original S2 trigger
planes. In the Summer and Fall of 2003 these planes were fully assembled
and installed in the HRS spectrometers; a picture of the completed S2 plane
for the Left HRS is shown in Figure~\ref{fig:s2m_pict}.

\begin{figure}[tbhp]
	\centering
	\includegraphics[angle=-90,width=\textwidth]{s2m_assembled.ps}
	\caption[Assembled S2m]{The fully assemble S2m scintillator plane in its
	frame, prior to installation into the LeftHRS. The on-frame discriminator
	can be seen mounted on the right.}
	\label{fig:s2m_pict}
\end{figure}

\subsubsection{Signal Processing}
The signal-path was designed so as to best preserve the timing of the
pulses. The signals from each photo-multiplier tube are sent to a passive
90/10\% splitter, with the greater portion sent to a P/S 706 discriminator
on the detector frame and the lesser portion sent to the Fastbus ADCs. To
form S2's contribution to the trigger, the first discriminator output for
each paddle's left- and right-side PMTs are logically AND'ed, and an OR
over these results is performed. The right-side PMTs determined the timing
of this trigger since their trigger-cables from the discriminator are 30ns
longer than those for the PMTs on the left side. The second output from the
discriminator is sent through a NIM-ECL converted and an active ECL-delay
module before being readout out by Lecroy 1875A TDC modules set to
50ps/channel.

\subsubsection{Results}
\begin{figure}[tbhp]
	\centering
	\includegraphics[width=\textwidth]{s2m_res.epsi}
	\caption[Time-difference resolution]{Distribution of the
	time-difference between the left- and right-PMTs on the upper eight
	paddles, after correcting for the signal propagation through the
	scintillator.}
	\label{fig:s2m_res}
\end{figure}

During the second half of the GDH (E97-110) experiment, the LeftHRS was
used as a luminosity monitor and data for the newly-installed S2 in the HRS
was taken parasitically. During this test, the TDC modules were set to
100ps/channel. To measure the inherent timing resolution, the time
difference between the left and right-PMTs of each paddle were
studied. After correcting for the signal propogation time through the
scintillator, the width of the timing distribution was typically $\sigma_{L-R}
\approx 210\mathrm{ps}$. The distributions for the upper eight paddles,
which are representative for all 16, are shown in
Figure~\ref{fig:s2m_res}. Under the assumption that both PMTs contribute
equally to the width of the distribution, such that
 $\sigma_{L-R} = \sqrt{2} \sigma_{PMT}$, an average timing resolution-per-PMT of
$\sigma_{PMT} \sim 170\mathrm{ps}$. The expected timing-resolution of the
the plane is then better than $\sigma_{s2} < 160\mathrm{ps}$.


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