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\title{ \makebox[\textwidth][r]{\normalsize CB-Note 144}\\[2cm]
LEAR Crystal Barrel Experiment, PS197 \\
Run Coordinator's Summary of the December 1990 Run}
\author{Curtis A. Meyer  \\ Universit\"{a}t Z\"{u}rich}
\date{05 January, 1991}
\maketitle
\begin{abstract}
The December 1990 run of the Crsytal Barrel Experiment began on
12 December, 1990 and
ended on 21 December 1990. The run was devoted to studies of {\em inflight}
annhilations at both 600 MeV/c and 1700 MeV/c. Trigger studies and optimization
were performed for these {\em inflight} data. Approximately 1.5 million
minimum bias events at 600 MeV/c and 55,000 minimum bias events at 1700 MeV/c
were collected over the run period. 
\end{abstract}
\end{titlepage}
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\section*{Status of the Apparatus}
\subsection*{Beam}
The beam first arrived at about 23:00 on the 14'th , and continued to
06:00 on the 21'st. The late arrival ( 8 hours ) on the 14'th was
due mainly to the ongoing strike at CERN. In general the beam was quite
good. The spot size was smaller than what we have had at 200 MeV/c, 
but the exact size was not determined. The first beam was at 600MeV/c,
while about 24 hours of 1800MeV/c beam was also taken. Two major 
improvements over previous runs were made by the LEAR staff. First
of all, we finally obtained a slit control in the experimental hut,
and second, the operators were able to start the spills with a trickle of
beam, and slowly build up to our desired rate, rather than overwhelming
the apparatus with an enourmous pulse, and then settling down to the
desired rate.
\subsection*{Silicon Counters}
Much work was done to get a combination of silicon counters on which we
could trigger. A major problem was a strong 70kHz noise signal which
was about the same size as the minimum ionizing signal from the anti-protons.
In the end we wound up using only the central and lower left counter. The
others either produced no signal, or the signal was down in the noise.
\subsection*{Target}
The target performed as it has always performed. However, due to the broad
stop distribution from the in flight data, the effects of the target 
bubbling were not noticible.
\subsection*{PWC}
The PWC functioned perfectly throughout the run. There were a couple
of voltage trips, but the chamber was quickly brought back to operational
voltage.
\subsection*{JDC}
The JDC functioned well during the entire run period. The High Voltages
and the preamplifier voltages were not changed during the run period.
The only troublesome area will be the gas mixture, which fluctuated
between 9.8\% Isobutane and 10.1\% Isobutane. As the {\sc hdc} was not
functioning, there was no inline measurement of the actual gas mixture,
only a measurement of what was going into the chamber, which probably 
has an eight hour time constant. The pressure also varied quite a bit
during the run, however during most of the data collection period it
was confined between 730 and 735 torr.
\subsection*{Crystals}
The problem with the Pedestal subtraction encountered in previous runs
was solved before this run started, which greatly reduced the amount of
Crystal data written to tape. Also, during the period of high momentum
running, (runs 5570 to 5579), the 2282 system was not working. This 
in general means that only FERA data are available during this period.
However, in general there were no problems
with the crystals or the crystal readout during the run.
\subsection*{Magnet}
No magnet problems were encountered during this run. However, due to the
LEAR shutdown procedure which occured on the 21'st, we have managed to 
test the temperature interlock on the magnet. This worked quite well,
and did prevent the magnet from melting when its cooling water was 
turned off. 
\subsection*{Monitor Drift Chamber}
The monitor drift chamber was not functioning during this run.
\subsection*{Slow Control}
Slow control was working, and was written to tape at a frequency of about
100 slow control event per data cartridge. Everything except the
monitor chamber were written out. It should be noted that without the 
monitor chamber, the gas mixture is basically unknown, or at least it lags
the mixture data by several hours.

\subsection*{Data Aquisition}
The data aquisition ran well during the entire run period. The only {\em serious}
problem was that {\sc table} events were not written on the data tapes
until after run number 5521.

\subsection*{Trigger}
Because we had never collected inflight data before, the trigger was
one of the areas which needed a lot of work. The signal seen in
the Si counters was often too small to be used as a trigger, and the
final combination of scintillator and silicon counters used was 
only ten to fifteen percent efficient. This precluded any meaningful
all neutral data because the event rate was far too small. During this
run, all of the data were taken with a combination of the following trigers:
\begin{itemize}
\item $\bar{p}$--stop as defined by a combination of silicon counters and
the plastic scintillator.
\item Silicon--veto as defined by a very large signal in a silicon counter,
indicative of annhilation in the silicon counter.
\item Pile--up veto from the downstream $\Pi$--counter.
\item Software Total energy sum, requiring at least 200 MeV of energy in
the barrel.
\end{itemize}


\section*{Data}
The data taken during the run were of continuously improving quality.
I will try to group runs into sections and describe the basic quality
of each block.
\subsection*{Runs 5422 to 5447}
These data were the very first {\em physics} data taken. Approximately
thirty percent of the events are empty, (no data), and of the remainder,
about half are coming from the silicon counters. These 240,000 events
would probably make a nice source of $\overline{p}{\textstyle Si}$
data, but it is not clear they are worth producing.
\subsection*{Runs 5448 to 5455}
These data were taken during optimisation of the trigger, and are of
widely varying quality. They should not be produced for physics 
purposes. 
\subsection*{Runs 5456 to 5494}
These 280,000 events were taken with our first good trigger. We have 
about fifty percent of the events coming from downstream of the target.
Probably from the light pipe on the veto counter. However, these data
are good, and should be produced.
\subsection*{Runs 5495 to 5569}
These 300,000 events were taken with the voltage on the veto counter
raised by 200 volts. Otherwise they are the same as the previous 
period. The data should be produced.
\subsection*{Runs 5570 to 5582}
These data are our sample of 1.77GeV/c events.  There are about 55,000
events in this set, however, most of them probably have no 2282 data.
\subsection*{Runs 5583 to 5653}
For these data taken at 600MeV/c, the trigger was again improved. 
The width of the veto gate was changed from 50ns to 600ns. This removed
almost all events coming from outside the target region. In this period,
are 632,000 events, and they are our best sample of data from this
run.

\section*{Conclusions and Recomendations}

Of major importance during this run was the ability to change trigger
conditions, and get a fairly quick response from the offline software
as to what happened. The analysis of several tapes per day with the
offline software also provided a major handle on what was going on
during the experiment. Without this fast turnaround, I feel that it
is quite questionable if we would have been able to acheive such
an optimum trigger at the end of the run. I would strongly reccomend that
during future runs, there is always someone who is able to analyze 
events with the offline program in a timely fashion.


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