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\title{ \makebox[\textwidth][r]{\normalsize addendum to CB-Note 283 }\\
        Experiments with the new SVX detector:\\
        First experience and implications}

\begin{document}
%
%\maketitle
\begin{center}
\begin{Large}
        {\bf \makebox[\textwidth][r]{\normalsize addendum to CB-Note 283 }\\
        Experiments with the new SVX detector:\\
        First experience and implication}
\end{Large}
\bigskip

P. Kammel, M. Doser and  C.A. Meyer~\footnote{Additional
information for this report was provided by 
C. Batty, K. Crowe, W. Duennweber, J. Luedemannn, C. Strassburger and C. Voelcker.}
 
\bigskip

September 25, 1995

\end{center}
\tableofcontents
%\newpage
%



%Contents:

%1.	Introduction
%2.	(Preliminary) facts from May and July $K_S$ runs
%2.1	SVX performance
%2.2	SVX target coverage
%2.3	Target performance
%2.4	Beam telescope and beam
%2.5	Trigger
%2.6	DAQ system
%2.7	Summary of achievements and problems

%3.	Some implications for upcoming runs
%3.1	In flight
%3.2	Stopped \pbar\ in gas target

%4.	Some ideas on possible improvements
%4.1	Liquid targets
%4.2	Gaseous targets

%5.	Conclusions


\section{Introduction}

During the May and July 1995 running periods,~\footnote{
First information about the ongoing in-flight experiment at 1940 MeV/c 
(September 95) was also included.}
 the new silicon vertex
detector (SVX) was employed instead of the PWC to provide improved
$K_S$ trigger capabilities. In general, the performance of the new
detector system was excellent. However, its insufficient geometrical
coverage of the region where \pbar 's annihilate created significant
problems.

%\newpage

The aim of this brief report is to compile:
\newcounter{counter}
\begin{list}{\roman{counter}}{\usecounter{counter}}
\item The (preliminary) facts from these initial runs (as a supplement to the Run Report).
\item Discuss some implications for our upcoming experimental program.
\item Sketch some possibilities for improvements.
\end{list}

In this way, the knowledge acquired during these runs should be
disseminated as quickly as possible to allow  decisions about
the best procedure (which we consider critical for the success of our
remaining research program). 


\section{(Preliminary) facts from our May and July $K_S$ runs}

\subsection{SVX performance}

During the main data run in July, both the signals of the 128 x 15
strips and the 15 back planes were fully recorded and used. Apart from
minor problems (1 dead viking and a few noisy strips) the whole system
was working stable and provided excellent resolution.
The problem with the viking was due to a short circuit in the vacuum
flange, which has been fixed before the September run.

\subsection{SVX target coverage}

It became clear already during the May test experiment that the SVX
does not sufficiently cover all the regions where the incoming \pbar\
stops. It extends physically only $\sim$ 1.5 cm  upstream from the target
center and preliminary analysis of the charged tracks indicated that
the sensitive region upstream is limited to $\sim$ 1 cm only. This leads to
the following consequence:

\begin{itemize}
\item For triggers requiring {\em no hit} in the SVX (neutral or 0-2 $K_S$
triggers\footnote{the former is about 3 \% and the latter only about 0.3 \% of all 
annihilations} all antiprotons stopping in this unprotected region ( in--flight 
annihilation in upstream part of the hydrogen filling and the entrance window,
stops in target entrance flange ) may create fake triggered events. 
The fraction of antiprotons stopping in this unprotected region is
only a few percent, but they get strongly enhanced in selective neutral 
triggers.
Thus, they saturate these
triggers and may lead to substantial background in the offline
reconstruction. These events would have been rejected with the PWC.
\end{itemize}

In order to substantially suppress this background (which dominated
the $K_S$ trigger rate by a factor of 10 during the May test run), two
modifications were made before the July run.

\begin{itemize}
\item[i.] The beam telescope was changed. A $\phi$=8mm, d=300$\mu$m  quadrant Si
detector was positioned $\approx$ 26.5 mm before the
target entrance window. A second small $\phi$=5mm, d=300$\mu$m Si counter
was placed inside the 10mm ID reentrant hole at a distance of
$\approx$ 4.5 mm to the target window.
This additional beam definition strongly reduced the background due to
stops in the entrance flange.

\item[ii.] The beam pipe before the target was shortened by 3mm. As
the SVX assembly is mounted on this pipe, the whole SVX moved upstream
as well. This compensated for the (3mm) shorter range of antiprotons
due to the additional Si counter.

\item[iii.] The \pbar\ stop trigger was changed from an {\sc or} of any 
of the 5 silicon counters, ( four quadrants plus the center) to and 
{\sc and} between the center silicon, and any of the four quadrant
counters.

\end{itemize}

\subsection{Target performance}

Unfortunately, one (or both) of these measures increased the heat loss
of the target to a very critical level. Even with the highest safe
(according to Bochaton) filling pressure of 1500mbar the hydrogen
target could be filled to $\approx$75\% at best. At this fill level,
$\approx$3\% of the \pbar\ beam transversed the target above the 
liquid level. These \pbar 's entered predominantly through the upper half of
 the quadrant
entrance counter and then stopped in the endcap of the insulation vacuum pipe
$\approx$13 cm downstream of the target center. Again, these events
were not seen by the SVX and produced a significant amount of
background.


In the last week of the run, the hydrogen was replaced with deuterium.
The pressure on the deuterium tank was 1700mbar, and due to the fact that
the boiling point of deuterium is somewhat warmer than hydrogen, the 
target was full (no downstream annihilations were visible in high statistics
data).


\subsection{Beam telescope and count rates}
	We define the four-fold quadrant detector as $\sum$Si and the
inner counter close to the target as Si$_c$. Because of its small size and
the non--optimal beam focus,
it reduced the incoming \pbar\ rate to 3-4  kHz, a factor of
$\approx$4 lower than the expected rate of 13 kHz. This severely
reduced the event statistics achieved during the July run.

Extensive and time-consuming beam--tuning efforts were performed. However,
the best beam ratio remained:
\vspace*{3mm}

\begin{tabular}{|l|c|}
\hline
Ken's chamber & $\approx$ 15kHz \\
$\sum$Si      & $\approx$  8kHz \\
Si$_c$        & $\approx$ 3-4kHz \\
\hline
\end{tabular}


\vspace*{3mm}
The limit of 15kHz in KC was necessary to keep JDC currents below
140$\mu$A. Moreover, these tuning fractions quickly deteriorated with
beam instabilities or changes of CP LEAR's polarity, requiring
additional time-consuming retunings.

\subsection{Trigger performance}

The CB hardware trigger had to be expanded and generalized for the
$K_S$ requirements. Given the high trigger selectivity, the basic idea
was to measure all (up to three) physics triggers simultaneously,
while keeping the deadtime at a low level. For hydrogen the combined
trigger ran at a rate of $\approx$15 Hz with $\approx$25\% deadtime.
It consisted of (simplified):

\begin{itemize}
\item 0-2 part (multiplicity 0 in SVX, multiplicity $\geq$2 in JDC)
\item 2-4 part (multiplicity 1-2 in SVX, multiplicity $\geq$4 in JDC)
\end{itemize}

The 0-2 trigger was only accepted if the \pbar\ entered through the
lower half of the $\sum$Si counter, to avoid trigger saturation from
events with \pbar\ passing through the incompletely filled target.

In case of deuterium a trigger combining 3 different trigger conditions
was used (0-2, 1-3, 3-5, c.f. run report). Since the target was 
sufficiently filled
0-2 triggers were accepted for all \pbar's passing the Si$_c$ counter.
Its rate of 22 Hz again was significantly below saturation.

The SVX trigger decision was based on the backplane signals only,
which was stable and had sufficient signal to noise separation. The 
hardware thresholds for $K_s$ triggering were set rather high
 (around 30meV).
For efficient vetoing in 0-pr. triggers they were reduced to 18mV
during the September run.  
 Cross
talk from the digital lines to the backplane preamps required the
introduction of 15$\mu$s dead time for every first level trigger (not
including read out).

In order to reduce the DAQ deadtime pile-up events (-10$\mu$s to +
4$\mu$s) were eliminated at the hardware level. This requirement
reduced the event rate significantly (the 2-4 trigger to 50\%) because
a large fraction of the events contained 0 or 2 prong events with a
subsequent pile-up. The tracks from the pile-up annihilation were
usually not seen by the SVX and thus faked  $K_S$ events.

\subsection{Performance of DAQ system}

The new SVX DAQ was reliable and efficient. However, there were several
problems with the JDC readout. Apparently many of the cpu's had recently
been upgraded, and there were a number of time--out problems. When things
were running smoothly with the combined $K_{S}$ trigger, typical DAQ times were
\begin{tabular}{ll}
hydrogen  &  $\approx$  12 minutes/tape @ 15 Hz event rate \\
deuterium &  $\approx$   7 minutes/tape @ 22 Hz event rate   \\
\end{tabular}

\begin{itemize}
\item[i.] At the start of every run it is necessary to recompute the
pedestals for the {\sc jdc}. If things were running smoothly, it was
possible to end one run, and have the next run started and taking data
within 2 minutes. However, one of the {\sc cpu}'s had a tendency to
hang, and even though the {\sc daq} did manage to restart itself, the
2 minutes was often stretched to 5 or 6 minutes. With the planned
change to Digital Linear Tapes (DLT) for data acquisition, actual run
stops and starts will be reduced. 

\item[ii.] During the run, a famous {\em j2xtal timeout} would often
kill the run. The recovery procedure was simple -- stop the run, and start
a new one, but this was then coupled with the deadtime between runs.
This particular problem seemed to be especially strongly correlated 
with the $K_{S}$ trigger. It was rarely seen in the more mundane 
minimum bias and two prong triggers. We were led to believe that the
$K_{S}$ triggers were extremely good at selecting rare garbage events
that were so large they managed to overflow some buffer in the {\sc jdc}
system. Work on this during the run appears to have solved some of this,
but the exact origin is still unknown.

\end{itemize}

\subsection{Summary of Achievements and Problems}

\subsubsection{Achievements}

\begin{itemize}
\item[i.] Efficient SVX with stable operation. Backplanes and strip
side provide enough redundancy to compensate for (the unlikely
case of ) failing vikings.

\item[ii.] Efficient and reasonably fast trigger system based on
backplane.

\item[iii.] Increased acceptance, reduced systematic bias for
detection of low momentum $K_S$

\item[iv.] Improved vertex and momentum resolution due to strip
information
\end{itemize}

\subsubsection{Problems}

\begin{itemize}
\item[i.] Data collection rate too small by factor 4-5. This loss is
due to effects related to


\begin{tabular}{|ll|l|}
\hline
 SVX & &                                       (loss factor) \\
& Small center counter &                    (2-3) \\
& Incomplete target filling               & 2 (for 0-2 triggers) \\
& High sensitivity to beam steering       & 1.2  \\
\hline
Others & & \\
& Problems with JDC readout               & 1.5 \\
\hline
\end{tabular}

The factor of 1.5 from the {\sc jdc} will probably be mostly achieved 
by going to DLT's for data taking. The problem with the high sensitivity
to the beam steering could be somewhat improved by taking a larger 
fraction of the primary beam, and then using the mass slits to control
the current in the {\sc jdc}, as well as further work on the beam
focus itself.

\item[ii.] Systematic problems \\
 Fake triggered events and higher background from \pbar\ stops outside SVX 
  (mainly for neutral SVX triggers).  \\
  Loss of solid angle for charged track in backward angles. \\

\end{itemize}

\section{Some implications for upcoming runs}

\subsection{In flight experiments}

As compared to the at rest experiments described above, the in flight
situation differs in the following respects. On the one hand, the
total hydrogen target forms an extended annihilation source, which
makes the insufficient SVX coverage more problematic (than for the
well located and centered source of the at rest experiments). On the
other hand, the Lorentz boost (in particular at high incident
momentum) will enhance the probability that even for upstream
annihilations a charged track reaches the SVX. Quantitative estimates
were performed by M. Doser for \pbar\ momenta of 1940 MeV/c (see appendix).
The extent of these problems depends on the physics program. Probably, it
is most significant for  0-pr or $K_S$ 0-2 triggers at low incident momenta,
whereas it seems
much less important for charged triggers (experience of the July run
indicates that background is small if 2 or more tracks are required in
the SVX) or high \pbar\ momenta.

\subsection{Experiments with stopped \pbar\ in gas targets}

New cells for the gas targets have been ordered which have $\phi$=20 mm and
l=96 mm, compared to the dimensions $\phi$= 28 mm , l=119 mm, of these used in the
November 1994 run. These cells would
completely fit into the SVX, so that the SVX could be symmetrically
positioned with respect to the gas target, thus eliminating most of
the problems inherent in the liquid target experiments.


The price for this smaller cell diameter is an estimated increase of
wall stops from $\sim 1 \%$ to $\sim 4 \%$, according to Chris Batty`s
calculations.
%These numbers can also be directly estimated from the
%stopping distributions of November 1994 (Christopher
%Strassburger?). 
Again, the increased number of wall stops will be most
serious for neutral events, where the vertex cannot be defined by JDC
tracking.

Recent calculations by Chris Batty indicate that a 20bar target would
significantly improve the stop distributions (only $\sim 0.6 \%$ wall stops ).
An optimization of the various target parameters (K. Huettmann, summer 
student from Munich) led to the following design values: 
distance Ti-window - target entrance 40 mm, Si trigger counter 5 mm before 
target entrance, SVX  active area 15 mm before target entrance, 
p= 13 to 14 bar. This will result in 0.4 \% annihilations in the wall,
16 \% missing the inner (7 mm) Si trigger sector, negligible contribution 
from annihilation outside target window,
in flight contribution from target window of about 0.1~\% ( fully seen by
SVX) and a sigma z of the stop distribution of 3.5 mm.

\section{Some ideas for improvements }

This section summarizes improvement ideas primarely concerning the liquid target setup
(the improved design of the gas cells has been discussed above).

\begin{itemize}


\item[i.] Target.
The fact that the SVX is positioned asymmetric relative to the target
and does not cover its entrance flange and part of the upstream liquid
filling, causes considerable problems. Accordingly, a redesign of the
liquid target such that the whole target fits into the SVX, would be the 
cleanest solution.
An initiative to design a new liquid target has been started by K. Crowe and
led to the following designs:\\
The cooling flange of the target remains unchanged, but the
reentrant cylinder is extended by 1.5 cm.  The target cell is lengthened
by the same amount. This gives a similar stopping distribution as the 
existing liquid cell, but 1.5 cm more SVX coverage of the upstream
region of the target. The wall thickness of the target is
increased (from 75 $\mu$m to 120--150 $\mu$m) to allow  higher
target filling pressures.\\
As a additional option, also the cooling flange 
could be redesigned, so that its diameter is reduced to 17 mm (the same as
the cell diameter) while keeping the cross sections
 of the heat exchange pipes the
same as in the present target.\\
The technical feasibility of these options are currently evaluated by 
the CERN target group.


\item[ii.] The beam telescope must be improved. Two options were discussed:
\begin{itemize}
\item[-] A central Si counter of $\phi$=(8--9?) mm inserted into the ID=10mm
reentrant cylinder. It mounted on an insulating tube and slipped into
the reentrant hole.

\item[-] A beam telescope where the central Si counter is replaced by
a $\approx$9mm hole counter immediately before the target flange
(outside the reentrant port). The hope is that its veto function
provides sufficient collimation, although it remains to be
demonstrated that such an arrangement will completely suppress stops
in the reentrant cylinder. 
\end{itemize}

The critical question with both options is that their performance can only be tested with beam. Any subsequent modifications will be very time-consuming.

\item[iii.] The anticounters for exiting antiprotons must be improved,
since their reactions are not vetoed by the SVX. We propose to mount
an additional Si counter in the insulation vacuum directly after the
SVX as an efficient veto detector with large solid angle. Segmentation
of this detector is possible if there are strong physics arguments in
favor of it.

\item[iv.] The heat exchange of the cold target must be reduced. We
hope this can be achieved by using less superinsulation carefully
wrapped around the critical regions to avoid the thermal contact. 

For the September in flight run the small silicon counter was removed
from the reentrant hole and the wrapping of superinsulation was
reduced. For this run the target temperature was back to normal and
the target could be filled completely.

\item[v.] All efforts must be made to reduce the JDC readout problems
in order to bring the DAQ efficiency back to normal. A lot of progress
was made during the run, and with the installation of the DLT system
we will probably come close to recovering most of these losses.

Latest news from the September run indicate that operation with 
the new DLT's is highly successful.

\item[vi.] The event rate could possibly be increased by $\approx$25\%
by additional beam collimation just before the CB detector or by
tolerating higher JDC currents. The feasibility of this option should
be investigated. 

\item[vii.] Some specific remarks concerning the in flight experiments: As
long as combined 0-pr 2-pr triggers are used and the JDC runs at 100\%
HV, the JDC veto is probably sufficient to define clean all neutrals.
For high rate 0-pr running (with JDC
reduced to 90\% HV) we should investigate inserting the PWC2 in
addition to the SVX. This seems technically feasible, but involves a
touchy assembly operation. For the $K_S$ triggers, PWC2 is of no help.\\
First information from the 1940 MeV/c run indicates that
the 0-pr. data based on the SVX backplane rejection looks reasonably clean.

\item[viii.] Work on a better beam focus from LEAR is in progress. Test
measurements with a MWPC in the magnet confirmed a larger beam size
than expected from the beam profile and multiple scattering,
consistent with the rates observed during the July run. Further
measurements are planned for December.\\
As measures to reduce multiple scattering, we also should consider removing
Ken's chamber and reducing the diameter of the 100 $\mu$m Be beam window,
which would allow a reduction of its thickness.

\item[ix.] A scintillating tube located external to the SVX containment
and upstream to the target could specifically reject tracks originating
from upstream annihilations. 

\end{itemize}

%\subsection{Gas targets}

%The necessary improvement are already incorporated in the present design.


\section{Conclusions}

As discussed in this note, the specific improvements and the choice of
optimal detector components (SVX versus PWC) depends on the details of
the experimental program. While the PWC would have allowed higher
event rates in the last $K_S$ run, we hope that the missing factor of
4-5 can be regained by suitable improvements so that the advantages of
the SVX can be fully utilized. Other programs, like in flight
experiments or gas target runs, require a careful discussion of their
own. Though we advocate that the PWC should be revived as a backup
detector, we also see a considerable danger if important detector
components are exchanged every other run.

We refrain from any specific recommendation in this respect, but
rather ask the collaboration to study this information and work out the
necessary details so that we can arrive jointly at an optimal
procedure.

\newpage
\appendix
\section{ MC study of SVX triggers for in--flight annihilations}
M. Doser simulated the  acceptance for real events and for background 
due to misidentified events 
in different trigger conditions.
\pip \pim \piz\ and \pip\pim\pip\pim\piz\ final states were generated
by GENBOD according to phase space for incoming \pbar 's of 1.9 GeV/c and
a uniform source distribution between -2 and 2 cm. The calculated acceptances
correspond to the detection of charged tracks and include only geometrical effects.
The detector positions assumed and the results of the simulation  are given in the 
tables below.
\bigskip

\begin{tabular}{|l|c|c|} \hline
 detector & radial distance (cm)  &  active region longitudinal (cm)   \\
\hline
 SVX      & 1.2                   &  -1 to 6          \\
 JDC$_{in}$ (layer 3)  & 7.9                   &  -20 to 20 \\
 JDC$_{out}$ (layer 19)  & 20.7       &           -20 to 20 \\
\hline
\end{tabular}
\bigskip


\begin{tabular}{|l|c|c|} \hline
 trigger                           & acc.(\%) for \pip\pim\piz
   & acc.(\%) for  \pip\pim\pip\pim\piz    \\
\hline
0-pr. (SVX=0, JDC$_{in}$=0)        &    0.5                    &    $\le$ 0.1                     \\
2-pr. (SVX=2, JDC$_{out}$=2)       &    33                     &       6                  \\
K$_S$ 0-2 (SVX=0, JDC$_{out}\ge 2$)&    6.8                    &     2.9                    \\
K$_S$ 2-4 (SVX=2, JDC$_{out}\ge 4$)&                           &     2.7                    \\ 
\hline
\end{tabular}

                
\end{document}

