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The Cerenkov detector response will be used
in the level 1 trigger.
One of the questions that arise is the
optimal number of
neighbouring Cerenkov detector elements (PMTs)
that should be added in a pre-trigger
scheme. This number should be large enough not to diminish
the electron detection efficiency and small enough to
allow
selection of certain polar angle regions
( e.g. to select high
or low
events).
It is natural to sum signals from both PMT of each segment,
since one electron can produce a signal in both PMTs.
Four trigger schemes were analyzed:
Just one segment in a group.
Two segments in a group.
Group # 1 includes segments 1 & 2;
Group # 2 includes segments 2 & 3;
.....................................
Group # 17 includes segments 17 & 18;
Three segments in a group.
Group # 1 includes segments 1 & 2 & 3
Group # 2 includes segments 3 & 4 & 5
.....................................
Group # 9 includes segments 17 & 18;
Group # 1 includes segments 1 & 2 & 3 & 4
Group # 2 includes segments 3 & 4 & 5 & 6
.....................................
Group # 8 includes segments 15 & 16 & 17 & 18
The groups overlap so that there is always at least one group which contains the total number of photo-electrons, generated by the electron.
How many segments should be added depends on the electron
momentum. For low momentum electrons this number will be larger.
Fig 11 a) shows the distribution of the number of hit
CC segments
for electrons of 0.8 GeV energy. Typically
detector segments fire. Figures 11 b), 11 c) and 11 d)
show a comparison of different trigger schemes for few different
PMT detection threshold
(in terms of photoelectrons). The trigger
efficiency is defined here as the ratio of events
satisfying the trigger condition and the number of events with
at least one photoelectrons produced.
It is clear from the figure, that
segments in a group
are sufficient - there is virtually no difference
between the TRIG=3 and TRIG=4 schemes.
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