As some people have noticed on shift, ttl0 shows a double peak, where
all the other TOFs have a single spike. The second peak on ttl0 is
broad and shifted from the first peak by something like 15-20ns. See
the first attachment, upper left and lower left for L0 and R0. Looking
at the raw data, the second peak is present in both L0Top and L0Bottom.
The second peak is most prominent in events with trig=3, that is the
non-ep elastic triggers. If you look only at trig=1 events (left TOF 0
in coincidence with right TOFs 10-15), the peak is greatly suppressed
(first attachment, upper right). You can also get rid of it by demanding
at least 18 wires fire in both sectors (lower right). Looking at the
filtered lr ntuple, the second peak is not there (not suprisingly).
The second attachment shows that for the ttl0 second peak, the event
timing was given by the right TOF (upper right). The symmetric
combination (r0 and l5) shows most of the events clustered in the corner
of the "stair" (lower right), where it's a tossup whether left or right
determines the timing. For ttr0, there may be a hint of a second peak
(lower left), but the time offset is much smaller than for ttl0.
Another interesting piece of information is that prior to run 616
(5/30/03), ALL of the right TOFs had this second peak, and none of the
lefts. This feature on ttl0 only appeared after run 616. That is when
Tancredi and I increased the output width of the TOF CFDs, and delayed
the Common Strobe (CS) to the Trigger Supervisor (the OR of all TOF
Meantimes), to be sure that the CS came after the XMLU physics trigger,
so that the CS determines the event timing. We did this primarily to be
sure we would get TOF-neutron wall coincidences. Perhaps prior to run
616, the CS was sometimes early, so the later sector would determine the
event timing. But I don't understand why only one sector was always
late.
I've looked on the scope, and I can see no clear indication of any
problem. Since the second peak disappears for true ep elastic events,
and since nothing obvious shows up on the scope, I believe we can ignore
the second peak in ttl0.
Karen
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