To summarize:
John is right, the TDC start cables got shorter (by 91" or 11ns), and
so that should only make the TOF time differences safer. In fact, by
the end of the 9/11/02 evening shift, the efficiency was back in the
high 90's, with the short TDC start cables.
Adrian had taken a run with TOF thresholds at 40mV just before I
shortened the start cables. The efficiency was 99%. After the TDC
starts were moved earlier in time, the BLAST field was turned on (which
resulted in no signals from the start counter). After the field was
turned off, Pete measured 70%. He eventually got back to 98% by
dropping the TOF thresholds back to 21mV.
How could 41mV be a good threshold in the afternoon but not in the
evening? Adrian will test 41mV again today. The TDC start cables are
still short and should stay that way. The individual TOF TDC stop
signals are around 70ns after the start (from what Pete saw on the scope
last night), so that is comfortably beyond the minimum 45ns, yet not
near the 200ns full-scale.
Karen
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