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User name johna
Log entry time 21:18:49 on October 25,2007
Entry number 143395
Followups:
keyword=coincidence trigger issues.
When we started looking at coincidence events in the HMS, we saw some unusual things. First, about half of the HMS histograms weren't being filled for coincidence events (you need "group=gep" in the hist block definition, which has now been added for all standard histogram blocks).
More importantly, it looked as though the timing was very odd for coincidence events. Plot #1 is the drift chamber raw TDC spectrum, showing a peak where one expects (consistent with what is seen for singles triggers), and a second peak, about 300ns off from the correct position. Initially, we thought it was a difference between coincidence and singles timing, but coincidence-only runs showed that both peaks were there in coincidence events. Similar behavior was seen in plot #2, which is basically the coincidence time spectrum (HMS single trigger for start, coincidence trigger for stop). For both the DCs and hodo triggers, there are REAL events coming 300-400ns before the coincidence. Since it is both DCs and scintillators, it has to be real events, that are somehow forming coincidences with bigcal 300-400ns later. Only explanation we can come up with is a reflection in the HMS trigger (the cables between the HMS and electronics platform (the bunker), as well as those between BigCal and the bunker are about the right length to give a reflection ~400ns later). Mark has gone down to fix the problem (yet again), while Andrew and I sit and wait for word of success....
FIGURE 1
FIGURE 2
