Data and analysis of last night gave a strong indication that the
signal we observe in the ADC spectra was real: it moves up and broadens
with HV (see attachment 1, and 2, where the HV is 5% and 10% higher then
the commissioning values)
After having eliminated the possibility of picking up noise or having
flaky cables, we accepted the above fact and speculate this is true backgound
from south hall. Most of this noise contribution goes away by requiring a
Cerenkov (or any other detectors as far as we understand it) in the cosmic
trigger. Results, for the highest voltage are shown in attachment 2,
that is to be compatred with 3.
In conclusion we are back on track to finish gain matching. The high
setting will be 1500 channels above pedestal. Gain matching for
for 1300 channels above pedestal (where the voltage is such that we
don't see these peaks, at high thresholds) was already close to be
finished and should be continued (at convenient times, i.e. beam off)
Note, the mean and the peak value could be 100's of channels away while
we want the peak. At +1500, we should have enough ADC range for 40-50
MeV, assuming an energy deposition of 8-10 MeV by cosmics.
The rate of the above background may be 30-40 Hz per tof's, and stays very
low on a dosimeter. RPO's experience is that such rates are possible although
they can't really tell where they are from, when using more sensitive
detectors (scintillators). We can tell it hits all TOF's, sooner or later.
It could be in the chambers or elsewhere in the detector.
We have never run at high voltages and we have no reasons to believe that
this noise could be a problem in a ep trigger. It was a problem to gain
match at a higher gain which is what we set out to do this weekend. We
did run higher voltages only in the DTF (where we did not see this
problem)
The Cerenkovs are in the cosmic trigger now. The cerenkov can be monitored
with scalers, that also show the coinc between "bottom" and "top" tubes.
The Cerenkov HV is to be adjusted to optimize the cosmic rate, which
hopefully also means higher efficiency and at some point gain matching (I
don't expect all boxes to be close to be matched, by inthe new set-up they
should be reasonably efficient).
regards,
-- tancredi
________________________________________________________________________________
Tancredi Botto, phone: +1-617-253-9204 mobile: +1-978-490-4124
research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949
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