We spent the day chasing TOF problems and here is a status report.
_ For R5, L9 that are counting significantly less (30 %) we could correlate
their top/bottom TDC spectrum with an attenuated connection to the CFD.
For R5 we could trace this problem to a lose ground connection inside the
(short) lemo-to-ribbon cable to the CFD. The other differences in rate
are smaller and thought to be gain-related.
_ For the noise peak in the adc spectra: we see on the scope cross-talks
between different PMT's in neighbouring TOFs. This already at the base of the
bottom pmt for L2 and L3, which tends to exclude most sources for pick-up.
It shows as a small analagoe pulse with no clear frequency signature.
The rate is higher than that of cosmics hitting neighbouring tofs however
the amplitude seems to be small to explain a peak 300 channels above ped
but it could still go above threshold.
It may be a false track nevertheless we are studying how the ADC peaks correlate
with odd TDC multiplicities in the data stream. Some events indeed have 3 TDC
hits, but most of the times this happens with R0bottom only (?). This could be
just noise in that tdc channel. A clean cosmic event has only 2 hits, top and
bottom, on the same tof of course.
Finally, the single tube rates (on VME) are too low to explain a random
coinc on noise. But we will check this also "by hand" in the D-tunnel.
_ Gain matching: we have decided to go ahead and find voltages for a
target around 1600 channels above pedestal anyways. If the analysis can
cut the peak its for the better. Else we can do it by just looking at the
cosmic peak in each ADC.
One could hope that this noise will fall out of the ep trigger, or at worst,
we'll just cut the peaks out in the analysis. Note, we have never seen these
effects with beam but we had lower voltages then. As soon as gain matching
is done we should be ready for beam, with the goal to measure the
efficiency of all tof's and cerenkovs first.
-- 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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