Hi,
it seems to me that the PMT signal is floating and I am missing
a reason for it. This may be the cause of the annoying pedestal shifts we
observe, and very likely of the charging up of the signal cables.
Right at the pmt base, I measure infinite resistance across the BNC shield
and signal. If HV is on, I actually measure some (fluctuating) number of
Mohms. The signal cables do not look good, but so far the ones I have
measured (with a terminator at one hand) always showed 50 - 60 Ohms.
I will put a 50 Ohms terminator on a "bad" PMT today and I want to see if
that changes at all over the weekend. Note that this does not attenuate
the signal.
-- t
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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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