Hello,
last week we could check the absolute calibration of the charge
measurement performed by the beam-charge scalers. This was done again by
injecting a "known" current into a calibration loop of the dcct toroid.
The "known" charge was measured independently by a HP high precision
multi-meter (with sensitivity down to 1 uA).
I repeated 10 measurements for each data point, at intervals of 5 mA. The
fitted curve is shown in red, in the first attachment. For comparison, the
light blue curve is the calibration from spring 03.
The difference is below 1 % for beam currents above 20 mA. Below 20 mA the
difference is larger but the previous calibration may not have been very
good. This is shown in the top panel of the 2nd attachment (y-coord is the
now/before ratio, the x axis is scaler counts). The bottom panel shows the
beam current vs scaler rate.
The bulk of our data has been taken at beam currents > 20 mA. Within error
bars (and 0.5 % of the absolute charge value) the calibration is the same
for both the gated/not-gated charge scaler. There is also a small shift in
the "pedestal" value (scaler charge with no beam) which brings an
additional 0.1-0.2 % effect at beam currents > 20 mA.
Unless your sample favours fills with very low beam currents (<20 mA) the
effect of these differences should be invisible. I would not expect that
the scaler-charge calibration is a significant contribution to systematic
effects in the data.
-- 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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