We have taken data for different H, V nominal beam positions
at the target, with the collimator in.
The horizonthal scan went from -1 to +2 mm in steps of 1 mm. Spectra
of the (charge normalized) vertex position distribution were already sent
out yesterday, and are shown as a function of beam H-position in the
first attachment.
While the total yield changes and gets worse the shape of the background
does not seem to change much. This was indeed what we hoped for. Note that
we will always measure the "stuff" between -30 -40 also with gas in the
target.
In the second attach - as a function of nominal Hor beam position (Hpos) -
top-left = total integrated yield of reconstructed tracks
top-right = BQM yields, *after* slit optimization
bot-left = Beam H position measured at lpmx01, lpmx02 (beam pos monitors)
bot-right = The vertical beam position (measured) during the scan
Here we come to a consistent result wrt bqms. However now, at each beam
pos, we re-optimized the slits. This was not the case of previous bqm
scans (slits out). Scanning with the slits out is time efficient. However
it seem to only give indication of a broad minimum in BQM rate, whereas
the reconstructed background can still change considerably (compare a
green and black in the first attach: they have the same slit out bqm rate).
It is also good news that with this caveat the bqms are still useful.
Given that the shape of our background proves indeed to be "universal",
than we should be able to use the yield of events at -30-40 as a
"normalization" for a future reference. This is shown in attach #3, where
the two runs (gas/empty) were taken for the same beam conditions.
After analysis of the V scan we should be able the tell about the best
beam position.
-- Tb,JS,NM
________________________________________________________________________________
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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