In the first two plots attached I show
proton (mean)time - electron (mean)time = proton TOF
the first peak corresponds to background events (that are higher
in number on the BL side, facing the ring) while the small peak
at about 300 channels away (15 ns) is due to elastic protons.
In the second plot I show the coplanarity (correlated TDC differences).
Our trigger was paddles 12,13 AND 2,3,4,5, symmetrically for the L and R
sector. Each row corresponds to a "proton paddle", respectively 12L, 13L,
12R, 13R and each column to an electron paddle. DAta was taken without the
start counter (physically removed). It shows that there is a rather
symmetric distribution of protons in both the left and right sectors.
However, this is not perfect. As you can see, protons are missing from
paddle 12L (in coinc with 2R), while this is not true for 12R&2L
One possible explanation could be alignement. Presently there are no hard
stops for the detector subframes, and one can expect their positioning is
good to within (or up to) an inch to the beam line and releatively to each
other. At a 3 mt distance from the target this could correspond to up to a
0.6 deg difference in the hadron angle, which in turn is a 20 % difference
in cross section. Things may not be so tight as we have an extended
target.
However, in future runs this evening we will "enlarge" the number of
hadron paddles in the trigger and continue analysis. The missing protons
should show up in the relevant neighbours. THis study however is not
strictly required to move on to the next part of the commissioning
program.
In the third plot I show raw physics trigger rates (for the trigger
configuration above) with an empty and a 1 sccm target. There are clearly
non linear, which is not surprising. Note how they correlate with the
inverse lifetime. Injection current today was up to 120 mA, but the beam
has a very short lifetime in the 80 -120 mA range. The above empty target
rate at 110 Hz was 1 KHz (!).
No effort was put in optimizing the background, as we stayed without beam
for quite some part of our shift (frequent ring RF trips).
Also, It was not possible to move the start counter back to the beam left
side as a flange in the scattering chamber has been modified such that
there is no room for the start counter anymore. However, recent data and
the analysis at page 47 of the logbook shows clearly that the retime issue
is dead. Going back at this point would mean opening the vacuum on beam
left.
regards
-- tancredi
________________________________________________________________________________
Tancredi Botto, phone: +1-617-253-9204 fax: +1-617-253-9599
research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949
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