Motivated by the plots shown by Philippe in the last beam test VRVS meeting on the distribution of CalZDir (see http://polywww.in2p3.fr/~bruel/cernps_20060801.pdf
) I spent some time inspecting the reconstruction of the direction of muons (which is far simpler than electrons) in the calorimeter of the calibration unit. In this page you find a comparison of the FRED event displays, as well as the distributions of Trk1ZDir and CalZDir, for muon events detected with the Calibration Unit (Pisa, July 2006) and with the Full LAT detector (SLAC, April-May 2006).
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The image MuonWithCU shows the distribution of energy deposited in the calorimeter when a muon goes through itthe Calibration Unit. This is a typical image; not a rare event. Note that the track is "brocken"; every other layer there is a "hole" in the track, and the energy deposited in that layer seems to show up located off the track. The FRED Images 1,2,3 and 4 show the first 4 consecutive events from that run (after the application of the selection criteria described above). You can see the same behaviour in all them. The FRED image Evt0_4Views shows the event 0 from 4 different perspectives. Note that the Z-X perspective is different from the Z-Y perspective. In the Z-X view the layers 2, 4, 6, 8 are alined and the layers 1, 3, 5, 7 are off; whereas in the Z-Y view the opposite occurs.
Anders and Leon atributed this effect to a wrong crystal diode relative calibration; since the position of the event along the crystal is determined comparing the signal produced in each of the diodes at the end of the crystal. Thus a wrong relative calibration of these two diodes will shift the position determined for that energy deposition. The fact that the position positions determined for in all layers are wrong (in a composed Z-Y and Z-X image) seems to point to a wrong relative calibration of all diode-pairs from each crystal.
This effect deteriorates the reconstruction of the direction of the event in the calorimeter; which is clearly observed in the distributions of the variables Trk1ZDir and CalZDir shown in MuonPisaReconstruction.gif, MuonPisaReconstruction.ps and MuonPisaReconstruction.root (gif, ps and root files). Note that (for this data and selection applied) the muon direction in the calorimeter (CalZDir) is properly reconstructed for ONLY a TINY FRACTION (about 3%) of the events.
In the case of electron events, there is not a single particle crossing the calorimeter but a bunch (shower) of them distributed over a given region (at least two crystals wide) in the calorimeter. Therefore the impact of this effect on the reconstruction of CalZDir will be somewhat lower. Yet it will probably be one of the reasons for the broad distributions of CalZDir found for run 700000811 that were shown by Philippe in the past VRVS beam test meeting (2006/08/01).
Additional muon runs
From the list of muon runs in the Beam test data base, only 2 runs are shown as Vertical Muon Runs. I took run number 700000322, which has 10^5 events (5962 seconds of data, taken on 2006/07/15). Before inspecting the data with FRED i applied the same selection criteria used above (vertical events within 5 deg which deposit some energy in teh cal) and ONLY 2 events survived. Later on i noticed that indeed, most events are empty (CalEnergyRaw < 10 MeV). Does anybody know why ?
Anyway, the FRED display for the two events which survived can be foundat found at Evt0 and Evt1. Note that the behaviour is similar to the one described above.
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