Pibeta MWPC alignment
For a good event reconstruction it is necessary to have the chambers well aligned. Both chambers consist of an outer and an inner cylinder (see also chamber specifications). The inner cylinder is fixed to the anode wires, but the outer cylinder can be rotated to the inner. Both chambers have to be aligned in z direction and the complete chamber system has to be aligned to the coordinate system, which is given by the CsI calorimeter.
Here you will find the results for a alignment check of the chambers. All checks have been made with data from cosmic runs 20818...20821.
Intrinsic alignment of the chambers
The intrinsic rotation of the outer cylinder relative to the inner cylinder can be checked by calculating the phi coordinate from the anode wires and from the cathode strip crossing. The difference between this phi coordinates indicates the intrinsic rotation of the outer cylinder. Figure 1 shows the difference of these phi coordinates for both chambers.
Figure 1: Intrinsic chamber alignment. For both chambers the mean of the Gaussian is close to 0. A further correction is not necessary. The MWPC2 peak is smaller, because it has more wires and strips (=> better resolution). The picture is also available as ps-file.
z shift between the chambers
When the chambers have been built in, it is possible, that chamber 1 is not exactly centered relative to chamber 2. The check was done by projecting the track from a cosmic particle, which was measured in chamber 2 onto chamber 1. The difference between the projected z-position and the measured z-position indicates a shift in the z-coordinate between the two chambers. Figure 2 shows this histogram (both the same, but on different scales). The center of the peak is at ~-0.25 mm. This correction can be built in into the analyzer as an additional shift parameter.
Figure 2: z shift between the two chambers. The picture is also available as ps-file.
Phi rotation ans z-shift between chambers and CsI calorimeter
When the chambers have been built in, it is possible, there can be a rotation or it can be, that both chamber are not exactly centered relative to the calorimeter, which defines the Pibeta coordinate system. The check was done by projecting the track from a cosmic particle, which was measured in both chambers from the chambers onto the calorimeter sphere. The difference between the projected phi-coordinate (z-position) and the measured phi-coordinate (z-position) indicates a rotation in phi (shift in the z-coordinate) between the two chambers and the sphere. Figure 3 shows this histogram (both the same, but on different scales) for the phi rotation. Figure 4 shows the z-shift of the chambers relative to the calorimeter sphere.
Figure 3: phi rotation between the two chambers and the calorimeter sphere. The picture is also available as ps-file.
Figure 4: z-shift between the two chambers and the calorimeter sphere. The picture is also available as ps-file.
Figure 3 shows, that the chambers are rotated by 1 degree to the sphere. This can be corrected by a software parameter in the analyzer. The z-shift between the chambers and the sphere is very small and needs not to be corrected.
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H.P. Wirtz , 11-Nov-1999
last update: 11-Nov-1999
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