Details on alignment and calibration

To obtain 100 micron transverse accuracy in the large trackers the following considerations are important:

• Internal rigidity of a single chamber
  A single chamber consists of 12 layers glued together. The best stability can be obtained with so called mono-layer planes, where each plane consists of 2 folded foils forming the honeycomb cells and 1 flat foil for stiffness glued together (figure 3.1). The position of the anode wires is the essential information needed to extract the track position from the measured drift times. The anode wires are positioned by the V groves in the plastic blocks at the ends of each honeycomb cell with an rms accuracy of 10 m within a block for 8 wires. These blocks are glued together while positioned on a template, which provides relative positioning of all blocks and the folded foils. Under appropriate thermal conditions a final position accuracy of 18 m has been obtained over several meters.

  The plastic blocks as well as the pocalon foils have a large thermal expansion coefficient of m m K , which will effect the location accuracy under experimental conditions. Therefore, even with a rigid honeycomb structure, optical monitoring of alignment references on the chamber is important to obtain 100 m location accuracy.

      
  Figure:   Geometry with 4 identical modules around the beam hole.
  Figure:   Chamber module consisting of 12 layers honeycomb cells.
  

  Independent of the overall thermal expansion of a single chamber, with all planes glued together, the relative positions around a local track stay fixed and this is important for the drift time calibration.

• Auto-calibration of the drift time
  From a sample of local tracks in a single chamber a global fit can be made for the function r(t), which relates the distance r between track and anode wire to the measured drift time t. This is possible due to the redundancy of planes and the fixed local positions inside a single chamber [18] No special calibration runs are needed; the auto-calibration can be done with the same tracks as used in the analysis.
• Alignment of a chamber with respect to other chambers
  In the previous EMC, NMC and SMC experiments the chambers were aligned with respect to each other by using data from special alignment triggers on halo particles and beam particles at low intensities and with the magnetic fields switched off. Straight tracks through reference chambers were used to align the other chambers. The large chambers were aligned with respect to the small chambers using tracks in the overlap regions. Some proportional chambers had their dead regions in the beam activated during these measurements. This procedure defined a fixed relation between the chambers at the moment of the alignment measurements. No changes were assumed until the next alignment measurement or when chambers were physically moved.
• Optical monitoring of the chamber coordinates
  New with respect to the previous muon experiments is that we intend to monitor the positions of the honeycomb chambers during data-taking. In this way temperature or other temporal changes of the relative positions can be automatically taken into account in the track reconstruction. External optical alignment monitoring can be done with RASNIK, a system consisting of infrared illuminated pattern masks, lenses and small CCD cameras. Due to the open structure of the spectrometer, where most of the chambers can be sled in from the top and from the sides, several options for monitoring the chamber positions are still open. The precision of these monitoring systems are in all cases better than the 100 m needed.