Central Drift Chamber Commissioning

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Introduction

This document describes the commissioning process for the GlueX central drift chamber (CDC).

Commissioning and Alignment with Cosmic Events

The CDC consists of 3522 8mm radius aluminum plated kapton straw tubes. Each tube contains a 20 micron diameter gold-plated tungston wire strung in the middle of the straw. The wires are operated at positive potential, about 2kV, above the grounded straw tubes. The chamber is filled with a 50--50 mixture argon and carbon dioxide, and the gas mixture combined with the nominal atmospheric pressure of the gas determines the voltage operating point of each wire. Each wire has been brought to high voltage, and cosmic signals observed several times during the commissioning process. First at Carnegie Mellon University, before the chamber was transported to Jefferson Lab. Again, at Jefferson Lab when the chamber was in a test lab, and finally, after the chamber was installed and hooked up in its final position in Hall D. As of the date of this report, all channels are operating, and considered to be at their nominal voltage.

A trigger has been set up to collect events which are seen by both the barrel calorimeter (BCAL) and the CDC. This trigger runs at a rate of few Hz, with about 0.5Hz, yielding CDC data. In the time between the middle of July and the middle of October, it should be possible to run these trigger on nights an weekends. A reasonable estimate is probably 50 hours per week of data collection, which yields about 90,000 events per week. This should yield sufficient data to run initial passes of the calibration and alignment procedures, but a dedicated cosmic run should be planned after the November commissioning run. It is currently estimated that about 300,000 reasonable CDC events are needed for alignment. This is probably just compatible with the night and weekend running prior to the November run. The following activities are expected to be done using cosmic events.

1. Study and minimize noise observed on the CDC as installed. This study does not require cosmic events, but work on making sure that the grounding and isolation of the CDC is fully optimized and set up according to specifications.
2. Measure efficiency of hits on individual wires being used on tracks. This study can be carried out with about 50,000 events with good tracks in the CDC.
3. Final relative alignment between the CDC and the BCAL. An initial study can be carried out with on the order of 50,000 events with good tracks in the CDC
4. Optimization of the time and amplitude algorithms that will run on the FPGAs in the Flash 125 modules. This study requires around 10,000 events with good tracks in the CDC.
5. Final alignment of relative wire positions in the CDC. This requires ~300,000 good cosmic events.

Commissioning and Alignment with Photon Beams

Initial beam into Hall D is expected in November 2014. The data collected during this running will be utilized to continue commissioning and alignment of the CDC.

Zero Magnetic Field

It is expected that running with either no magnetic field, or with very low magnetic field will be available for alignment studied of the detectors. At this point in time, it is not clear if the anticipated electromagnetic backgrounds during this running will allow operation of the CDC, or some fraction of the CDC. The following is predicated on being able to operate some fraction of the CDC during this running. From the perspective of the CDC commissioning, the primary purpose of this running will be for alignment, both of the CDC itself, and especially of the CDC with respect to the forward drift chambers (FDC). For the latter activity, the location of the scattering target should be upstream of its nominal position, perhaps near the up-stream end of the CDC.

1. Straight tracks originating from a well-defined target location in the CDC can be used to tune the locations of the wires found from cosmic events.
2. Straight tracks provide the optimal way to align the CDC with the FDC, both possible rotations of the detectors with respect to each other, but also as a check of the relative positions along the beam axis. The events necessary to calibrate the FDC alone will be sufficient to also carry out these alignment studies between the two chamber systems.

Non-zero Magnetic Field

In Fall of 2014, the CDC will be operated in magnetic field for the first time. Two big changes will occur relative to running in zero magnetic field, both of which have been carefully simulated using the GARFIELD program. First, the trajectories of the ionization electrons produced along the primary track will follow a curved path as they drift to the wire at the center of the straw tube. Thus, for a given radius of the primary track, the signal will take longer to reach the CDC, and a different time-to-distance look-up table will need to be used. Second, the magnetic field is not uniform along the length of the CDC, and these variations will affect the time-to-distance relations as well. Corrections for this effect based on the known magnetic field and GARFIELD studies have been built into the reconstruction code, but these also need to be checked.

1. Do all channels function in magnetic field. This can be determined from the first hour of beam.
2. Electromagnetic background levels in the CDC. This involves looking at raw hits in the detector as a function of layer in the CDC. This can happen with the first day of beam.
3. Basic low-level performance studies of the CDC in magnetic field. For reconstructed tracks, drift-time distributions, amplitude distributions, occupancy of the straws, occupancy of the detector. This will require at least one day worth of data.
4. Check and optimize the CDC alignment with tracks in magnetic field. Particularly those tracks that are seen by other detectors.
5. Calibrate the CDC by optimizing the choice of look-up-table for the data. This will be performed by reconstructing tracks in the CDC and optimizing on the tracking chi-square between the hit positions and the reconstructed track position. This will require several days of data.
6. Calibrate the magnetic-field dependent effects in the chamber, this will require at least a week of data to assess.

Calibration and Detector Performance during Normal Running

During normal experimental running, the main calibration effort will be making sure that the correct time-to-distance look-up-table is used for reconstruction. Verification of the correct choice can be made by looking at the the tracking chi-square between the hit positions and the reconstructed track positions. This can be run on normal data, possibly with some preselection by the level-3 software trigger towards events with long tracks in the CDC. The external factors that can affect the choice of the look-up-table are the density of the gas in the CDC, the exact high-voltage setting for the wires, and the magnetic field in the CDC. The gas density is probably the most dynamic. It depends on the atmospheric pressure in the Hall and the temperature of the chamber gas. Both of these are monitored variables that will be placed in the data stream at regular intervals with the period being measured in minutes. It is expected that the reconstruction software will ultimately follow these variables and mak dynamic corrections during processing. The tracking chi-square will be used as a variable to both tune this procedure, and monitor the quality of the reconstruction.

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