Forward Drift Chamber Commissioning

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The main procedures and configurations needed for the commissioning of the Forward Drift Chamber (FDC) system of the GlueX experiment are described below. The document assumes a certain knowledge of the detector as described in the Technical Construction Report.

Commissioning activities before the beam run

All the chambers have been tested and checked channel-by-channel multiple times during the production. Each of the four packages, after its completion, has been tested in horizontal position triggering on cosmic events with scintillator paddles. For one of the packages (the third) this test was performed with all the channels connected to the electronics.

The FDC is the only detector in the GlueX experiment that is not serviceable in its final position: once installed in the bore of the magnet, there's no access to the detector, front-end electronics, cable connections, grounding, gas and cooling system connections etc.. Pulling the FDC out of the magnet is a major operation requiring also removing of the target, the Start Counter, and the Central Drift Chamber (CDC). Therefore, all the channels have been tested multiple times during the installation and after the detector has been installed.

The main challenge is that the chambers are in vertical position and only basic performance features can be tested with cosmic rays. Triggering the DAQ system is also a problem. The FDC back-end electronics, fADC125, and F1TDCV3 modules do not allow for self triggering. So far only a trigger-less DAQ system (i.e. with a random trigger) has been used to collect data from the detector reading all the samples from the fADC125 modules. To minimize the huge amount of data, sparsification has been done in the ROCs of the crates. All this required weeks of data taking in order to have some statistics that allows to evaluate the performance of all the channels.

The following activities are expected to be done before the start of the photon beam run:

• Check the operation of the gas system, cooling system, HV and LV systems and make sure all the interlocks, trips, and alarms are working as required.
• Keep the FDC under HV and study the long term stability of the detector. Correlate the results with changes in the atmospheric pressure and ambient temperature, and try to optimize the detector performance.
• Study the noise observed on some of the FDC channels as installed. The study needs data from the DAQ but does not require cosmic events. Measure the width of the pedestals. Correlate the noise with the one observed on the CDC and try to find and suppress the source of the noise.
• Take cosmic data from all the channels. Measure the pedestals. Optimize the thresholds. Troubleshoot faulty electronics and cable connections. We assume that we will use a trigger from the Barrel Calorimeter made as a coincidence of the top and bottom halves of the detector. We expect also that during this commissioning phase, the firmware for the fADC125 will be developed so that the data sparsification is done on the module itself.

Commissioning and alignment with photon beam

We are scheduled for a three week beam commissioning in November-December 2014. We expect significant amount of time to be devoted to the tuning and aligning of the photon beam in the Hall, for which the FDC is needed. Along the beam axis the FDC chambers have a $2.6$~cm diameter low-mass area, and a $6$ to $7.8$~cm diameter insensitive area. The photon beam has to be aligned through these areas by minimizing the background rates on the central strips. During these studies one has to adjust the HV to avoid high currents on the wires.

The beam run will allow us to check all the characteristics of the FDC and precisely align the chambers. We anticipate to study the detector performance without and with magnetic field, with different gas mixtures, as well as using targets at different positions. Therefore, we expect that for the FDC, this running period will be dominated by configuration changes rather than the actual beam-on-target time. Different studies will be done with zero and non-zero magnetic filed that are summarized below.

Studies With No Magnetic Field

It is expected that running with either no magnetic field, or with very low magnetic field will be available. By that time the beam should be well tuned to allow for the operation of the FDC at the nominal HV. Collect data with a thin target in a position placed as far as possible upstream from the detector (to be decided where exactly). Two gas mixture will be used: 90/10 Ar/CO2 and the nominal one 40/60. The first gas mixture has almost linear time-to-distance function allowing for a better chamber alignment.

• Measure the pedestals of all fADC125 channels. Calibrate the strip gains. Reconstruct the wire positions using cathode strip information only. Check the strip resolution at different HV and compare with previous results.
• Adjust the wire threshold. Collect drift time spectra and compare with previous results. Determine the "zero-time" for all the TDC channels.
• Use the straight tracks coming from the target to determine the offset, pitch and rotation corrections for the cathode plane in each package, using straight tracks coming from the target.
• Adjust the time-to-distance function. Estimate the wire resolution.
• Determine the relative positions of all the packages also with respect to the CDC.

Studies With Magnetic Field

The magnetic field will affect the time-to-distance function, and will offset the avalanche position that is seen by the cathode strips. These two effects will be studied with the standard gas mixture of 40/60 Ar/CO2 to make sure they are well described by the Garfield calculations. The ultimate goal is to correct for the above effects and achieve a tracking resolution that is compatible to the one obtained without magnetic field. The target is supposed to be at the nominal position.

Expected Rates and Beam Time Request

At 10^-4 R.L. radiator, 50nA electron beam and 5mm collimator, we expect 5x10^7 Hz photons with E>1GeV. With 2mm plastic target (at the vacuum exit after pair spectrometer) we expect 180 Hz total charged particles.

Assuming ~18Hz rate in 2 beam-on-target days we will have 3M events, needed for alignment (~1M was almost enough to align 3rd package).

Trigger rate???, raw sample mode allows for max 30Hz readout.