Drift chambers detect ionization of gas atoms by charged particles. By applying high voltage between wires and foils, ionized electrons can be collected, used to generate gain through an avalanche , and electronically amplified and discriminated to generate logical signals.
In the VDCs (vertical drift chambers), the readout anode wire is between two two high voltage cathode planes. Particles generally pass through the cells at about a 45o angle. Because the cell is narrow, from wire to wire, but relatively high from wire to plane, most of the ions / electrons are generated far from the wires, and drift vertically towards them. Typically, particles pass through four or five cells, and, in the center one, pass very close to the wire. Thus, this geometry leads to a drift time spectrum that has a large prompt peak, followed by a long flat plateau about 200 ns wide.Because each particle should give several signals, the VDCs should be highly efficient for detecting particles. Wires in well comissioned wire chambers are generally no more than 1 or 2% inefficient. Tracking efficiencies in the VDCs generally run about 85%, if one restricts the analysis to clean events.
Wire chambers tend to be more sensitive devices than scintillators, both because they also have a gas supply system, and because there are a large number of electronics channels, which can lead to RF pickup and noise problems. These problems are generally identified and rectified during the commissioning, but subtle changes in, e.g., positioning of the readout cables, can lead to previously unseen noise pickup. Too, twisted pair cable connectors are sensitive. Pulling them can lead to internal shorts in the wires that lead to misshapen or lost signals, and nonlocking connectors can easily be not quite plugged into pins, so that signals from one or more channels on a cable are lost.
As part of the regular (each shift) maintenance, chamber gas supplies should be checked, and bottles should be replaced when they are about 90% used. The procedure for changing gas bottles can be found here . Gas output flow can be found through the Hall A EPICS screens; look under detectors / vdcs. High voltage for the VDCs can also be found through the Hall A EPICS screens; it is at the bottom of the Hall A Summary screen, or you can look under detectors / vdcs. Other things that can go wrong include:
The basic technique for checking the VDCs during runs is to look at histograms. This can be done with DHIST and / or ESPACE.
One standard set of histograms is the wire map , which shows the number of hits versus wire number for a plane of wires. Generally, one expects a smooth distribution, with edge wires only getting a small fraction of the number of hits of central wires. Hot wires have a large number of counts compared to their neighbors, usually as a result of some noise pickup, bad connection, or low threshold. Dead wires have very low efficiency, and usually result from a bad electrical connection or component in the circuitry.
Another standard histogram is the drift time distribution. This has been described above.
One important statistic that can be seen in the ESPACE cuts is the fraction of events that have a good track. This number is the single most significant indication in replay that the system is working properly.
Created October 21, 1996 Ronald Gilman