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detectors [2013/12/26 12:55] pereira [Cathode Readout Drift Chambers (CRDC)]		detectors [2013/12/26 12:57] pereira [Cathode Readout Drift Chambers (CRDC)]
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-	The principle of operation of a CRDC is illustrated in the figure below. The nuclei passing through the detector ionize the gas, dissociating electrons which drift towards an anode wire under the action of a vertical electric field. The collection of charge in the anode induces a positive charge on the cathode pads, which are read out individually. The //x// position is determined by fitting the charge distribution on the cathode pads with a Gaussian function. The drift time of the electrons to the anode wire, measured with respect to a trigger signal (typically from a scintillator), provides the //y// position. The resulting position resolution is less than 0.5 cm. Depending on the position of the track, the typical drift times of the electrons to the anode wires are 0 to 20 µs. The relatively long drift times limit the maximum rate that the detector can process properly to around 5000 counts per second. High rates affect also the aging of the anode wire. These problems can be partly amended by spreading the beam over a large portion of the active area, as it is done in focus mode. In addition, an electrostatic gate has been added above the Frisch grid of the CRDCs to enable the possibility to block primary electrons based on a trigger decision. This functionality is still under development, regarding in particular the recovery time of the preamplifiers after switching of the electrostatic gate. If successful, it should mitigate the effects of high rate on the CRDCs, by effectively reducing the rate of avalanches on the wire. Note that it will not eliminate pile-up effects. Pile-up effects can only be reduced by increasing the drift voltage (standard operating voltage: 800 V, maximum operating voltage: 2000 V).	+	The principle of operation of a CRDC is illustrated in the figure below. The nuclei passing through the detector ionize the gas, dissociating electrons which drift towards an anode wire under the action of a vertical electric field. The collection of charge in the anode induces a positive charge on the cathode pads, which are read out individually. The //x// position is determined by fitting the charge distribution on the cathode pads with a Gaussian function. The drift time of the electrons to the anode wire, measured with respect to a trigger signal (typically from the [[Plastic scintillators\|E1 scintillator]]), provides the //y// position. The resulting position resolution is less than 0.5 cm. Depending on the position of the track, the typical drift times of the electrons to the anode wires are 0 to 20 µs. The relatively long drift times limit the maximum rate that the detector can process properly to around 5000 counts per second. High rates affect also the aging of the anode wire. These problems can be partly amended by spreading the beam over a large portion of the active area, as it is done in focus mode. In addition, an electrostatic gate has been added above the Frisch grid of the CRDCs to enable the possibility to block primary electrons based on a trigger decision. This functionality is still under development, regarding in particular the recovery time of the preamplifiers after switching of the electrostatic gate. If successful, it should mitigate the effects of high rate on the CRDCs, by effectively reducing the rate of avalanches on the wire. Note that it will not eliminate pile-up effects. Pile-up effects can only be reduced by increasing the drift voltage (standard operating voltage: 800 V, maximum operating voltage: 2000 V).

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