| Both sides previous revision
Previous revision
Next revision
|
Previous revision
Next revision
Both sides next revision
|
detectors [2013/12/26 12:53]
pereira [Cathode Readout Drift Chambers (CRDC)] |
detectors [2013/12/26 12:59]
pereira [Cathode Readout Drift Chambers (CRDC)] |
| |
| ===== Cathode Readout Drift Chambers (CRDC) ===== | ===== Cathode Readout Drift Chambers (CRDC) ===== |
| Two Cathode Readout Drift Chamber (CRDC) are used to measure the transversal positions and angles in the [[Stations#Focal Plane station|focal plane]]. The first detector (CRDC1) is located at the nominal optical focal plane, and it is separated 1 m from the second downstream detector (CRDC2). Each detector has an active depth of 1.5 cm, an active area of 26 cm (non-dispersive direction) x 56 cm (dispersive direction), and [[Gas handling system|it is filled]] with a gas mixture consisting of 80% CF<sub>4</sub> and 20% C<sub>4</sub>H<sub>10</sub> at a typical pressure of 50 torr. The [[HV bias#CRDCs|operating high power depends on the charge of the measured nuclei. A schematic view of a CRDC can be seen in the figure below. | Two Cathode Readout Drift Chamber (CRDC) are used to measure the transversal positions and angles in the [[Stations#Focal Plane station|focal plane]]. The first detector (CRDC1) is located at the nominal optical focal plane, and it is separated 1 m from the second downstream detector (CRDC2). Each detector has an active depth of 1.5 cm, an active area of 26 cm (non-dispersive direction) x 56 cm (dispersive direction), and [[Gas handling system|it is filled]] with a gas mixture consisting of 80% CF<sub>4</sub> and 20% C<sub>4</sub>H<sub>10</sub> at a typical pressure of 50 torr. The [[HV bias#CRDCs|operating high power]] depends on the charge of the measured nuclei. A schematic view of a CRDC can be seen in the figure below. |
| |
| {{:wiki:crdc-drawing.jpg?600|Schematic view of the two S800 CRDCs.}} | {{:wiki:crdc-drawing.jpg?600|Schematic view of the two S800 CRDCs.}} |
| |
| |
| The principle of operation of a CRDC is illustrated in the figure. 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 [[Detectors#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). |
| |
| |
