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detectors [2015/04/12 13:30]
pereira [Cathode Readout Drift Chambers (CRDC)] |
detectors [2015/04/21 17:45]
pereira [Tracking Parallel Plate Avalanche Counters (TPPAC)] |
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| ===== Plastic scintillators ===== | ===== Plastic scintillators ===== |
| In order to determine the Time-Of-Flight for the particle identification, the S800 includes a plastic scintillator at the [[Stations#Object|object station]] (S800_OBJ) and at the [[Stations|focal-plane station]] (E1). The detector material typically used is | In order to determine the Time-Of-Flight for the particle identification, the S800 includes a plastic scintillator at the [[Stations#Object_station|object station]] (S800_OBJ) and at the [[Stations#focal_plane_station|focal-plane station]] (E1). The detector material typically used is |
| [[http://www.detectors.saint-gobain.com/uploadedFiles/SGdetectors/Documents/Product_Data_Sheets/BC400-404-408-412-416-Data-Sheet.pdf|BC-400]] or [[http://www.detectors.saint-gobain.com/uploadedFiles/SGdetectors/Documents/Product_Data_Sheets/BC400-404-408-412-416-Data-Sheet.pdf|BC-404]] made from polyvinyltoluene (>97% ) and organic fluors (<3%) with a density 1.032 g/cm<sup>3</sup> and a refractive index 1.58. The thickness of the detectors is chosen on the basis of the charge of the nuclei to be measured. The available thicknesses are __127 μm and 1 mm__ for OBJ_SCI and __1 mm and 5 mm__ for E1. The OBJ_SCI has an active area of __xxx__ and is connected to a photomultiplier __xxx__. The E1 scintillator is connected to photomultipliers [[EMI 98807B]] in both ends (up and down). The time signal from the E1 scintillator is calculated as the average time signal from each photomultipliers. | [[http://www.detectors.saint-gobain.com/uploadedFiles/SGdetectors/Documents/Product_Data_Sheets/BC400-404-408-412-416-Data-Sheet.pdf|BC-400]] or [[http://www.detectors.saint-gobain.com/uploadedFiles/SGdetectors/Documents/Product_Data_Sheets/BC400-404-408-412-416-Data-Sheet.pdf|BC-404]] made from polyvinyltoluene (>97% ) and organic fluors (<3%) with a density 1.032 g/cm<sup>3</sup> and a refractive index 1.58. The thickness of the detectors is chosen on the basis of the charge of the nuclei to be measured. The available thicknesses are __127 μm and 1 mm__ for OBJ_SCI and __1 mm and 5 mm__ for E1. The OBJ_SCI has an active area of __xxx__ and is connected to a photomultiplier __xxx__. The E1 scintillator is connected to photomultipliers [[EMI 98807B]] in both ends (up and down). The time signal from the E1 scintillator is calculated as the average time signal from each photomultipliers. |
| Different Time-of-flights can be constructed by combining the timing signals from these two detectors with the timing signals from the [[https://groups.nscl.msu.edu/a1900/|A1900]] focal plane, and the RF cyclotron. The E1 detector is also used to define a valid trigger from the S800. The timing resolution for a point-like beam spot in the focal plane is around 100 ps. However, this resolution worsens significantly (up to 1 ns) when the whole focal plane is illuminated, because of path length differences of the traversing nuclei. It can be recovered by tracking the position of each event on the scintillator from the position and angle information provided by the [[Detectors#Cathode Readout Drift Chambers (CRDC)|CRDC]] detectors. The plastic scintillators can withstand maximum rates up to 1 x 10<sup>6</sup> particles per second. | Different Time-of-flights can be constructed by combining the timing signals from these two detectors with the timing signals from the [[https://groups.nscl.msu.edu/a1900/|A1900]] focal plane, and the RF cyclotron. The E1 detector is also used to define a valid trigger from the S800. The timing resolution for a point-like beam spot in the focal plane is around 100 ps. However, this resolution worsens significantly (up to 1 ns) when the whole focal plane is illuminated, because of path length differences of the traversing nuclei. It can be recovered by tracking the position of each event on the scintillator from the position and angle information provided by the [[Detectors#Cathode Readout Drift Chambers (CRDC)|CRDC]] detectors. The plastic scintillators can withstand maximum rates up to 1 x 10<sup>6</sup> particles per second. |
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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 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, which 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 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 (dispersive direction) 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 (non-dispersive direction). 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, which can only be reduced by increasing the drift voltage (standard operating voltage: 800 V, maximum operating voltage: 2000 V). |
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