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detectors [2013/10/17 09:27]
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detectors [2013/10/17 09:40]
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| Some experiments are particularly sensitive to the incoming positions and angles of the nuclei impinging on the target. Two tracking parallel plate avalanche counters (PPAC) are installed in the [[Stations#Intermediate Plane|intermediate plane station]] of the [[Introduction#Analysis Line|analysis line]]. The position and angles measured with both PPACs are transformed into the corresponding coordinates in front of the target, using the transfer matrix of the second half of the analysis line. The analysis line [[Magnets#Spectrograph Dipole|dipole magnets]] downstream of the intermediate image plane filter the particles produced in the tracking detectors, which would otherwise contaminate the data.\\ | Some experiments are particularly sensitive to the incoming positions and angles of the nuclei impinging on the target. Two tracking parallel plate avalanche counters (PPAC) are installed in the [[Stations#Intermediate Plane|intermediate plane station]] of the [[Introduction#Analysis Line|analysis line]]. The position and angles measured with both PPACs are transformed into the corresponding coordinates in front of the target, using the transfer matrix of the second half of the analysis line. The analysis line [[Magnets#Spectrograph Dipole|dipole magnets]] downstream of the intermediate image plane filter the particles produced in the tracking detectors, which would otherwise contaminate the data.\\ |
| Each PPAC has an active area of 10 cm x 10 cm and is filled with isobutane at a typical __pressure of 5 torr__. The detector consists of a cathode foil with a series of aluminum strips oriented in the non-dispersive direction, followed by an anode plate and a second cathode foil with the strips oriented in the dispersive direction (see [[Fig xxx]]). A total of 128 pads are connected to the strips of each cathode foil. The x and y positions are determined from the charge distribution on the pads. The position calibration was done using the pad pitch of 1.27 mm.\\ | Each PPAC has an active area of 10 cm x 10 cm and is filled with isobutane at a typical __pressure of 5 torr__. The detector consists of a cathode foil with a series of aluminum strips oriented in the non-dispersive direction, followed by an anode plate and a second cathode foil with the strips oriented in the dispersive direction (see [[Fig xxx]]). A total of 128 pads are connected to the strips of each cathode foil. The x and y positions are determined from the charge distribution on the pads. The position calibration was done using the pad pitch of 1.27 mm.\\ |
| The particles transmitted through the PPAC ionize the gas, producing electrons and positive ions. The drift of electrons towards the central anode plane induces an image charge on the aluminum strips. The signal generated on a given pad is sent to a preamplifier, and processed by a switch capacitor array (SCA), which acts as an analogic memory. Since the signals of this detector are generated before a valid trigger occurs, they need to be temporarily recorded until the trigger is received and a proper correlation can be established. The SCA samples the signals from the detector with a period of 200 ns and saves the data on a continuous mode, generating an analogic buffer. When a valid trigger is received, the sampling stops and the SCA pointer moves back in the buffer by a number of samplings pre-defined according to the time passed between the tracking signal and the valid trigger. In this way, the valid trigger is correlated with the tracking signal corresponding to the same event. The reading algorithm is lead by an XLM72 FPGA module like the one used with the CRDCs. The PPACs can work at maximum rates in the range 0.1 – 1 MHz. The efficiency is significantly reduced for light nuclei (typically below Z=10). | The particles transmitted through the PPAC ionize the gas, producing electrons and positive ions. The drift of electrons towards the central anode plane induces an image charge on the aluminum strips. The signal generated on a given pad is sent to a preamplifier, and processed by a switch capacitor array (SCA), which acts as an analogic memory. Since the signals of this detector are generated before a valid trigger occurs, they need to be temporarily recorded until the trigger is received. The SCA samples the signals from the detector with a period of 200 ns and saves the data on a continuous mode, generating an analogic buffer. When a valid trigger is received, the sampling stops and the SCA pointer moves back in the buffer by a number of samplings pre-defined according to the time passed between the tracking signal and the valid trigger. In this way, the valid trigger is correlated with the tracking signal corresponding to the same event. The reading algorithm is lead by the FPGA chips of a XLM72 VME module. The PPACs can work at maximum rates in the range from 100 KHz to 1 MHz. The efficiency is significantly reduced for light nuclei (typically below Z=10). |
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| Fig xxx illustrates the principle of operation of a CRDC. 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 s. 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. | Fig xxx illustrates the principle of operation of a CRDC. 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 s. 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. |
| Both CRDCs are equipped with digital electronics, which consist of of seven front-end electronic boards (FEE) designed and developed by the STAR collaboration (RHIC), followed by interface boards connected to a programmable FPGA VME module (XLM72). Each FEE includes 32 channels of preamplifier shaper, followed by a switch capacitor array (SCA) and an ADC xxx. The processing of signals is driven by the FPGA module. Each SCA samples the signals after a valid trigger is received and sends the information into and ADC. The digitized data are then stored into the internal memory of the FPGA and read out in block mode. The sampling frequency and number of samples read out are adjustable; typical values are 20 MHz and 8 to 12 samples. The time needed for each sampling is around 16 s. Thus, the dead time of the electronics is directly proportional to the number of samples read out. The main advantage of the on-detector digitalization technique used with the CRDCs is the reduction of noise by avoiding the transmission of analog signals (448 from the two CRDCs) outside the vacuum chamber, and the possibility to record multi-hit events like in traditional TPC detectors. | Both CRDCs are equipped with digital electronics, which consist of of seven front-end electronic boards (FEE) designed and developed by the STAR collaboration (RHIC), followed by interface boards connected to a programmable FPGA VME module (XLM72) like the one used with the PPACs in the intermediate image station. Each FEE includes 32 channels of preamplifier shaper, followed by a switch capacitor array (SCA) and an ADC xxx. The processing of signals is driven by the FPGA module. Each SCA samples the signals after a valid trigger is received and sends the information into and ADC. The digitized data are then stored into the internal memory of the FPGA and read out in block mode. The sampling frequency and number of samples read out are adjustable; typical values are 20 MHz and 8 to 12 samples. The time needed for each sampling is around 16 s. Thus, the dead time of the electronics is directly proportional to the number of samples read out. The main advantage of the on-detector digitalization technique used with the CRDCs is the reduction of noise by avoiding the transmission of analog signals (448 from the two CRDCs) outside the vacuum chamber, and the possibility to record multi-hit events like in traditional TPC detectors. |
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