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--- detectors [2013/12/13 18:19]
pereira
+++ detectors [2013/12/13 18:37]
pereira [Tracking Parallel Plate Avalanche Counters (TPPAC)]
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 ====== Detectors ======
-The standard detection system of the S800 consists of a [[Detectors#Plastic scintillators|plastic scintillator]] at the object station; two tracking detector at the intermediate plane station ([[Detectors#Tracking Parallel Plate Avalanche Counters (TPPAC)|TPPAC]]) and a series of detectors at the focal plane station (see figure below), which include two cathode readout drift chambers ([[Detectors#Cathode Readout Drift Chambers (CRDC)|CRDC]]) [[[5]]] located about 1 m apart; a [[Detectors#Ionization Chamber|multi-segmented ionization chamber]], a thin [[Detectors#Plastic scintillators|plastic scintillators]] and a [[Detectors#Hodoscope|Hodoscope]] [[[6]]].
+The standard detection system of the S800 consists of a [[Detectors#Plastic scintillators|plastic scintillator]] at the object station; two tracking detector at the intermediate plane station ([[Detectors#Tracking Parallel Plate Avalanche Counters (TPPAC)|TPPAC]]) and a series of detectors at the focal plane station (see figure below), which include two cathode readout drift chambers ([[Detectors#Cathode Readout Drift Chambers (CRDC)|CRDC]]) located about 1 m apart; a [[Detectors#Ionization Chamber|multi-segmented ionization chamber]], a thin [[Detectors#Plastic scintillators|plastic scintillators]] and a [[Detectors#Hodoscope|Hodoscope]].
-{{:wiki:s800-fp-detectors.jpg?800|S800 detector station at the Focal Plane.}}
+{{:wiki:s800-fp-detectors.jpg?650|S800 detector station at the Focal Plane.}}
@@ Line 16: / Line 16: @@
 Some experiments are particularly sensitive to the incoming positions and angles of the nuclei impinging on the target. Two tracking parallel plate avalanche counters (TPPAC) are installed in the [[Stations#Intermediate Plane|intermediate plane station]] of the [[Introduction#Analysis Line|analysis line]]. The position and angles measured with both TPPACs 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 TPPAC 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 TPPAC 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. 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 TPPAC 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 [[http://wwwp.cord.edu/dept/physics/mona/manuals/XLM72UM.pdf|XLM72]] VME module. The TPPACs can work at maximum rates in the range from
@@ Line 24: / Line 24: @@
 ===== Cathode Readout Drift Chambers (CRDC) =====
-Two Cathode Readout Drift Chamber (CRDC) are used  to measure the transversal positions and angles in  the 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 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 operating high power depends on the charge of the measured nuclei.(e.g. __Xxx for xxx, and xxx for xxx__). A schematic view of a CRDC can be seen in [[Fig xxx]]. Each detector consists of two windows mounted on frames, two printed circuit boards (PCB) and an anode frame. Each PCB is made of un-masked G-10, and includes a field shaping foil to ensure a uniform field in the active region of the detector. Two G-10 spacers are laminated to the board on each side. The shaping foils are made of 1.9-mm pitch evaporated aluminum strips perpendicularly oriented to the electric field. The anode frame includes a glued cathode grounding plane, an anode wire running across the field, and a Frisch grid. Cathode pads are located in front of and behind the anode wire. The pads have a pitch of 2.54 mm. The anode frame is sandwiched between the two printed circuit boards with two spacers in between, as shown in [[Fig. xxx]].
+Two Cathode Readout Drift Chamber (CRDC) are used  to measure the transversal positions and angles in  the 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 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 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.}}
+Each detector consists of two windows mounted on frames, two printed circuit boards (PCB) and an anode frame. Each PCB is made of un-masked G-10, and includes a field shaping foil to ensure a uniform field in the active region of the detector. Two G-10 spacers are laminated to the board on each side. The shaping foils are made of 1.9-mm pitch evaporated aluminum strips perpendicularly oriented to the electric field. The anode frame includes a glued cathode grounding plane, an anode wire running across the field, and a Frisch grid. Cathode pads are located in front of and behind the anode wire. The pads have a pitch of 2.54 mm. The anode frame is sandwiched between the two printed circuit boards with two spacers in between, as shown in the figure below.
+{{:wiki:crdc-section.jpg?600|Cross section of a 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.
+{{:wiki:crdc-section-drift.jpg?600|Principle of operation of a CRDC.}}
-[[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 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.
 Both  CRDCs are equipped with digital electronics, which consist of seven front-end electronic boards (FEE) designed and developed by the [[http://www.star.bnl.gov/|STAR collaboration]] ([[http://www.bnl.gov/rhic/|RHIC]]), followed by interface boards connected to a programmable FPGA VME module ([[http://wwwp.cord.edu/dept/physics/mona/manuals/XLM72UM.pdf|XLM72]]) like the one used with the [[Detectors#Tracking Parallel Plate Avalanche Counters (TPPAC)|TPPACs]] 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 the 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 µs. 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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