tpc design
DESCRIPTION
TPC Design. Howard Wieman CCAST Student Lecture. Topics. TPC what it does TPC How it works Electron drift in gas Wire chamber/pad readout. BRAHMS. PHOBOS. PHENIX. STAR. TPC. I will cover TPC technology, but examples will be for the STAR TPC at RHIC - PowerPoint PPT PresentationTRANSCRIPT
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TPC
I will cover TPC technology, but examples will be for the STAR TPC at RHIC
The TPC is the main detector in the STAR experiment
STAR
BRAHMS
PHENIX
PHOBOS
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TPC what it does
Measures many of the charged particles radiating from the collision point» Measures momentum
vectors of the particles. This is done by electronically recording high resolution space points along the particle path as it curves through a magnetic field
» Determines the particle species from the density of ionization left by the particle as they travel through gas
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TPC – How it works – 3D from 2D The TPC is basically an empty volume of gas
with an electric field High velocity particles leave a trail of
ionization (electrons and positive ions) as they pass through the gas
The particles are detected using the released electrons they leave in their path
The electrons drift in through the gas driven by the electric field (typical drift velocities – centimeters per s)
The drifting electrons are detected at the end cap with a 2D array of detectors
Two dimensions of the path are obtained by the 2D pattern traced on the end cap
The third dimension is obtained from the arrival time, the time it takes the electrons to drift from the track creation point to the end cap detectors
The start time is know from the accelerator bunch timing or from fast detectors such as scintillators which detect the fast particles thus identifying the time of interaction
The end time is determined by the end cap array of detectors
The positive ions drift much slower ( ½ sec) than the electrons (10s of sec)
» The positive ions are not detected but they do affect TPC performance – a subject to be addressed later in this talk
Path of ionization left by a particle radiating from the collisionElectrons greenPositive ions red
E
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Topics to be covered xxxxx
overview» 3D tracking with dE/dx for particle ID
generating the signal track point reconstruction limits on momentum resolution
» multiple scattering » accuracy of track reconstruction
– readout resolution– distortions in the electron drift path
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More topicsxxxxxx
particle identification with dE/dx» limitations due to ionization fluctuations
front-end low noise electronics
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An excelent reference on signal generation and readout elelectronics
Ann. Rev. Nucl. Part. Sci. 1988. 38: 217
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TPC Capability
Tracking, momentum reconstruction for 4000 charged particles in 0<||<1.8p/p = 1.5% low p, p/p = 3% at 10 GeV/c
Particle ID by dE/dx, 6.7%
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Why we need a TPC
to get particle momentum and particle identification in a high track density environment one needs to over sample, i.e. lots of 3D pixels
the TPC provides by far the lowest cost per pixel of any detector
it does this by recording 3D space with 2D hardware
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gas gain amplification at the anode wire on the sector
avalanche in the high fieldregion near theanode wire
Er
2
1
0
r m10
E = ~200 kV/cm
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current induced in the wire as the avalanche positive ions drift away from
the wire
I tq
a b t t
ln
1
0
I(t) q(t)
the “one over t tail”
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charge induced on the pad plane surface by a line charge at the wire
location
a good measureof the true surfacecharge generated by the avalanchepositive ions
x
D x D
2
1
cosh
+
x
D
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pad signal as a function of distance between avalance and pad center
(pad response function)
0
x
ex
2
22
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extracting track position
the avalanche position is extracted assuming a gaussian pad response function if the cluster is narrow (~3 pads wide)» i.e. the track is close to perpendicular to
the wires ( = ~0) a weighted mean is used for wider
clusters where is larger
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sources of error in the hit position determination
electron cloud width due to diffusion while drifting
tan() effect due to clustering or non uniform deposition of charge along the track
EXB term in the drift velocity as the electrons approach the anode wire
electronic noise of the amplifier Total r direction error in STAR
TPC:
z zm
cmT
215
210 31cm mm .
20:1 signal/noise center pad
~ 500 m
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gating grid
reduces drift distorting space charge by preventing avalanche positive ions from reaching the drift volume
reduces wire aging by preventing electrons from non-trigger events from reaching the MWPC
symmetric + - on alternate wires largely prevents induced signals on the wires and pads
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measuring momentum
tracking in a magnetic field
sources of error» multiple coulomb
scattering» errors in hit
reconstruction» global distortion in
the electron drift path