09/01/2008ron settles mpi-munich tsinghua tpc school jan.2008 1 tsinghau/ccast tpc school january...
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Tsinghau/CCAST TPC SchoolJanuary 2008
Experience with the Aleph TPC
(and other things)Ron Settles MPI-Munich/Desy
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OutlineOutline
•What physics do we want to do, What physics do we want to do, where?where?
•What is the best detector?What is the best detector?
•TPC and the Aleph TPCTPC and the Aleph TPC
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International Linear ColliderWhere?: Technology decision: COLD superconducting à la TESLA chosen
• Baseline: 200 GeV < √s < 500 GeV Integrated luminosity ~ 500 fb-1 in 4 years80 % e- beam polarisationUpgrade to 1TeV, L = 1 ab-1 in 3 years2 interaction regionsConcurrent running with the LHC from 2015
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The Global Design Effort
Formal organization begun at LCWS 05 at Stanfordin March 2005 when Barry became director of the GDE
Technically Driven Schedule
ACFA’07 Beijing:
RDR(+cost), DCR
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Physics we want to do?Physics we want to do?•Keisuke gave a nice overview yesterdayKeisuke gave a nice overview yesterday•For example, For example, from my talk at Arlington WS Jan.2003:
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2018
2019
2020
2024
+y
2027
+y+SF
25
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Where are we with the Higgs? CERN Courier, Nov 2005
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Very latest electroweak combinations:
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Why do we think these indirect, precision meas. are telling us anything??? CERN Courier, Nov 2005 :
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The value of precision measurements…
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Polarization
Multipole expansion
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…the Higgs?
…the Higgs?
cmE recHM Expt (GeV) Decay
Channel (GeV/c2) ln(1+s/b)
115 GeV/c2
1 ALEPH 206.7 4-jet 114.3 1.73
2 ALEPH 206.7 4-jet 112.9 1.21
3 ALEPH 206.5 4-jet 110.0 0.64
4 L3 206.4 E-miss 115.0 0.53
5 OPAL 206.6 4-jet 110.7 0.53
6 Delphi 206.7 4-jet 114.3 0.49
7 ALEPH 205.0 Lept 118.1 0.47
8 ALEPH 208.1 Tau 115.4 0.41
9 ALEPH 206.5 4-jet 114.5 0.40
10 OPAL 205.4 4-jet 112.6 0.40
And in addition we have LEP events…And in addition we have LEP events…
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LCs
But this all may be a fata morgana…
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75% Dark matter
25%
Baryons
Speed of light in the filaments is slower than in the voids. Take this into account, ‘dark energy’ is a fata morgana? And in reality…?
Is this true??
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•What is the best detector?What is the best detector?
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High precision tracking…
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Highly efficient tracking, high granularity calorimetry…
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LDC/GLD=ILD Conceptor
A TPC for a Linear Collider Detector
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22GLD
LDC
HCalECal
TPC
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Now 2x10-5/(GeV/c)
Now 0.25/E @ ZpeakParticle Flow
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The Aleph Time Projection Chamber
The Aleph Time Projection Chamber
Ron Settles, MPI-Munich/DESY(talk at Mike Ronan’s
TPC Symposium@LBNL 2003)
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• TPC is a 3-D imaging chamber– Large volume, small amount of material.– Slow device (~50 s)
– 3-D ‘continuous’ tracking (xy 170 m, z 600 m for Aleph)
• Review some of the main ingredients • History
– First proposed in 1976 (Dave Nygren, PEP4-TPC)– Used in many experiments– Aleph as an example here– Now a well-established detection technique that is
still in the process of evolution…
SummarySummary
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OutlineOutline
• Examples• TPC principles of operation
– Drift velocity, Coordinates, dE/dx• TPC hardware ingredients
– Field cage, gas system, wire chambers, gating, laser calibration system, electronics
• The Aleph TPC• From the drawing board to the gadget• Performance• Some ‘features’ (i.e. trouble shooting…) • Conclusion
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Some TPC examplesSome TPC examples
TPC ReferencePEP4 PEP-PROPOSAL-004, Dec 1976TOPAZ Nucl. Instr. and Meth. A252 (1986) 423ALEPH Nucl. Instr. and Meth. A294 (1990) 121DELPHI Nucl. Instr. and Meth. A323 (1992) 209-212NA49 Nucl. Instr. and Meth. A430 (1999) 210STAR IEEE Trans. on Nucl. Sci. Vol. 44, No. 3 (1997)
Grand-daddy/mama of all TPCs
STAR FTPC
ALICE
ILD (future) …
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TPC principles of operationTPC principles of operation
A TPC contains:
– GasE.g.: Ar + 10-20 % CH4
– E-fieldE ~ few x 100 V/cm
– B-fieldas large as possible to measure momentum,to limit electron diffusion
– Wire chamber (those days)to detect projected tracks
y
z
x
E
B electron drift
chargedtrack
wire chamber to detect projected tracks
gas volume with E & B fields
Now trying out new techniques--►
•
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TPC Characteristics
– Only gas in active volume,small amount of material
– Long drift ( > 2 m ) therefore slow detector (~50 s)want no impurities in gasuniform E-fieldstrong & uniform B-field
– Track points recorded in 3-D(x, y, z)
– Particle Identification by dE/dx
– Large track densities possible
y
z
x
E
B drift
chargedtrack
0BE
•
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Drift velocity
22
2
)()()(
)(1 B
BBE
B
BEEvd
Drift of electrons in E- and B-fields (Langevin) mean drift time between collisions
me particle
mobility
mceB cyclotron
frequency1)( Vd along E-field lines
1)( Vd along B-field lines
Typically ~5 cm/s for gases like Ar(90%) + CH4(10%)
Electrons tend to follow the magnetic field lines () >> 1
•
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3-D coordinatesz
x
y
wire plane
track
projected track
–Z coordinate from drift time–X coordinate from wire number–Y coordinate?
• along wire direction• need cathode pads •
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Coordinate from cathode Pads
projected track
pads
drifting electrons
avalanche
y
x
y
z
– Measure Ai
– Invert equation to get y
)222)(( prwi
iyyAeA
Amplitude on ith pad
y avalanche position
iy position of center of ith pad
prw pad response width •
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TPC Coordinates: Pad Response Width
Normalized PRW: 2
2
2ˆ
prw
Distance between pads
is a function of: •
– the pad crossing angle • spread in r
– the wire crossing angle • ExB effect, lorentz angle
– the drift distance• diffusion
2̂
22 tan~ˆ
cos)tan(tan~ˆ 22
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TPC coordinate resolutionSame effects as for PRW are expected but statistics of • drifting electrons must be considered
zz
z
D
r
)(
cos)tan(tan
tan
),,(
2
222
22
2
0
2
electronics, calibration
angular pad effect (dominant for small momentum tracks)
angular wire effect
forward tracks -> longer pulses -> degrades resolution
)_(22 angledipzZ
“diffusion” term
(…disappears with new technologies…)
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Particle Identification by dE/dx
– Energy loss (dE/dx) depends on the particle velocity.
– The mass of the particle can be identified by measuring simultaneously momentum and dE/dx (ion pairs produced)
– Particle identification possible in the non-relativistic region (large ionization differences)
– Major problem is the large Landau fluctuations on a single dE/dx sample.
• 60% for 4 cm track
• 120% for 4 mm track
2)1(
2ln
1 22
2
22
J
mv
A
ZKz
dx
dE
Energy loss (Bethe-Bloch)
m mass of electron
vz, charge and velocity of incident particle
J mean ionization energydensity effect term
•
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TPC ingredients (Aleph example)
TPC ingredients (Aleph example)
• Wire chambers – Gating– Cooling– Mechanics
• Field cage• Gas system• Laser system• Electronics
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Wire Chambers3 planes of wires •
– gating grid– cathode plane (Frisch
grid)– sense and field wire
plane
– cathode and field wires at zero potential
pad size– various sizes &
densities– typically few cm2
gas gain– typically 3-5x103
pad plane
field wire
sense wire
gating grid
Drift region
cathode plane
V=0
x
z
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Wire Chambers: ALEPH36 sectors, 3 types •
– no gaps extend full radius
wires– gating spaced 2 mm – cathode spaced 1 mm – sense & field spaced 2
mm, interleaved
pads– 6.2 mm x 30 mm– ~1200 per sector– total 41004 pads
readoutpads and wires
•
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GatingProblem: Build-up of space charge in the drift region by ions.
– Grid of wires to prevent positive ions from entering the drift region
“Gating grid” is either in the open or closed state
– Dipole fields render the gate opaque •
Operating modes:– Switching mode (Aleph)– Diode mode
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Cooling, Mechanics
• Terribly mundane but terribly important (everything is important)
• Cooling:– Combined air and water cooling to completely
insulate the gas volume• Mechanics:
– 25% X_0 for sectors, preamps, cooling (but before cables)
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E-field produced by Field Cage
HV
Ewires at ground potential
planar HV electrode
potential strips encircle gas volume
–chain of precision resistors with small current flowing provides uniform voltage drop in z direction
–non uniformity due to finite spacing of strips falls exponentially into active volume
z
y
•
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Field cage: ALEPH exampleDimensions
cylinder 4.7 x 1.8 mDrift length
2x2.2 mElectric field
110 V/cmE-field tolerance
V < 6VElectrodes
copper strips (35 m & 19 m thickness, 10.1 mm pitch, 1.5 mm gap) on Kapton
Insulatorwound Mylar foil (75m)
Resistor chains2.004 M (0.2%)
Nucl. Instr. and Meth. A294 (1990) 121
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Laser Calibration SystemPurpose
Measurement of drift velocity Determination of E- and B-field distortions
Drift velocity Laser system ∂(v_drift) ~ 1‰ Hookup tracks to Vdet ∂(v_drift)~a few times 0.01‰ …used after Vdet installation
ExB Distortions Laser used only in early days to get firstcorrections. After, tracks (mostly μ pairs from Z decays) used exclusively (read on…)
Laser tracks in the ALEPH TPC
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Gas system
Properties:Drift velocity (~5cm/s)Gas amplification (~7000)Signal attenuation my electron attachment (<1%/m)
Parameters to control and monitor:Mixture quality (change in amplification)O2 (electron attachment, attenuation)
H2O (change in drift velocity, attenuation)
Other contaminants (attenuation)
Typical mixtures: Ar91%+CH49%
Ar93%+CH45%+CO22% Ar93%+CF43%+IsoB1%
Operation at atmospheric pressure
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Influence of Gas Parameters (*)
Parameterchange
Drif t velocity, vd Eff ect on gasamplifi cation, A
Signal ettenuation byelectron attachment
0.1% CH4 0.4 % -2.5% f or A = 1x104
10 ppm O2 Negligible up to 100 ppm Negligible up to 100 ppm 0.15%/ m of drif t
10 ppm H2O 0.5 % Negligible at 100 ppm < 0.03% / m of drif t
1 mbar Negligible if at max. -(0.5%-0.7%)
(*) from ALEPH handbook (1995)
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Electronics: from pad to storageTPC pad
amp
FADC
zerosuppression
featureextraction
DAQ
Pre-amplifiercharge sensitive, mounted on wire chamber
Shaping amplifier:pole/zero compensation. Typical FWHM ~200ns
Flash ADC:8-9 bit resolution. 10 MHz. 512 time buckets
Multi-event buffer
Digital data processing: zero-suppression.
Pulse charge and time estimates
Data acquisition and recording system
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Analog Electronics
ALEPH analog electronics chain
–Large number of channels O(105)–Large channel densities–Integration in wire chamber–Power dissipation–Low noise
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More details about Aleph…More details about Aleph…
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Wire Chambers: ALEPH
Long pads for better coordinate precision
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After 3 man-centuries
(or more, depending on how you count)…
From TPC90…↑
…as usual, lots of meetings…↓
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From the drawing board to the gadget…
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A Detector with TPC
…where…
you end up with-----------►
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Thanks to many people… (and to Pere Mato and Werner Wiedenmann for help on
these slides)
…you need a few cables, cooling, etc…
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It finally started working…
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ALEPH Event, early days…
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And towards the end…
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Coordinate Resolution(1): ALEPH TPC
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Coordinate Resolution(2): ALEPH TPC
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dE/dx: ResultsGood dE/dx resolution requireslong track lengthlarge number of samples/trackgood calibration, no noise, ...
ALEPH resolutionup to 334 wire samples/tracktruncated (60%) mean of samples4.5% (330 samples)
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But, there were ‘FEATURES’…
Werner’s talk contains many details, see alephwww.mppmu.mpg.de/~settles/tpc • here a few examples…
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Historical Development (1)• LEP start-up: 1989-1990
– Failure of magnet compensating power supplies in 1989 required development of field-corrections methods
• derived from 2 special laser runs (B on/off)• correction methods described in NIM A306(1991)446
– Later, high statistics Z->μμ events give main calibration sample• LEP 1: 1991-1994
– VDET 1 becomes operational in 1991– Development of common alignment procedures for all three
tracking detectors– Incidents affect large portions of collected statistics and require
correction methods based directly on data• 1991-1993, seven shorts on field cage affect 24% of data• 1994, disconnected gating grids on 2 sectors affect 20% of
data– All data finally recuperated with data-based correction
methods
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Historical Development (2)• LEP 1/2: 1994-1996
– Tracking-upgrade program (LEP 1 data reprocessed)• Improved coordinate determination requires better
understanding of systematic effects• Combined calculations for field and alignment distortions,
reevaluation of B-field map– All methods for distortion corrections now based directly on data– Development of “few”-parameter correction models to cope with
drastically reduced calibration samples at LEP 2• LEP 2: 1995-2000
– New VDET with larger acceptance– Calibrations@Z at beginning of run periods have limited statistics– Frequent beam losses cause charge-up effects and new FC
shorts• Superimposed distortions• Short-corrections with Z -> μμ;time-dep. effects tracked with
hadrons
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Examples from Werner’s slides…
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e.g. (see Werner’s slides…)
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e.g., non-linear F.C. potential
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e.g., disconnected gating grids
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e.g., field-cage shorts
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(N.B., design your detector to be easily accessible…)
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σ ~ 0.54E-03
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the bottom line (e.g., momentum resolution)
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ConclusionConclusion
• We’d better learn from these past lessons so that the new TPC will evolve to a much better main tracker for the future LC its performance will then improve by an order of magnitude relative to that at Lep…