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8.882 LHC Physics
Experimental Methods and Measurements
Detectors: Tracking
[Lecture 7, February 25, 2009]
Physics
Colloquium Series
�09
The Physics Colloquium Series
Spring
Thursday, February 26 at 4:15 pm in room 10-250
Zoltan Fodor
University of Wuppertal, Eotvos University of Budapest, John von Neumann Institute for Computing, DESYZeuthen, and Forschungszentrum-Juelich
"The Origin of Mass of the Visible Universe"
For a full listing of this semester’s colloquia,
please visit our website at
web.mit.edu/physics
Organizational Issues
Nothing from my side....
Remember though
в—Џ project 1 due March 12 (2.3 weeks)
C.Paus, LHC Physics: Detectors: Tracking
3
Lecture Outline
п‚·Detectors: Tracking
п‚· gas tracking detectors
Sauli paper CERN 77-09
п‚· the Central Outer Tracker (COT) at CDF
п‚· silicon detectors
п‚· the silicon tracking system at CDF
п‚· the tracker at CMS
C.Paus, LHC Physics: Detectors: Tracking
4
To Remember: Gas Detectors
п‚·Design is complex.. or simply black magic
п‚·Things you should remember
п‚· ionization, avalanche development
п‚· gain
п‚· proportional chamber, multi wire chamber
п‚· outline of gas choices
п‚· resolution
п‚·Pretty complete overview in Sauli's paper,
impossible to copy in this lecture.
C.Paus, LHC Physics: Detectors: Tracking
5
Ionization Reminder
п‚·Ionization: process which causes
п‚· usually kick electron out
п‚· breaking ionization potential barrier
п‚·Charged particle causes ionization in detector
п‚· ion-electron pair (called ion pair)
п‚· separate ion and electron in electric field
п‚· electron drifts to anode
п‚· ion drifts to cathode
п‚· round geometry:
C.Paus, LHC Physics: Detectors: Tracking
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Ionization continued
п‚·Factors for ionization
 electric field = “voltage”, but not only parameter
п‚· affected by
п‚·
п‚·
п‚·
п‚·
gas temperature
gas pressure
electric field
gas composition
п‚· mean free path an important parameter
п‚· ionization depends on the material's ionization potential
п‚· some gases eat up electrons (quenchers)
C.Paus, LHC Physics: Detectors: Tracking
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Ionization as a Function of Energy
п‚·Ionization probability
quite gas dependent
п‚·General features
 threshold (≈20 eV)
п‚· fast turnon
 maximum (≈100 eV)
п‚· soft decline
C.Paus, LHC Physics: Detectors: Tracking
8
Mean Free Path
п‚·Mean free path
п‚· average distance an electron travels until it hits a target
 half of ionization is due to “last mean free path”
п‚·Some typical numbers
Low vacuum
Pressure [hPa] Molecules/ccm mean free path [m]
1013
2.7 * 1e19
68 * 1e-9
300..1
1e19..1e16
1e-7 – 1e-4
Medium vacuum
1..1e-3
1e16..1e13
1e-4 – 1e-1
High vacuum
1e-3..1e-7
1e13..1e9
1e-1 – 1e3
Ultra high vacuum
1e-7..1e-12
1e9..1e4
1e3 – 1e8
<1e4
> 1e8
Vacuum range
Ambient pressure
Extremely high vacuum <1e-12
C.Paus, LHC Physics: Detectors: Tracking
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What Happens after Ionization?
п‚·After collision ions/electrons thermalize quickly and
travel until neutralized
п‚·Ions
п‚· neutralize through electron, wall, negative ion
п‚· travel slowly through diffusion process
п‚· diffusion velocity depends on gas, important for design
п‚·Electrons
п‚· neutralize through ions, wall, attach to some molecules
п‚· mean free path about 4 times longer than for ions
п‚· diffuse very quickly, accelerate in E field (avalanche)
п‚· drift velocity strongly depends on gas mixture
C.Paus, LHC Physics: Detectors: Tracking
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The Avalanche
п‚·Electrons diffuse to anode
п‚· ionize atoms they hit
п‚· spreading laterally
 electron drift fast about 1 ns ↔ ions slower (heavier)
п‚· leave positive ion cloud behind
п‚·
C.Paus, LHC Physics: Detectors: Tracking
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Gas Tracking Detectors
п‚·Ionization Chamber
п‚· lowest voltage
п‚· no secondary ionization, just collect ions
п‚·Proportional Chamber
 higher voltage – tuned
п‚· avalanches develop but independently
п‚· total charge proportional to particle's kinetic
Smoke Detector
energy
п‚·Geiger-MГјller Counter
п‚· highest voltage
п‚· avalanche maximal, saturation
Geiger Counter
C.Paus, LHC Physics: Detectors: Tracking
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Regimes in a Tracking Chamber
п‚·Characteristics
п‚· ionization
п‚· proportional
п‚· Geiger-MГјller
п‚·Transitions not
abrupt
C.Paus, LHC Physics: Detectors: Tracking
13
Multiplication Factor / Gains
п‚·Strong signal is important
п‚· detection efficiency
п‚· precision of pulse height / energy relation
п‚·Multiplication factor, M (Np.i. в€— M)
full derivation Sauli paper
п‚·For V0 >> VT expression
can be approximated as
C.Paus, LHC Physics: Detectors: Tracking
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Quantities from Equation
п‚·k - material constant (avalanche development)
п‚·N - number of molecules per unit volume
п‚·C - system capacitance (ne/V)
п‚·a - wire radius
ε0- dielectric constant of gas (≈8.85 pF/m)
п‚·V0 - operating voltage between anode and cathode
п‚·VT - voltage threshold for proportional amplification
C.Paus, LHC Physics: Detectors: Tracking
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(Multi) Wire (Proportional) Chamber
п‚·Principle design
п‚· single anode wire в†’ wire plan
п‚· cathode plane: mostly foils
п‚· forces homogeneous field,
sufficiently far from anode wire
п‚· field around wires very sensitive
to positioning of the wires
п‚· 25 Ојm wire
2mm
C.Paus, LHC Physics: Detectors: Tracking
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What Measures a Wire Chamber?
Running in “Geiger” amplification
п‚· pulse time & drift velocity в†’ position, ambiguous
п‚· brings up issue of t0 calibration (per event)
п‚· remove ambiguity with another wire under angle, stereo
п‚· axial wires and stereo wires
п‚·Running in proportional amplification
п‚· in addition measure pulse height
п‚· determines energy and thus allows dE/dx measurement
п‚· talk more about this in another lecture
п‚· momentum of track more precise from curvature in B
п‚·Resolution:
use large radius
with L = router - rinner
C.Paus, LHC Physics: Detectors: Tracking
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Wire Chamber Design
п‚·Constraints
п‚· precise position measurements require precise and wire
spacing and small wire spacing
п‚· homogeneous fields require small wire spacing
п‚· large fields (high amplification) requires thin wires
п‚· rigorous calculations available (see Sauli's paper)
п‚· geometric tolerances cause gain variations
п‚·Geometry and problems
п‚· sub millimeter precision required
п‚· long chambers need strong wire tungsten/gold plated
п‚· long chamber: large force to minimize sagging
п‚· fixing wires becomes a difficult task
C.Paus, LHC Physics: Detectors: Tracking
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Choice of Gas System - Magic
п‚·Factors for gas system choice
п‚· low working voltage
п‚· high gain operation
п‚· good proportionality
п‚· high rate capability
п‚· long lifetime
п‚· fast recovery
п‚· price
п‚· etc.
C.Paus, LHC Physics: Detectors: Tracking
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The No-Brainers
п‚·Typical gas pressures for tracking detectors
п‚· slightly over atmosphere:
 higher then atmosphere to minimize incoming gas “polution”
п‚· remember a large tracker is not really air tight
п‚· not too high (difficult to maintain), but reasonable ionization
п‚·Typical temperatures
п‚· most important: avoid large temperature differences
п‚· slightly lower then room temperature
п‚· affected by environment (silicon at T < -10в—‹C at LHC)
п‚· dew point is always dangerous....
C.Paus, LHC Physics: Detectors: Tracking
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Some Gas Properties
п‚·From Sauli's paper
C.Paus, LHC Physics: Detectors: Tracking
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Choice of Gas
п‚·Noble gas
п‚· lowest electrical field necessary for multiplication
п‚· suggests to be the main component
п‚· Krypton/Xenon are too expensive
п‚· Argon is fine and has highest specific ionization
п‚· high gains do not work, consider energy balance:
п‚·
п‚·
п‚·
п‚·
excited noble gases radiate (Ar, 11.6 eV) to dissipate energy
radiation causes electron extraction from cathode
secondary current develops в†’ discharge
gains up to 103-104 are possible
п‚·Need to catch photons and low energy electrons
C.Paus, LHC Physics: Detectors: Tracking
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Choice of Gas
п‚·Polyatomic molecules (ex. hydrocarbons, alcohols)
п‚· more than 4 atoms per molecule preferred
п‚· various non-radiative excited states (rotational,
vibrational modes)
п‚· thermal or chemical energy dissipation
п‚· thermal: through elastic collisions, heating environment
п‚· chemical: split molecules into radicals
п‚· excitation modes cover spectrum of noble gas radiation
п‚· photons get captured в†’ quenched
п‚· also low energy electrons get absorbed
п‚· neutralization at the cathode does not create radiation
п‚· gains higher than 106 are achieved
C.Paus, LHC Physics: Detectors: Tracking
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Choice of Gas
п‚·Polyatomic molecules, disadvantages
п‚· radicals created in dissociation
п‚· for high ionization gas characteristics changes rapidly
п‚· requires sufficient gas exchange in the chamber
п‚· open system design
п‚· closed system design with cleaning, separate cleaning cycle
п‚· worse, liquid and solid polymers can be created in
neutralization – insulator layer on cathode/anode wires
п‚· chamber performance suffers, Malter effect (1937):
п‚· charge builds up on insulator and potential difference causes
ionization of the wire
п‚· ionization leads to a current, independent of the particles
causing primary ionization в†’ discharge
п‚·
C.Paus, LHC Physics: Detectors: Tracking
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Limitations of Chambers
п‚·High occupancy no problem
п‚· Alice uses huge chamber for tracking: 15k tracks/event
п‚· uses Time Projection Chamber (TPC), 3m radius
п‚·Radiation hardness manageable
п‚· can be managed though it is tough depending on
design
п‚·Drift speed is limiting factor
п‚· high luminosity requirement at LHC (for pp operation)
п‚· bunch crossing rate is 25 ns
п‚· ion drift is to slow
 chamber would be “glowing”
п‚·Alternative: GEM (http://cerncourier.com/main/article/38/9/10) in
Micro Strip Gas Chambers
C.Paus, LHC Physics: Detectors: Tracking
25
CDF: Central Outer Tracker
п‚·Open Cell Design (at 396 ns bunch crossing)
check it out: http://fcdfwww.fnal.gov/~burkett/COT/newhome.html
C.Paus, LHC Physics: Detectors: Tracking
26
Silicon Detectors
п‚·Main purpose
п‚· determine 3 dimensional vertex of tracks precisely
п‚· improve momentum resolution for large momenta
п‚·Also
п‚· improve momentum resolution in general
п‚·Basic operation principle same as gas detectors
except E field now in a solid
C.Paus, LHC Physics: Detectors: Tracking
27
Why (Semi) Conductors?
п‚·Why go to solids?
п‚· increase dq/dE
п‚· fast response
п‚·Semi conductors?
п‚· high electric field (drift)
п‚· large signal charge
п‚· small DC current (depletion)
C.Paus, LHC Physics: Detectors: Tracking
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Silicon Strips
п‚·1 dimensional ambiguity
C.Paus, LHC Physics: Detectors: Tracking
(resolve with stereo, 90deg)
29
Silicon Pixels
п‚·Full 3 dimensional point
п‚·Features
п‚· very small, many channels
п‚· close to beam
п‚· radiation hardness crucial
 readout tricky, “bonding”
п‚· established technology:
п‚· camera, night vision devices
C.Paus, LHC Physics: Detectors: Tracking
30
Radiation Hardness
CDF Run I
п‚·What does it mean?
п‚· particle damages silicon
structure
п‚· band gap changes
п‚· leakage currents increase
п‚· gain drops
п‚· detector looses efficiency and
precision
п‚· detector needs exchanging
п‚· already well planned for CMS
п‚· diamond detector extremely
radiation hard, but difficult
C.Paus, LHC Physics: Detectors: Tracking
31
CDF Silicon Detector
Design (0.75M channels, ≈3 m2)
п‚·Features (all strips):
п‚· up to 8 layers, innermost 1.2 cm, outermost 29 cm
 resolution at PV per track ≈30 μm (x,y) ≈40 μm (z)
C.Paus, LHC Physics: Detectors: Tracking
32
CMS (Silicon) Tracker
п‚·Design:
 10M chan., ≈100 m2
п‚·Barrel: 3 pixel, 10 strip
п‚·EndCap: 2 pixel, 9 strip
C.Paus, LHC Physics: Detectors: Tracking
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Large Silicon Detectors
C.Paus, LHC Physics: Detectors: Tracking
34
Conclusions
п‚·Tracking detectors
п‚· detect charged particles only
п‚· measures: arrival time and charge deposition
п‚· derives: 3 dimensional location and energy
п‚·Sensitivities
п‚· innermost measures vertex (best hit resolution, needed)
п‚· overall radius measures momentum
п‚·Design
п‚· inside, always silicon (best pixels), highest track density
resolution: tens of Ојm
п‚· outside, if possible gas detector (low material budget)
resolution: hundreds of Ојm
C.Paus, LHC Physics: Detectors: Tracking
35
Next Lecture
Track reconstruction and fitting
в—Џ
в—Џ
в—Џ
в—Џ
в—Џ
в—Џ
general idea of track reconstruction
particle hypothesis
multiple scattering
energy loss
magnetic field
calibration of the tracking
C.Paus, LHC Physics: Detectors: Tracking
36
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