Detectors

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Detectors
1. Accelerators 2. Particle detectors overview 3.
Tracking detectors
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Why do we accelerate particles ?

• (1) To take existing objects apart
• 1803 J. Daltons indivisible atom
• atoms of one element can combine with atoms of
  other element to make compounds, e.g. water is
  made of oxygen and hydrogen (OH)
• 1896 M. P. Curie find atoms decay
• 1897 J. J. Thomson discovers electron
• 1906 E. Rutherford gold foil experiment
• Physicists break particles by shooting other
  particles on them

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Why do we accelerate particles ?

• (2) To create new particles
• 1905 A. Einstein energy is matter Emc2
• 1930 P. Dirac math problem predicts antimatter
• 1930 C. Anderson discovers positron
• 1935 H. Yukawa nuclear forces (forces between
  protons and neutrons in nuclei) require pion
• 1936 C. Anderson discovers pion muon
• First experiments used cosmic rays that are
  accelerated for us by the Universe
• are still of interest as a source of extremely
  energetic particles not available in laboratories

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Generating particles

• Before accelerating particles, one has to create
  them
• electrons cathode ray tube
• (think your TV)
• protons cathode ray tube
• filled with hydrogen
• Its more complicated for other particles (e.g.
  antiprotons), but the main principle remains the
  same

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Basic accelerator physics

• Lorentz Force F qE q(v?B)
• magnetic force perpendicular to velocity, no
  acceleration (changes direction)
• electric force acceleration

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Accelerators Cockroft-Walton

• A (series of) voltage gap(s)
• Maximum energy of a single gap is 200 kV, limited
  by discharge
• CW accelerator at Fermilab 750 kV

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Accelerators Van de Graaf

• Van de Graaf generator an electrostatic machine
  which uses a moving belt to accumulate very high
  voltages on a hollow metal globe

1 metallic sphere 2 electrode connected to 1 3
upper roller 4 belt (positive side) 5 belt
(negative side) 6 lower roller 7 lower
electrode (ground) 8 spherical device, used to
discharge the main sphere 9 spark
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Surfing the electromagnetic wave

• Charged particles ride the EM wave
• create standing wave
• use a radio frequency cavity
• make particles arrive on time
• Self-regulating
• slow particle ? larger push
• fast particle ? small push

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Surfing the electromagnetic wave
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How to create a standing wave ?

• Klystron (S. R. Varian)
• electrons flow into cavity, excite eigen modes
• creates standing electromagnetic waves
• A similar device (magnetron) found in your
  microwave oven

325 MHz Klystron for Proton Driver Linac
(Fermilab)
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Cyclotron

• 1929 E.O. Lawrence
• The physics centripetal force mv2/r Bqv
• Particles follow a spiral in a constant magnetic
  field
• A high frequency alternating voltage applied
  between D-electrodes causes acceleration as
  particles cross the gap
• Advantages compact design (compared to linear
  accelerators), continuous stream of particles
• Limitations synchronization lost as particle
  velocity approaches the speed of light

the world largest cyclotron at TRIUMF (520 MeV
protons)
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Synchrotron

• The idea both magnetic field strength and
  electric field frequency are synchronized with
  the traveling particle beam
• particle trajectories confined to a thin vacuum
  beamline ? no large magnets, expandable
• synchrotron radiation limits its use for
  electrons
• Currently, accelerators of this type provide
  highest particle energies in the world

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Summary on accelerator types

• Electrostatic accelerators
• acceleration tube breakdown at 200 keV
• Cockroft-Walton improves to 800 keV
• AC driven accelerators
• linear cavity design and length critical
• circular accelerators
• cyclotron big magnet, non-relativistic
• synchrotron vacuum beamline, expandable, small
  magnets and cavities
• synchrotron radiation large for light particles

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Hadron vs electron colliders
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Large Electron-Positron collider

• Location CERN (Geneva, Switzerland)
• accelerated particles electrons and positrons
• beam energy 45?104 GeV, beam current 8 mA
• the ring radius 4.5 km
• years of operation 1989?2000

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Tevatron

• Location Fermilab (Batavia, IL)
• accelerated particles protons and anti-protons
• beam energy 1 TeV, beam current 1 mA
• the ring radius 1 km
• in operation since 1983

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Large Hadron Collider

• Location CERN (Geneva, Switzerland)
• accelerated particles protons
• beam energy 7 TeV, beam current 0.5 A
• the ring radius 4.5 km
• scheduled start 2007

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Future of accelerators

• International Linear Collider 0.5?3 TeV
• awaiting directions from LHC findings
• political decision of location
• Very Large Hadron Collider (magnet development
  ?) 40?200 TeV
• Muon Collider (source ?) 0.5?4 TeV
• lepton collider without synchrotron radiation
• capable of producing many more Higgs particles
  compared to an ee? collider

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Conclusions

• Motivation for particle acceleration
• understand matter around us
• create new particles
• Particle accelerator types
• electrostatic limited energy
• AC driven linear or circular
• Modern accelerators
• TeVatron, LHC
• accelerators to come ILC, VLHC, muon collider

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Detectors
1. Accelerators 2. Particle detectors overview 3.
Tracking detectors
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Detectors and particle physics

• detectors allow one to detect particles ?
• experimentalists study their behavior
• new particles are found by direct observation or
  by analyzing their decay products
• theorists predict behavior of (new) particles
• experimentalists design the particle detectors

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Overview of particle detectors

• What do particle detectors measure ?
• spatial location
• trajectory in an EM field ? momentum
• distance between production and decay point ?
  lifetime
• energy
• momentum energy ? mass
• flight times
• momentum/energy flight time ? mass

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Natural particle detectors

• A very common particle detector the eye
• detected particles photons
• sensitivity high (single photons)
• spatial resolution decent
• dynamic range excellent (1?1014)
• energy range limited (visible light)
• energy discrimination good
• speed modest (10 Hz, including processing)

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Photographic paper

• 1895 W. C. Röntgen sensitivity to high energy
  photons (X-rays) invisible to the eye
• working medium emulsion
• Properties
• detected particles photons
• sensitivity good
• spatial resolution very good
• dynamic range good
• no online recording
• no speed resolution

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The Geiger counter

• 1908 H. Geiger
• passing charge particles ionize the gas
• ions (electrons) drift towards cathode (anode)
• cause an electric pulse, can be heard in a
  speaker
• Properties
• detected particles charged particles (electrons,
  ?,)
• sensitivity single particles
• spatial resolution none (detector size) can be
  fixed
• dynamic range none can be fixed
• speed high (determined by charge drift velocity)

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The cloud chamber

• 1911 C. T. R. Wilson (1927 Nobel Prize)
• the first tracking detector (trackingmany
  spatial measurements per particle)
• Principle of operation
• an air volume is saturated with water vapor
• pressure lowered to generate super-saturated air
• charge particles cause saturation of vapor into
  small droplets ? can be observed as a track
• photographs allow longer inspection

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The cloud chamber

• Properties
• detected particles charged particles (electrons,
  ?,)
• sensitivity single particles
• spatial resolution excellent
• dynamic range good
• as particle slows down, droplets occur closer to
  each other
• if placed inside a magnet, can observe curled
  trajectories
• speed limited (need time to recover the
  super-saturated state)

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Photographic emulsions

• Rarely used in modern experiments due to
  principal restrictions
• cannot be read out electronically
• used to need a lot of technicians looking at
  photographs by eye inefficient, boring, and
  error prone
• today using pattern recognition software (think
  OCR)
• cannot be used online
• One advantage is excellent spatial resolution (lt1
  ?m)
• Were used in the ?-neutrino discovery (DONUT,
  2000)

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Modern detector types

• Tracking detectors
• detect charged particles
• principle of operation ionization
• two basic types gas and solid
• Scintillators
• sensitive to single particles
• very fast, useful for online applications
• Calorimeters
• measure particle energy
• usually measure energy of a bunch of particles
  (jet)
• modest spatial resolution
• Particle identification systems
• recognize electrons, charged pions, charged
  kaons, protons

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Tracking detectors

• A charged track ionizes the gas
• 1040 primary ion-electron paris
• multiplication ?34 due to secondary ionization
• typical amplifier noise 1000 e
• the initial signal is too weak to be effectively
  detected !
• as electrons travel towards cathode, their
  velocity increases
• electrons cause an avalanche of ionization
  (exponential increase)
• The same principle (ionization avalanche) works
  for solid state tracking detectors
• dense medium ? large ionization
• more compact ? put closer to the interaction
  point
• very good spatial resolution

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Calorimetry

• The idea measure energy by total absorption
• also measure location
• the method is destructive particle is stopped
• detector response proportional to particle energy
• As particles traverse material, they interact
  producing a bunch of secondary particles
  (shower)
• the shower particles undergo ionization (same
  principle as for tracking detectors)
• It works for all particles charged and neutral

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Electromagnetic calorimeters

• Electromagnetic showers occur due to
• Bremsstrahlung similar to synchrotron radiation,
  particles deflected by atomic EM fields
• pair production in the presence of atomic field,
  a photon can produce an electron-positron pair
• excitation of electrons in atoms
• Typical materials for EM calorimeters large
  charge atoms, organic materials
• important parameter radiation length

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Hadronic calorimeters

• In addition to EM showers, hadrons (pions,
  protons, kaons) produce hadronic showers due to
  strong interaction with nuclei
• Typical materials dense, large atomic weight
  (uranium, lead)
• important parameter nuclear interaction length
• In hadron shower, also creating non detectable
  particles (neutrinos, soft photons)
• large fluctuation and limited energy resolution

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Muon detection

• Muons are charged particles, so using tracking
  detectors to detect them
• Calorimetry does not work muons only leave
  small energy in the calorimeter (said to be
  minimum ionization particles)
• Muons are detected outside calorimeters and
  additional shielding, where all other particles
  (except neutrinos) have already been stopped
• As this is far away from the interaction point,
  use gas detectors

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Detection of neutrinos

• In dedicated neutrino experiments, rely on their
  interaction with material
• interaction probability extremely low ? need huge
  volumes of working medium
• In accelerator experiments, detecting neutrinos
  is impractical rely on momentum conservation
• electron colliders all three momentum components
  are conserved
• hadron colliders the initial momentum component
  along the (anti)proton beam direction is unknown

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Multipurpose detectors

• Today people usually combine several types of
  various detectors in a single apparatus
• goal provide measurement of a variety of
  particle characteristics (energy, momentum,
  flight time) for a variety of particle types
  (electrons, photons, pions, protons) in (almost)
  all possible directions
• also include triggering system (fast
  recognition of interesting events) and data
  acquisition (collection and recording of
  selected measurements)
• Confusingly enough, these setups are also called
  detectors (and groups of individual detecting
  elements of the same type are called detector
  subsystems)

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Generic HEP detector
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D? detector at Fermilab

• D? detector is one of two large multipurpose
  detectors at Fermilab (another one is CDF)
• name one of six intersection points

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D? fairly typical HEP detector
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D? tracking system (1)

• Vertex detector Silicon Microstrip Tracker
• four layers of silicon detectors intercepted with
  twelve disks (recent addition) Layer 0

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D? tracking system (2)

• Outer tracking detector Central Fiber Tracker
• sixteen double layers of scintillating fibers

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D? calorimeter

• Liquid argon / uranium calorimeter, consisting of
  central and two end calorimeters

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D? outer muon system

• The outermost part of the detector, surrounds the
  whole thing
• Proportional Drift Tubes, Mini Drift Tubes
• Central (Forward) muon SCintillators

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D? other elements

• Magnet a central solenoid magnet (2 T) and outer
  toroid magnet
• Luminosity scintillating counters
• Central and forward preshower
• Forward proton detector (Roman pots)
• Data acquisition, trigger system,

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Conclusions

• Particle detectors follow simple principles
• detectors interact with particles
• most interactions are electromagnetic
• imperfect by definition but have gotten pretty
  good
• crucial to figure out which detector goes where
• Three main ideas
• track charged particles and then stop them
• stop neutral particles
• finally find the muons which are left

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Detectors
1. Accelerators 2. Particle detectors overview 3.
Tracking detectors
47
Gas detectors

• As a charged particle crosses a gas volume, it
  creates ionization
• electrons get kicked out of atoms
• the rest of atom becomes electrically charged
  (ion)
• In absence of external field, ions and electrons
  recombine back to neutral atoms
• electrons drift to anode
• ions drift to cathode

E V/r ln(b/a)
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Ionization

• Affected by many factors
• gas temperature
• gas pressure
• electric field
• gas composition
• Important parameters
• ionization potential
• mean free path
• Some gases eat up electrons (quenchers)

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Ionization as a function of energy

• Ionization probability gas dependant
• General features
• threshold (20 eV)
• fast turn on
• maximum (100 eV)
• soft decline

eV
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Mean free path

• Average distance an electron travels before it
  hits an atom determined by gas density
• At ambient pressure (1013 hPa), air density is
  2.7?1019 molecules/ccm, and mean free path is
  68 ?m
• At high vacuum (103107 hPa), mean free path is
  0.11000 m

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What happens after ionization ?

• After collision, ions (electrons) thermalize and
  travel until neutralized through electron (ion),
  wall, negative ion (other molecule)
• Mean free path for electrons 4 times longer than
  for ions
• Ions diffuse slowly, electrons diffuse quickly
• Diffusion velocity depends on gas

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Avalanche

• Steps of an avalanche
• a primary electron proceeds towards the anode,
  experiencing ionizing collisions
• due to the lateral diffusion, a drop-like
  avalanche, surrounding the wire, develops
• electrons are collected during 1 ns
• a cloud of positive ions slowly migrates towards
  the cathode

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Ionization chamber

• Low voltage, no secondary ionization just
  collect ions
• example smoke detector
• radiation source (Am-241) emits ?-particles
• they pass through ionization chamber, creating
  current
• smoke absorbs ?-particles and interrupts current

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Proportional counter

• Higher voltage, tuned to provide proportional
  regime
• each avalanche is created independently from
  others ? total amount of charge created remains
  proportional to the amount of charge liberated in
  the original event, which in turn is proportional
  to the particles kinetic energy

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Spark chamber

• Device similar to Geiger counter
• Ionizing particles produce sparks along its way
  that can be photographed and used later for
  reconstruction of tracks
• My diploma work was done on the ITEPs 3m magnet
  spectrometer equipped with spark chambers

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Regimes in a tracking chamber
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Gas tracking detectors summary
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Multi Wire Proportional Chamber

• 1968 G. Charpak (1992 Noble Prize)
• the idea make a proportional counter with a lot
  of anodes placed between two cathode planes
• by looking at which wires were fired, can
  determine position of the particle
• if the proportional mode is used, can determine
  particles energy improve position resolution
  (by interpolation)
• drift chambers measure time of arrival of the
  electron avalanche ? improve position resolution
  provide a timing reference point

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MWPC electric field

• Homogeneous field away from anode wires
• Field near wires very sensitive to their position

from G. Charpaks Noble lecture
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MWPC design

• Constraints
• precise position measurements require precise and
  small wire spacing
• homogeneous fields require small wire spacing
• large fields require thin wires
• geometric tolerances cause gain variations
• Geometry and problems
• required precision sub millimeter
• long chambers need strong wires (W/Au plated) and
  high tension to minimize sagging

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Choice of gas

• Its a magic
• low working voltage
• high gain operation
• good proportionality
• high rate capability
• long lifetime
• fast recovery
• price

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Operation conditions

• Pressure slightly above atmospheric
• avoid incoming gas pollution
• a large tracker is not really air tight
• not too high (difficult to maintain)
• Temperature slightly lower than room t.
• avoid large temperature gradients
• affected by environment (e.g. cooling of nearby
  systems)

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Limitations of chambers

• High occupancy OK
• used in Alice (heavy ion collisions at LHC)
• Radiation hardness
• tough but manageable (need gas flow)
• Speed
• is a problem for LHC applications (25 ns bunch
  crossing)
• ion drift is limiting factor
• can be addressed with special technologies (GEM)

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Time Projection Chamber (RHIC)

• Brookhaven Natl Lab, Relativistic Heavy Ion
  Collider
• Shown Gold-Gold collision

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Solid state detectors

• Basic operation principle same as gas detectors

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Silicon detectors

• Solid state tracking detectors semiconductor
  diodes with reverse bias
• normally there is no current (except very low
  dark current)
• a charged particle creates a track of carriers
  (electron-hole pairs) along its way ? charge pulse

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Why silicon ?

• Low band gap width 1.12 eV (large number of
  charge carriers / unit energy loss)
• Energy to create an e/h pair 3.6 eV (an order of
  magnitude smaller than ionization energy for
  gases)
• high carrier yield
• low Poisson noise
• no gain stage required
• better energy resolution and high signal

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Why silicon ? (contd)

• High density and atomic number
• reduced range of secondary particles
• can build thin detectors
• better spatial resolution
• High carrier mobility
• typical charge collection times lt30 ns
• no slow component (ions)
• Excellent mechanical rigidity
• Industrial fabrication techniques
• Detector and electronics can be integrated

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Problems

• Cost
• proportional to area covered
• most of the cost is moving to read out channels
• Material budget
• for complex detectors can be as large as 12
  radiation lengths
• affects calorimeters behind the detector
• affects tracking accuracy (multiple scattering)
• Typically need cooling to reduce leakage current
  (thermal energy 1/40 eV)

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Radiation hardness

• What is it ?
• particles damage silicon crystal structure
• band gap decreases
• leakage currents increase
• gain drops
• detector looses efficiency and precision
• What to do ?
• exchange detectors
• ATLAS replace inner detector after 3 yrs of
  operation
• switch to radiation hard technology (e.g.
  diamonds)

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Diode strip detectors

• Idea (1980s) divide the large-area diode into
  many small strip-like regions and read them out
  separately
• Typical strip pitch p 20few hundred ?m
• Position measurement precision
• digital readout ? p/?12
• analog readout ? p/(S/N) (S signal, N
  noise)

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?-function

• Let a particle pass the detector between two
  strips (i) and (i1) at coordinate x xixip
• If strip (i) collects charge qi, and strip (i1)
  collects charge qi1, ?(x) qi/(qiqi1)
• ideally, ?(x) 1, xltxip/2, and ?(x) 0,
  xgtxip/2
• in practice, its not true
• finite charge cloud size (5 ?m)
• charge capacitance between strips
• non-uniform electric field

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Lorentz shift

• If a detector is placed in magnetic field
  (parallel to its strips), charge careers are
  deflected as they drift towards the strips
• introduces systematic shift of the measured
  position
• signal gets spread between several strips
• increases cluster sharing (bad)
• with analog readout, improves spatial resolution
  (good)

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Double sided readout detectors

• Idea use both types of carriers to make two
  position measurements for the same amount of
  material
• Usually cross strips ? 2-dim measurement
• From charge correlation can resolve ambiguities

n-side charge
p-side charge
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Pixel detectors

• Provide 3-dim points with very high precision
• main issue is readout
• can read out individual pixels or entire
  rows/columns
• Electrodes are close !
• low full bias
• low collection distance
• no charge spreading
• fast charge sweep out

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Pixel vs strip detector operation
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W2D
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strip detector
pixel detector
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Pixel detector at ATLAS
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Conclusions

• Tracking detectors
• detect charged particles
• measure arrival time and charge deposition
• derive 3 dimensional location and energy
• Design
• inner detectors silicon (strip/pixel), highest
  track density resolution (tens of ?m)
• outer detectors gas detectors, lower resolution
  (hundreds of ?m)