Detectors
1. Accelerators
2. Particle detectors overview
3. Tracking detectors
Why do we accelerate particles ?
(1) To take existing objects apart1803 J. Dalton’s 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 decay1897 J. J. Thomson discovers electron1906 E. Rutherford: gold foil experiment
Physicists break particles by shooting other particles on them
Why do we accelerate particles ?
(2) To create new particles1905 A. Einstein: energy is matter E=mc2
1930 P. Dirac: math problem predicts antimatter
1930 C. Anderson: discovers positron1935 H. Yukawa: nuclear forces (forces between
protons and neutrons in nuclei) require pion1936 C. Anderson: discovers pion muon
First experiments used cosmic rays that are accelerated for us by the Universeare still of interest as a source of extremely
energetic particles not available in laboratories
Generating particlesBefore accelerating particles, one has to
create themelectrons: cathode ray tube (think your TV)protons: cathode ray tube filled with hydrogen
It’s more complicated for other particles (e.g. antiprotons), but the main principle remains the same
Basic accelerator physicsLorentz Force: F = qE + q(vB)
magnetic force: perpendicular to velocity, no acceleration (changes direction)
electric force: acceleration
Accelerators: Cockroft-WaltonA (series of) voltage gap(s)Maximum energy of a single gap is 200
kV, limited by dischargeCW accelerator at Fermilab: 750 kV
Accelerators: Van de GraafVan 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
Surfing the electromagnetic wave
Charged particles ride the EM wavecreate standing waveuse a radio frequency cavitymake particles arrive on time
Self-regulating:slow particle larger pushfast particle small push
Surfing the electromagnetic wave
How to create a standing wave ?
Klystron (S. & R. Varian)electrons flow into cavity, excite eigen modescreates standing electromagnetic waves
A similar device (magnetron) found in your microwave oven
325 MHz Klystron for Proton Driver Linac (Fermilab)
Cyclotron1929 E.O. Lawrence
The physics: centripetal force mv2/r = BqvParticles follow a spiral in a constant magnetic fieldA 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 cyclotronat TRIUMF (520 MeV protons)
SynchrotronThe idea: both magnetic field strength and
electric field frequency are synchronized with the traveling particle beamparticle trajectories confined to a thin vacuum
beamline no large magnets, expandablesynchrotron radiation limits its use for electrons
Currently, accelerators of this type provide highest particle energies in the world
Summary on accelerator typesElectrostatic accelerators
acceleration tube: breakdown at 200 keVCockroft-Walton: improves to 800 keV
AC driven acceleratorslinear: cavity design and length criticalcircular accelerators:
cyclotron: big magnet, non-relativisticsynchrotron: vacuum beamline, expandable, small
magnets and cavitiessynchrotron radiation large for light particles
Hadron vs electron colliders
electron
proton
Point-like particle yes no
Uses full beam energy yes no
Transverse energy sum zero zero
Longitudinal energy sum zero non-zero
Synchrotron radiation large small
Large Electron-Positron colliderLocation: CERN (Geneva, Switzerland)
accelerated particles: electrons and positronsbeam energy: 45104 GeV, beam current: 8
mAthe ring radius: 4.5 kmyears of operation: 19892000
TevatronLocation: Fermilab (Batavia, IL)
accelerated particles: protons and anti-protons
beam energy: 1 TeV, beam current: 1 mAthe ring radius: 1 kmin operation since 1983
Large Hadron ColliderLocation: CERN (Geneva, Switzerland)
accelerated particles: protonsbeam energy: 7 TeV, beam current: 0.5 Athe ring radius: 4.5 kmscheduled start: 2007
Future of acceleratorsInternational Linear Collider: 0.53 TeV
awaiting directions from LHC findingspolitical decision of location
Very Large Hadron Collider (magnet development ?): 40200 TeV
Muon Collider (source ?) 0.54 TeVlepton collider without synchrotron radiationcapable of producing many more Higgs
particles compared to an e+e collider
ConclusionsMotivation for particle acceleration
understand matter around uscreate new particles
Particle accelerator typeselectrostatic: limited energyAC driven: linear or circular
Modern acceleratorsTeVatron, LHCaccelerators to come: ILC, VLHC, muon
collider…
Detectors
1. Accelerators
2. Particle detectors overview
3. Tracking detectors
Detectors and particle physicsdetectors allow one to detect particles
experimentalists study their behaviornew particles are found by direct observation
or by analyzing their decay productstheorists predict behavior of (new) particlesexperimentalists design the particle detectors
Overview of particle detectorsWhat do particle detectors measure ?
spatial locationtrajectory in an EM field momentumdistance between production and decay point
lifetime
energymomentum + energy mass
flight timesmomentum/energy + flight time mass
Natural particle detectorsA very common particle detector: the eye
detected particles: photonssensitivity: high (single photons)spatial resolution: decentdynamic range: excellent (11014)energy range: limited (visible light)energy discrimination: goodspeed: modest (~10 Hz, including processing)
Photographic paper1895 W. C. Röntgen: sensitivity to high
energy photons (X-rays) invisible to the eyeworking medium: emulsion
Properties:detected particles: photonssensitivity: goodspatial resolution: very gooddynamic range: goodno online recordingno speed resolution
The Geiger counter1908 H. Geiger
passing charge particles ionize the gasions (electrons) drift towards cathode (anode)cause an electric pulse, can be heard in a speaker
Properties:detected particles: charged particles (electrons, ,
…)sensitivity: single particlesspatial resolution: none (detector size) – can be
fixeddynamic range: none – can be fixedspeed: high (determined by charge drift velocity)
The cloud chamber1911 C. T. R. Wilson (1927 Nobel Prize)
the first tracking detector (tracking=many spatial measurements per particle)
Principle of operation:an air volume is saturated with water vaporpressure lowered to generate super-saturated
aircharge particles cause saturation of vapor into
small droplets can be observed as a “track”photographs allow longer inspection
The cloud chamberProperties:
detected particles: charged particles (electrons, ,…)
sensitivity: single particlesspatial resolution: excellentdynamic 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)
Photographic emulsionsRarely 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 (<1 m)
Were used in the -neutrino discovery (DONUT, 2000)
Modern detector types Tracking detectors
detect charged particlesprinciple of operation: ionization two basic types: gas and solid
Scintillatorssensitive to single particlesvery fast, useful for online applications
Calorimetersmeasure particle energyusually measure energy of a bunch of particles (“jet”)modest spatial resolution
Particle identification systems recognize electrons, charged pions, charged kaons,
protons
Tracking detectors A charged track ionizes the gas
10—40 primary ion-electron parismultiplication 3—4 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 increaseselectrons cause an avalanche of ionization (exponential increase)
The same principle (ionization + avalanche) works for solid state tracking detectorsdense medium large ionizationmore compact put closer to the interaction pointvery good spatial resolution
CalorimetryThe idea: measure energy by total absorption
also measure locationthe method is destructive: particle is stoppeddetector 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
Electromagnetic calorimetersElectromagnetic 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 atomsTypical materials for EM calorimeters: large
charge atoms, organic materialsimportant parameter: radiation length
Hadronic calorimetersIn 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
Muon detectionMuons are charged particles, so using
tracking detectors to detect themCalorimetry 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
Detection of neutrinosIn dedicated neutrino experiments, rely on
their interaction with materialinteraction probability extremely low need
huge volumes of working mediumIn accelerator experiments, detecting
neutrinos is impractical – rely on momentum conservationelectron colliders: all three momentum
components are conservedhadron colliders: the initial momentum
component along the (anti)proton beam direction is unknown
Multipurpose detectors Today people usually combine several types of
various detectors in a single apparatusgoal: 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”)
Generic HEP detector
D detector at FermilabD detector is one of two large
multipurpose detectors at Fermilab (another one is CDF)name = one of six intersection points
D: fairly typical HEP detector
D: tracking system (1)Vertex detector: Silicon Microstrip Tracker
four layers of silicon detectors intercepted with twelve disks + (recent addition) Layer 0
D: tracking system (2)Outer tracking detector: Central Fiber
Trackersixteen double layers of scintillating fibers
D: calorimeterLiquid argon / uranium calorimeter,
consisting of central and two end calorimeters
D: outer muon systemThe outermost part of the detector,
surrounds the whole thingProportional Drift Tubes, Mini Drift TubesCentral (Forward) muon SCintillators
D: other elementsMagnet: a central solenoid magnet (2 T)
and outer toroid magnetLuminosity scintillating countersCentral and forward preshowerForward proton detector (Roman pots)Data acquisition, trigger system, …
ConclusionsParticle detectors follow simple principles
detectors interact with particlesmost interactions are electromagneticimperfect by definition but have gotten pretty
goodcrucial to figure out which detector goes where
Three main ideastrack charged particles and then stop themstop neutral particlesfinally find the muons which are left
Detectors
1. Accelerators
2. Particle detectors overview
3. Tracking detectors
Gas detectorsAs a charged particle crosses a gas
volume, it creates ionizationelectrons get kicked out of atomsthe rest of atom becomes electrically charged
(ion)In absence of external field, ions and
electrons recombine back to neutral atomselectrons drift to anodeions drift to cathode
E = V/r ln(b/a)
IonizationAffected by many factors
gas temperaturegas pressureelectric fieldgas composition
Important parameters:ionization potentialmean free path
Some gases eat up electrons (“quenchers”)
Ionization as a function of energy
Ionization probability gas dependantGeneral features:
threshold (~20 eV)fast turn onmaximum (~100 eV)soft decline
eV
Mean free pathAverage distance an electron travels
before it hits an atom – determined by gas density
At ambient pressure (1013 hPa), air density is 2.71019 molecules/ccm, and mean free path is 68 m
At high vacuum (10—3…10—7 hPa), mean free path is 0.1…1000 m
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
AvalancheSteps 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 nsa cloud of positive ions slowly migrates towards the
cathode
Ionization chamberLow voltage, no secondary ionization –
just collect ionsexample: smoke detector
radiation source (Am-241) emits -particlesthey pass through ionization chamber, creating
currentsmoke absorbs -particles and interrupts current
Proportional counterHigher 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 particle’s kinetic energy
Spark chamberDevice similar to Geiger counterIonizing particles produce sparks along its
way that can be photographed and used later for reconstruction of tracksMy diploma work was done on the ITEP’s 3m
magnet spectrometer equipped with spark chambers
Regimes in a tracking chamber
Gas tracking detectors: summary
detector voltage avalanches regime
ionization chamber
low nosingle ion collection
proportional counter
medium isolatedproportiona
l
Geiger-Müller
counterhigh maximal saturated
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 particle’s energy + improve position resolution (by interpolation)
drift chambers: measure time of arrival of the electron avalanche improve position resolution + provide a timing reference point
MWPC electric fieldHomogeneous field away from anode
wiresField near wires very sensitive to their
position
from G. Charpak’s Noble lecture
MWPC designConstraints
precise position measurements require precise and small wire spacing
homogeneous fields require small wire spacinglarge fields require thin wiresgeometric tolerances cause gain variations
Geometry and problemsrequired precision: sub millimeterlong chambers need strong wires (W/Au
plated) and high tension to minimize sagging
Choice of gasIt’s a magic
low working voltagehigh gain operationgood proportionalityhigh rate capabilitylong lifetimefast recoveryprice…
Operation conditionsPressure: slightly above atmospheric
avoid incoming gas “pollution”a large tracker is not really air tightnot too high (difficult to maintain)
Temperature: slightly lower than room t.avoid large temperature gradientsaffected by environment (e.g. cooling of
nearby systems)
Limitations of chambersHigh 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 factorcan be addressed with special technologies
(GEM)
Time Projection Chamber (RHIC)
Brookhaven Nat’l Lab, Relativistic Heavy Ion Collider
Shown: Gold-Gold collision
Solid state detectorsBasic operation principle same as gas
detectorsgas liquid solid
Density low moderate high
Atomic number low moderate moderate
Ionization energy moderate moderate low
Signal speed moderate moderate high
Silicon detectorsSolid state tracking detectors:
semiconductor diodes with reverse biasnormally 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
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 yieldlow Poisson noiseno gain stage required
better energy resolution and high signal
Why silicon ? (cont’d)High density and atomic number
reduced range of secondary particlescan build thin detectors
better spatial resolution
High carrier mobilitytypical charge collection times <30 nsno slow component (ions)
Excellent mechanical rigidityIndustrial fabrication techniquesDetector and electronics can be integrated
ProblemsCost
proportional to area coveredmost of the cost is moving to read out channels
Material budgetfor complex detectors can be as large as ~1—2
radiation lengthsaffects calorimeters behind the detectoraffects tracking accuracy (multiple scattering)
Typically need cooling to reduce leakage current (thermal energy = 1/40 eV)
Radiation hardnessWhat is it ?
particles damage silicon crystal structureband gap decreasesleakage currents increasegain 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)
Diode strip detectorsIdea (1980’s): divide the large-area diode
into many small strip-like regions and read them out separatelyTypical strip pitch p = 20—few hundred mPosition measurement precision:
digital readout: = p/12analog readout: = p/(S/N) (S = signal, N =
noise)
-functionLet a particle pass the detector between two
strips (i) and (i+1) at coordinate x = xi…xi+p
If strip (i) collects charge qi, and strip (i+1) collects charge qi+1, (x) = qi/(qi+qi+1)ideally, (x) = 1, x<xi+p/2, and (x) = 0,
x>xi+p/2in practice, it’s not true:
finite charge cloud size (~5 m)charge capacitance between stripsnon-uniform electric field
Lorentz shiftIf a detector is placed in magnetic field
(parallel to its strips), charge careers are deflected as they drift towards the stripsintroduces systematic shift of the measured
positionsignal gets spread between several strips
increases cluster sharing (bad)with analog readout, improves spatial resolution
(good)
Double sided readout detectorsIdea: use both types of carriers to make
two position measurements for the same amount of materialUsually cross strips 2-dim measurementFrom charge correlation can resolve
ambiguities
p-side charge
n-s
ide
charg
e
Pixel detectorsProvide 3-dim points with very high
precisionmain issue is readoutcan read out individual pixels or entire
rows/columnsElectrodes are close !
low full biaslow collection distanceno charge spreadingfast charge sweep out
Pixel vs strip detector operation
h+
e-
-ve +veSiO2
W3D
h+
e-
+ve
W2D
+ve -ve-ve -ve
p+
n+
n EE
pixel detector
strip detector
Pixel detector at ATLAS
ConclusionsTracking detectors
detect charged particlesmeasure arrival time and charge depositionderive 3 dimensional location and energy
Designinner detectors: silicon (strip/pixel), highest
track density resolution (tens of m)outer detectors: gas detectors, lower
resolution (hundreds of m)