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)