introduction: detector tasks

Particle DetectorsTools of High Energy and Nuclear Physics

Goal: to detect Individual Particlesand reconstruct their 4-vectors

Thanks to H. Fenker (Jlab) and B. Surrow (MIT)

Introduction: detector tasks

• Position measurement: localize hits of a charged particle(eg: wire chambers, segmented scintillators or calorimeters)

• Momentum measurement: by measuring the particle deflection in a magnetic field (eg: magnetic spectrometer)

• Energy measurement: deposition of energy in a localized volume (eg: calorimeters)

• Particle identification: mass and charge of the particle (eg: Cerenkov detectors)

• Triggering: select events of interest• Data acquisition system: readout of an event and storage

after positive decision.

Introduction: detector performances

• Time– Response time: time which is required to produce a signal after the

passage of a particle in the detector (from ~10 ns to ~100 s).– Deadtime: time that must elapse following the passage of a particle

before the detector is ready for the next particle.

• EfficiencyThat is (event registered)/(events emitted by source)The efficiency is the product of at least two quantities:– Intrisic efficiency: (event registered)/(events impinging on detector)– Geometric efficiency: (event impinging on detector)/(event emitted by

source)

• Resolution/Accuracy

Example of position detectors performance

• Avalanche multiplication• Detectors that see the

electrons– Wire chambers

– Time Projection Chamber

– Gas Electron Multiplier

• Detectors that see the light– Scintillators

– Cerenkov Detectors

– Calorimeters

Outline of class Interactions of Particles with Matter

Charged particles

Photons

Using the Interactions:Particle Detectors

Avalanche multiplication

Detectors that sense Charge

Detectors that sense light

Aside: magnetic spectrometers

Putting it all together

Interaction of particles with matter

Charged particles:Energy and trajectories get

degraded as they pass through matter:

• Ionization• Bremsstrahlung• Cherenkov radiation

Photons:Flux gets decreased as they go

through matter. All or nothing interactions

• Photo-electric effects• Compton scattering• Pair production


Charged particle

Photons







Charged particles- Ionization

Atomic electron is knocked free from the atom.

The remaining atom is now an ion or left in an excited state (will decay by emitting a photon)

ChargedParticle

Free Electron

Electric Field

IonIonization

Ionization: Bethe-Bloch formula

where , , relate to particle speed, z is the particle’s charge..

The other factors describe the medium (Z/A, I, ), or are physical constants.

dE/dx units is MeV cm2/g

E= dE/dx * x /

A value to keep in mind is 2 MeV.g-1.cm2

Charged Particles: Bremsstrahlung

Radiation of real photons in the Coulomb field of a nuclei of the absorber.

Electron

Nucleus

Photon

Define X0 the radiation length: length during which the particle looses a fraction e of its initial energy

Critical energy (Ec) is the energy at which Bremstrahlung loss equal ionization loss

Ec(e- Cu)=20 MeV

Ec(- Cu)=800 GeV

Charged particles: Multiple Coulomb Scattering(both for ionization and radiation)

Detectors scatter particles even without much energy loss…

MCS theory is a statistical description of the scattering angle arising from many small interactions with atomic electrons.

MCS alters the direction of the particle in average.

Most important at low energy.

is particle speed,

z is its charge,

X0 is the material’s Radiation Length.

0

Charged particles- Cerenkov radiationThe electric field of a particle has a long-range interaction with material as it passes

through a continuous medium. It does create a shock wave that causes the material to emit light when it’s speed is large

enough

v = c > c/n where n is the index of refraction of the material

Also the light is emitted at the angle = cos-1 (1/n)

Photon energy ~ few eV (UV to visible) (not an efficient way to loose energy)

1/n1/n

Cerenkov light produced by fuel elements of a nuclear power plant.


Charged particle

Photons







Interaction of photon with matterPhoto-electric effect

Compton scattering

Pair production

Interaction of photon with matter

Total probability for interaction:

Probability per unit length or total absorption coefficient

Such that the flux of photon is

Absorption length=(absorption coeff)-1

Interactions of Particles with Matter - Summary

When particles pass through matter they usually produce either free electric charges (ionization) or light (photoemission).

Most “particle” detectors actually detect the light or the charge that a particle leaves behind.

In all cases we finally need an electronic signal to record.


Charged particle

Photons


Aside: Avalanche multiplication





Particle Detectors… Avalanche Multiplication

When a particle passes through matter, it creates just a fewelectrons/ions or photons

But the best we can do is to detect signals of the order of nV in a 100 Ohm resistor which

correspond toI= U/R= 10-9 /100 = 10-11 A = 108 electrons. s-1

Which means: The detectors need to amplify the charge produced by particles going through matter. By giving

the charges a push, we can make them move fast enough so that they ionize other atoms when they collide. After this has happened several times we have a sizeable free charge that can be sensed by an electronic circuit.

Particle Detectors: amplification with the photo multiplier

PMTs are commercially produced and very sensitive.•One photon --> up to 108 electrons!•Fast! …down to ~ few x 10-9 seconds.

Photoelectric Effect: A photon liberates a single electron

Secondary Emission

Energetic electrons striking some surfaces can liberate MORE electrons. Those, in turn, can be accelerated onto another surface … so on.

• V is the voltage• rc radius of the outer cathode plane• ra radius of the inner anode

Close to the wire the E field is veryIntense, the charged particle is Accelerated and as a result creates secondaryParticles.

Particle Detectors: amplification with the E-Field

Particle Detectors… Gas Electron Multiplier (GEM)

Gas Ionization and Avalanche Multiplication again, but…

… a different way to get an intense electric field,

… without dealing with fragile tiny wires

http://gdd.web.cern.ch/GDD/

--V

~400v0.002”

GEM

To computer

Dense Material => Lots of Charge. Typically no Amplification

Noble LiquidLiquid Argon Calorimeter

Signals toComputer

Electrons are knocked loose in the silicon and drift throughit to electronics.

Readout strips maybe VERY NARROW

0.001”

0.012”

SemiconductorSilicon

Diamond

StripsPixelsDrift

Particle Detectors: bypassing the amplification (new methods)

Particle detectors






– Calorimeters

Particle Detectors: Gas Filled Wire Chamber

Let’s use Ionization and Avalanche Multiplication to build a detector…

Make a Box. Fill it with some gas: noble gases are more likely to ionize than

others. Use Argon. Insert conducting surfaces to make an intense electric field: The

field at the surface of a small wire gets extremely high, so use tiny wires.

Attach electronics and apply high voltage. We’re done!!

Straw Tube Tracker for the COSY-TOF Experiment Julich Institute (Germany)

Central Drift Chamber (Hall D Jlab)

Straw chambers: better resolution

2D solution:

The wire touched gives X position info

The time of transit between the two amplis gives the Y position info

X

y

Multi-Wire Gas Chamber

start

stop

Best 1D solution:

-Use an external trigger to start a clock

-Measure the time it takes for the electron to drift from the initial ionization to the wire.

Resolution ~ 10 m

TPC: 3D position information.

Cathode Anode(gain)

Readoutelectrodes

Time Projection Chamber (TPC): Drift through a VolumeJust a box of gas with

Electric Field andReadout Electrodes

Readout elements only on one surface.Ionization Electrons drift to Surface for

AmplificationCharge Collection

Readout Electrode Position gives (x,y)Time of Arrival gives (z).

Particletrack

X

Y

Z

Particle detectors






– Calorimeters

Particle Detectors… Cerenkov Counter

If =v/c > 1/n, there will be light.If not, there won’t.

Can be used for trigger systemIf you know the momentum of the particle: can be used for PID

Cerenkov Counters – sensitive to

TRD Counters – sensitive to

= (1- 2)-1/2 = E/m

= v/c = p/E

Momentum (GeV/c)

Rich detector : Ring imaging Cherenkov

Light is emitted at the angle

= cos-1 (1/n)

Scintillators

Scintillation Counters are probably the most widely used detectors in Nuclear and High Energy Physics.

Scintillator material are special material that

-emit light when traversed by energetic particles and

-can shift the wavelength of this light

to be harnessed by PMTs

They can be solid, liquid (even gas)

They can be molded in all kind of shapes

Solids

Liquid

Saint Gobin Inc

Sample experimentMIT/Bates

Detecting scintillationIn Air.

Particle Detectors: Calorimeter

Used to measure energy.

Based on Bremsstrahlung effect

Suppose an initial photon of energy E0

After t radiation lengths, the average energy of secondary is:

The number of secondaries is

The shower stops when

EM (or hadronic) calorimeters are used because:- They can detect charged or neutral particle- They produce a really fast signal (10-100 ns) that is ideal for making trigger decision

The energy resolution of the calorimeter therefore goes as

Alice’s ZDC calorimeterCERN

Hall A/DVCS calorimeter(50*30 cm2)JLAB


Charged particle

Photons







Particle Detectors:aside: magnetic spectrometer

Just as light of different wavelength is bent differently by a prism...

Nature lets us measure the Momentum of a chargedparticle by seeing how much its path is deflected bya magnet.

(x1,y1)(x2,y2)

Magnet

MAMI (Germany)

Four quantities measured in the IN frame and 4 in the OUT/VDC frame.The best is to design the magnet to be insensitive to xIN and with <x|> as the main coefficient

Central trajectory

OUT frame usually equipped with a Vertical Drift Chamber (VDC)

IN frameUsually a“short” target


Charged particle

Photons







Putting it all Together: A Detector SystemHall C/JLAB

Putting it all Together: A Detector System

QWEAK: conceptual overview

• Elastic e-p scattering on liquid hydrogen target• Toroidal magnet to provide momentum dispersion• Collimator system to select elastic events only• Lower energy inelastic events bent outside of the detector acceptance


Charged particle

Photons







introduction: detector tasks

Documents

particles charge

particle speed

particle deflection

cm2charged particles

gevcharged particles

deposition of energy

sense chargedetectors

sense lightaside