particle detectors - indico · particle detectors history of instrumentation ↔ history of...

Particle Detectors

History of Instrumentation ↔ History of Particle Physics

The ‘Real’ World of Particles

Interaction of Particles with Matter, Tracking detectors

Photon Detection, Calorimeters, Particle Identification

Detector Systems

Summer Student Lectures 2007Werner Riegler, CERN, [email protected]

W. Riegler/CERN 1

Gas DetectorsW. Riegler/CERN 2

Detectors based on Ionization

Gas Detectors:

• Transport of Electrons and Ions in Gases

• Wire Chambers

• Drift Chambers

• Time Projection Chambers

Solid State Detectors

• Transport of Electrons and Holes in Solids

• Si- Detectors

• Diamond Detectors

• Principle: At sufficiently high electric fields (100kV/cm) the electrons gain

energy in excess of the ionization energy secondary ionzation etc. etc.

• Elektron Multiplication:

– dN = N α dx α…’first Townsend Coefficient’

– N(x) = N0 exp (αx) α= α(E), N/ N0 = A (Amplification, Gas Gain)

– N(x)=N0 exp ( (E)dE )

– In addition the gas atoms are excited emmission of UV photons can ionize

themselves photoelectrons

– NAγ photoeletrons → NA2 γ electrons → NA2 γ2 photoelectrons → NA3 γ2 electrons

– For finite gas gain: γ < A-1, γ … ‘second Townsend coefficient’


Gas Detectors with internal Electron Multiplication

Wire Chamber: Electron Avalanche

Electric field close to a thin wire (100-300kV/cm). E.g.

V0=1000V, a=10m, b=10mm, E(a)=150kV/cm

Electric field is sufficient to accelerate electrons to energies which are

sufficient to produce secondary ionization electron avalanche signal.

Wire with radius (10-25m) in a tube of radius b (1-3cm):


bb

a Wire

W. Riegler/CERN 5

From L. Ropelewski

Gas Detectors with internal Electron Multiplication

Proportional region: A103-104

Semi proportional region: A104-105

(space charge effect)

Saturation region: A >106

Independent from the number of primary

electrons.

Streamer region: A >107

Avalanche along the particle track.

Limited Geiger region:

Avalanche propagated by UV photons.

Geiger region: A109

Avalanche along the entire wire.

Wire Chamber: Electron Avalanches on the Wire


The electron avalanche happens very close to the wire. First multiplication only

around R =2x wire radius. Electrons are moving to the wire surface very quickly

(<<1ns). Ions are difting towards the tube wall (typically 100s. )

The signal is characterized by a very fast ‘spike’ from the electrons and a long Ion

tail.

The total charge induced by the electrons, i.e. the charge of the current spike due

to the short electron movement amounts to 1-2% of the total induced charge.


Wire Chamber: Signals from Electron Avalanches

Rossi 1930: Coincidence circuit for n tubes Cosmic ray telescope 1934

Geiger Mode

Position resolution is determined

by the size of the tubes.

Signal was directly fed into an

electronic tube.


Detectors with Electron Multiplication

Charpak et. al. 1968, Multi Wire Proportional Chamber


Classic geometry (Crossection) :

One plane of thin sense wires is placed

between two parallel plates.

Typical dimensions:

Wire distance 2-5mm, distance between

cathode planes ~10mm.

Electrons (v5cm/s) are being collectes

within in 100ns. The ion tail can be

eliminated by electroniscs filters pulses

100ns typically can be reached.

For 10% occupancy every s one pulse

1MHz/wire rate capabiliy !

In order to eliminate the left/right

ambiguities: Shift two wire chambers by

half the wire pitch.

For second coordinate:

Another Chamber at 900 relative rotation

Signal propagation to the two ends of

the tube.

Pulse height measurement on both ends

of the wire. Because of resisitvity of the

wire, both ends see different charge.

Segmenting of the cathode into strips or

pads:

The movement of the charges induces a

signal on the wire AND the cathode. By

segmengting and charge interpolation

resolutions of 50m can be achieved.


Charpak et. al. 1968, Multi Wire Proportional Chamber

1.07 mm

0.25 mm

1.63 mm

(a)

C1 C1 C1 C1 C1

C2C2C2C2

Anode wire

Cathode s trips

(b)

C1

Cathode strip:

Width (1) of the charge

distribution DIstance

‘Center of gravity’ defines the

particle trajectory.

Avalanche


Multi Wire Proportional Chamber

Drift Chambers 1970:

In an alternating sequence of wires with different potentials one finds an electric field

between the ‘sense wires’ and ‘field wires’.

The electrons are moving to the sense wires and produce an avalanche which induces a

signal that is read out by electronics.

The time between the passage of the particle and the arrival of the electrons at the wire is

measured.

The drift time T is a measure of the position of the particle !

By measuring the drift time, the wire distance can be reduced (compared to the Multi Wire

Proportional Chamber) save electronics channels !

E

Scintillator: t=0

Amplifier: t=T


Drift Chambers, typical Geometries

W. Klempt, Detection of Particles with Wire Chambers, Bari 04

Electric Field 1kV/cm


The Geiger counter reloaded: Drift Tube

Primary electrons are drifting to the wire.

Electron avalanche at the wire.

The measured drift time is converted to a radius by a (calibrated) radius-time correlation.

Many of these circles define the particle track.

ATLAS MDTs, 80m per tube

ATLAS Muon Chambers

ATLAS MDT R(tube) =15mm Calibrated Radius-Time correlation


Atlas Muon Spectrometer, 44m long, from r=5 to11m.

1200 Chambers

6 layers of 3cm tubes per chamber.

Length of the chambers 1-6m !

Position resolution: 80m/tube, <50m/chamber (3 bar)

Maximum drift time 700ns

Gas Ar/CO2 93/7


The Geiger counter reloaded: Drift Tube

ATLAS Muon Chamber Front-End Electronics

Single Channel Block Diagram3.18 x 3.72 mm

• 0.5m CMOS technology

– 8 channel ASD + Wilkinson

ADC

– fully differential

– 15ns peaking time

– 32mW/channel

– JATAG programmableHarvard University, Boston University


Designed around in 1997, produced in 2000, today – 0.17um process … rapidly changing technologies.

Large Drift Chambers: Central Tracking Chamber CDF Experiment

660 drift cells tilted 450

with respect to the

particle track.

Drift cell


y

z

x

E

B drift

charged track

wire chamber to detect projected tracks

gas volume

Time Projection Chamber (TPC):

Gas volume with parallel E and B Field.

B for momentum measurement. Positive effect:

Diffusion is strongly reduced by E//B (up to a

factor 5).

Drift Fields 100-400V/cm. Drift times 10-100 s.

Distance up to 2.5m !


• Gas Ne/ CO2 90/10%

• Field 400V/cm

• Gas gain >104

• Position resolution = 0.2mm

• Diffusion: t= 250m

• Pads inside: 4x7.5mm

• Pads outside: 6x15mm

• B-field: 0.5T

cm


ALICE TPC: Detector Parameters

ALICE TPC: Konstruktionsparameter

• Largest TPC:

– Length 5m

– diameter 5m

– Volume 88m3

– Detector area 32m2

– Channels ~570 000

• High Voltage:

– Cathode -100kV

• Material X0

– Cylinder from composit

materias from airplane

industry (X0= ~3%)


ALICE TPC: Pictures of the construction

Precision in z: 250m

Wire chamber: 40m

End plates 250m


ALICE : Simulation of Particle Tracks

• Simulation of particle tracks for a

Pb Pb collision (dN/dy ~8000)

• Angle: Q=60 to 62º

• If all tracks would be shown the

picture would be entirely yellow !

• TPC is currently under

Commissioning !


ALICE TPC


My personal

contribution:

A visit inside the TPC.

Solid State DetectorsW. Riegler/CERN 24

Detectors based on Ionization

Gas detectors:

• Transport of Electrons and Ions in Gases

• Wire Chambers

• Drift Chambers

• Time Projection Chambers


• Transport of Electrons and Holes in Solids

• Si- Detectors

• Diamond Detectors

Originally:

Solid state ionization chambers in Crystals (Diamond, Ge, CdTe …)

Primary ionization from a charged particle traversing the detector moves

in the applied electric field and induced a signal on the metal electrodes.

Principle difficulty:

Extremely good insulators are needed in order to suppress dark currents

and the related fluctuations (noise) which are hiding the signal.

Advantage to gas detectors:

1000x more charge/cm (density of solids 103 times density of gas)

Ionization energy is only a few eV (up to times smaller than gas).



Diamond Detector


Velocity:

μe=1800 cm2/Vs, μh=1600 cm2/Vs, 13.1eV per e-h pair.

Velocity = μE, 10kV/cm v=180 μm/ns Very fast signals of only a few ns length !

Charges are trapped along their path. Charge collection efficiency approx 50%.

Diamond is an extremely interesting material. The problem is that large size single crystals cannot be grown

at present. The technique of chemical vapor deposition can be used to grow polycrystalline diamonds only.

The boundaries between crystallites are probably responsible for incomplete charge collection in this

material.

Typical thickness – a few 100μm

Silicon Detector


Velocity:

μe=1450 cm2/Vs, μh=505 cm2/Vs, 3.63eV per e-h pair.

~11000 e/h pairs in 100μm of silicon.

However: Free charge carriers in Si:

T=300 K: n = 1.45 x 1010 / cm3 but only 33000e-/h in 300m produced by a

high energy particle.

Why do we use Si as a solid state detector ???

n-type

p-type

doping

p n

Silicon Detector used as a Diode !


At the p-n junction the charges are

depleted and a zone free of charge

carriers is established.

By applying a voltage, the depletion

zone can be extended to the entire

diode highly insulating layer.

If an ionizing particle produced free

charge carriers in the diode they

drift in the electric field an produce

an electric field.

As silicon is the most commonly

used material in the electronics

industry, it has one big advantage

with respect to other

materials, namely highly developed

technology.


Si-Diode used as a Particle Detector !

300m

SiO2

passivation

readout capacitances

ca. 50-150 m

Silicon Detector


Fully depleted zone

N (e-h) = 11 000/100μm

Position Resolution down to ~ 5μm !


Silicon Detector

Every electrode is connected to an amplifier

Highly integrated readout electronics.

Two dimensional readout is possible.

Outer Barrel module

Picture of an CMS Si-Tracker Module


CMS Tracker Layout

Outer Barrel --

TOB-

Inner Barrel & Disks

–TIB & TID -

End Caps –TEC

1&2-

2,4

m

Total Area : 200m2

Channels : 9 300 000


W. Riegler/CERN 34

CMS Tracker

ionizing particle

Collection

drift cathodespull-up

cathode

bias HV divider

Silicon Drift Detector (like gas TPC !)


Drift distance (mm)

Res

olu

tion

(

m) Anode axis (Z)

Drift time axis (R-F)


Silicon Drift Detector (like gas TPC !)

Pixel-Detectors


Problem:

2-dimensional readout of strip detectors results in ‘Ghost Tracks’ at

high particle multiplicities i.e. many particles at the same time.

Solution:

Si detectors with 2 dimensional ‘chessboard’ readout. Typical size 50

x 200 μm.

Problem:

Coupling of readout electronics to the detector.

Solution:

Bump bonding.

Bump Bonding of each Pixel Sensor to the Readout Electronics


ATLAS: 1.4x108 pixels

Pixel Detector Application: Hybrid Photon Detector


Elektro-Magnetic Interaction of Charged Particles

with Matter

1) Energy Loss by Excitation and Ionization

2) Energy Loss by Bremsstrahlung

3) Cherekov Radiation and 4) Transition Radiation are only minor

contributions to the energy loss, they are however important effects for

particle identification.

Classical QM

W. Riegler/CERN 40

A charged particle of mass M and

charge q=Z1e is deflected by a

nucleus of Charge Ze.

Because of the acceleration the

particle radiated EM waves

energy loss.

Coulomb-Scattering (Rutherford

Scattering) describes the deflection

of the particle.

Maxwell’s Equations describe the

radiated energy for a given

momentum transfer.

dE/dx

Bremsstrahlung, semi-classical:


Proportional to Z2/A of the Material.

Proportional to Z14 of the incoming

particle.

Proportional zu of the particle.

Proportional 1/M2 of the incoming

particle.

Proportional to the Energy of the

Incoming particle

E(x)=Exp(-x/X0) – ‘Radiation Length’

X0 M2A/ ( Z14 Z2)

X0: Distance where the Energy E0 of

the incoming particle decreases

E0Exp(-1)=0.37E0 .

W. Riegler/CERN 42

Elektron Momentum 5 50 500 MeV/c

Critical Energy: If dE/dx (Ionization) = dE/dx (Bremsstrahlung)

Myon in Copper: p 400GeV

Electron in Copper: p 20MeV

W. Riegler/CERN 43

Critical Energy

For the muon, the second

lightest particle after the

electron, the critical

energy is at 400GeV.

The EM Bremsstrahlung is

therefore only relevant for

electrons at energies of

past and present

detectors.

For E>>mec2=0.5MeV : = 9/7X0

Average distance a high energy

photon has to travel before it

converts into an e+ e- pair is

equal to 9/7 of the distance that a

high energy electron has to

travel before reducing it’s energy

from E0 to E0*Exp(-1) by photon

radiation.

44W. Riegler/CERN

45W. Riegler/CERN

Electro-Magnetic Shower of High Energy Electrons and Photons

W. Riegler/CERN 46

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