Slides - Indico - Cern

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 2007 

Werner Riegler, CERN, werner.riegler@cern.ch 

W. Riegler/CERN 1

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 

W. Riegler/CERN Gas Detectors 

2

Gas Detectors with internal Electron Multiplication 

• 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) = N 0 exp (αx) α= α(E), N/ N 0 = A (Amplification, Gas Gain) 

– N(x)=N 0 exp ( (E)dE ) 

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

themselves photoelectrons 

– NAγ photoeletrons → NA 2 γ electrons → NA 2 γ 2 photoelectrons → NA 3 γ 2 electrons 

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


3

Wire Chamber: Electron Avalanche 

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

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

V 0=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. 

b 

a 

b 

Wire 


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From L. Ropelewski 

Gas Detectors with internal Electron Multiplication 

W. Riegler/CERN 5

Wire Chamber: Electron Avalanches on the Wire 

Proportional region: A10 3 -10 4 

Semi proportional region: A10 4 -10 5 

(space charge effect) 

Saturation region: A >10 6 

Independent from the number of primary 

electrons. 

Streamer region: A >10 7 

Avalanche along the particle track. 

Limited Geiger region: 

Avalanche propagated by UV photons. 

Geiger region: A10 9 

Avalanche along the entire wire. 


6

Wire Chamber: Signals from Electron Avalanches 

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 

(

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 


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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 ! 


9

Charpak et. al. 1968, Multi Wire Proportional Chamber 

In order to eliminate the left/right 

ambiguities: Shift two wire chambers by 

half the wire pitch. 

For second coordinate: 

Another Chamber at 90 0 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. 


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Multi Wire Proportional Chamber 

(a) 

1.07 mm 

0.25 mm 

1.63 mm 

(b) 

Cathode strip: 

Width (1) of the charge 

distribution DIstance 

‘Center of gravity’ defines the 

particle trajectory. 

C 1 C 1 C 1 C 1 C 1 

C 2 

Avalanche 

C 2 

C 2 

C 2 

Anode wire 

Cathode strips 


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C 1

Drift Chambers 1970: 

E 

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 ! 

Amplifier: t=T 

Scintillator: t=0 

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

Proportional Chamber) save electronics channels ! 


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Drift Chambers, typical Geometries 

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

Electric Field 1kV/cm 


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The Geiger counter reloaded: Drift Tube 

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

correlation 

ATLAS Muon Chambers 

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 


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The Geiger counter reloaded: Drift Tube 

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,

ATLAS Muon Chamber Front-End Electronics 

3.18 x 3.72 mm 

Harvard University, Boston University 

Single Channel Block Diagram 

• 0.5m CMOS technology 

– 8 channel ASD + Wilkinson 

ADC 

– fully differential 

– 15ns peaking time 

– 32mW/channel 

– JATAG programmable 

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


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Large Drift Chambers: Central Tracking Chamber CDF Experiment 

660 drift cells tilted 45 0 

with respect to the 

particle track. 

Drift cell 


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y 

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 ! 

z 

x 

B drift 

E 

charged track 

gas volume 

wire chamber 

to detect 

projected tracks 


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• Gas Ne/ CO 2 90/10% 

• Field 400V/cm 

• Gas gain >10 4 

• Position resolution = 0.2mm 

• Diffusion: t= 250m cm 

• Pads inside: 4x7.5mm 

• Pads outside: 6x15mm 

• B-field: 0.5T 

ALICE TPC: Detector Parameters 


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• Largest TPC: 

– Length 5m 

– diameter 5m 

– Volume 88m 3 

– Detector area 32m 2 

– Channels ~570 000 

• High Voltage: 

– Cathode -100kV 

• Material X 0 

– Cylinder from composit 

materias from airplane 

industry (X 0= ~3%) 

ALICE TPC: Konstruktionsparameter 


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Precision in z: 250m 

ALICE TPC: Pictures of the construction 

Gas Detectors 

End plates 250m 

Wire chamber: 40m 

W. Riegler/CERN 21

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 ! 


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ALICE TPC 

My personal 

contribution: 

A visit inside the TPC. 


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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 

W. Riegler/CERN Solid State Detectors 

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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 10 3 times density of gas) 

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


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Typical thickness – a few 100μm 

Diamond Detector 

Velocity: 

μ e=1800 cm 2 /Vs, μ h=1600 cm 2 /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. 


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Silicon Detector 

Velocity: 

μ e=1450 cm 2 /Vs, μ h=505 cm 2 /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 10 10 / cm 3 but only 33000e-/h in 300m produced by a 

high energy particle. 

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


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Silicon Detector used as a Diode ! 

doping 

n-type 

p-type 

p n 


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Si-Diode used as a Particle Detector ! 

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. 


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Fully depleted zone 


readout capacitances 

N (e-h) = 11 000/100μm 

Position Resolution down to ~ 5μm ! 

ca. 50-150 m 

SiO 2 

passivation 

300m 


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Every electrode is connected to an amplifier 

Highly integrated readout electronics. 

Two dimensional readout is possible. 


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Picture of an CMS Si-Tracker Module 

Outer Barrel module 


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2,4 

m 

Inner Barrel & Disks 

–TIB & TID - 

CMS Tracker Layout 

Outer Barrel -- 

TOB- 

End Caps –TEC 

1&2- 

Total Area : 200m 2 

Channels : 9 300 000 


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CMS Tracker 

W. Riegler/CERN 34

Silicon Drift Detector (like gas TPC !) 

drift cathodes 

ionizing particle 

bias HV divider 

Collection 

pull-up 

cathode 


35

Resolution (m) 

Silicon Drift Detector (like gas TPC !) 

Anode axis (Z) 

Drift time axis (R-F) 

Drift distance (mm) 


36

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. 


37

Bump Bonding of each Pixel Sensor to the Readout Electronics 

ATLAS: 1.4x10 8 pixels 


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Pixel Detector Application: Hybrid Photon Detector 


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Elektro-Magnetic Interaction of Charged Particles 

with Matter 

Classical QM 

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. 

W. Riegler/CERN 40

Bremsstrahlung, semi-classical: 

A charged particle of mass M and 

charge q=Z 1e 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 


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Proportional to Z 2 /A of the Material. 

Proportional to Z 1 4 of the incoming 

particle. 

Proportional zu of the particle. 

Proportional 1/M 2 of the incoming 

particle. 

Proportional to the Energy of the 

Incoming particle 

E(x)=Exp(-x/X 0) – ‘Radiation Length’ 

X 0 M 2 A/ ( Z 1 4 Z 2 ) 

X 0: Distance where the Energy E 0 of 

the incoming particle decreases 

E 0Exp(-1)=0.37E 0 . 

W. Riegler/CERN 42

Critical Energy 

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 

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. 

W. Riegler/CERN 43

W. Riegler/CERN 

For E>>m ec 2 =0.5MeV : = 9/7X 0 

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 E 0 to E 0*Exp(-1) by photon 

radiation. 

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Electro-Magnetic Shower of High Energy Electrons and Photons 

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W. Riegler/CERN 46


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