Particle Detectors
Summer Student Lectures 2007 Werner Riegler, CERN, [email protected]
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
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) = 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’
W. Riegler/CERN Gas Detectors 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.
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.
b a b Wire
W. Riegler/CERN Gas Detectors 4 Gas Detectors with internal Electron Multiplication
From L. Ropelewski
W. Riegler/CERN 5 Wire Chamber: Electron Avalanches on the Wire
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.
W. Riegler/CERN Gas Detectors 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 (<<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.
W. Riegler/CERN Gas Detectors 7 Detectors with Electron Multiplication
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.
W. Riegler/CERN Gas Detectors 8 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 !
W. Riegler/CERN Gas Detectors 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 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.
W. Riegler/CERN Gas Detectors 10 Multi Wire Proportional Chamber Cathode strip:
Width (1) of the charge distribution DIstance
‘Center of gravity’ defines the particle trajectory.
Avalanche
(a) (b) Anode wire
1.07 mm Cathode strips
0.25 mm C1 C1 C1 C1 C1 C1
1.63 mm C2 C2 C2 C2
W. Riegler/CERN Gas Detectors 11 Drift Chambers 1970: Amplifier: t=T
E
Scintillator: t=0
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 !
W. Riegler/CERN Gas Detectors 12 Drift Chambers, typical Geometries Electric Field 1kV/cm
W. Klempt, Detection of Particles with Wire Chambers, Bari 04
W. Riegler/CERN Gas Detectors 13 The Geiger counter reloaded: Drift Tube
Primary electrons are drifting to the wire.
ATLAS MDT R(tube) =15mm Calibrated Radius-Time correlation 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 Muon Chambers
ATLAS MDTs, 80m per tube W. Riegler/CERN Gas Detectors 14 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, <50m/chamber (3 bar) Maximum drift time 700ns
Gas Ar/CO2 93/7
W. Riegler/CERN Gas Detectors 15 ATLAS Muon Chamber Front-End Electronics
Single Channel Block Diagram 3.18 x 3.72 mm
• 0.5m CMOS technology – 8 channel ASD + Wilkinson ADC – fully differential – 15ns peaking time – 32mW/channel – JATAG programmable Harvard University, Boston University
Designed around in 1997, produced in 2000, today – 0.17um process … rapidly changing technologies.
W. Riegler/CERN Gas Detectors 16 Large Drift Chambers: Central Tracking Chamber CDF Experiment
660 drift cells tilted 450 with respect to the particle track.
Drift cell
W. Riegler/CERN Gas Detectors 17 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 volume
B drift
E y x z charged track
wire chamber to detect projected tracks
W. Riegler/CERN Gas Detectors 18 ALICE TPC: Detector Parameters
• Gas Ne/ CO2 90/10% • Field 400V/cm • Gas gain >104 • Position resolution = 0.2mm
• Diffusion: t= 250m cm • Pads inside: 4x7.5mm • Pads outside: 6x15mm • B-field: 0.5T
W. Riegler/CERN Gas Detectors 19 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%)
W. Riegler/CERN Gas Detectors 20 ALICE TPC: Pictures of the construction
Precision in z: 250m End plates 250m
Wire chamber: 40m W. Riegler/CERN Gas Detectors 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 !
W. Riegler/CERN Gas Detectors 22 ALICE TPC
My personal contribution:
A visit inside the TPC.
W. Riegler/CERN Gas Detectors 23 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 Solid State Detectors 24 Solid State 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).
W. Riegler/CERN Solid State Detectors 25 Diamond Detector
Typical thickness – a few 100μm
Velocity: 2 2 μe=1800 cm /Vs, μh=1600 cm /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.
W. Riegler/CERN Solid State Detectors 26 Silicon Detector
Velocity: 2 2 μe=1450 cm /Vs, μh=505 cm /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 ???
W. Riegler/CERN Solid State Detectors 27 Silicon Detector used as a Diode !
p n
doping
n-type
p-type
W. Riegler/CERN Solid State Detectors 28 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.
W. Riegler/CERN Solid State Detectors 29 Silicon Detector
ca. 50-150 m readout capacitances
Fully depleted zone SiO2 passivation
300m
N (e-h) = 11 000/100μm Position Resolution down to ~ 5μm !
W. Riegler/CERN Solid State Detectors 30 Silicon Detector
Every electrode is connected to an amplifier Highly integrated readout electronics.
Two dimensional readout is possible.
W. Riegler/CERN Solid State Detectors 31 Picture of an CMS Si-Tracker Module
Outer Barrel module
W. Riegler/CERN Solid State Detectors 32 CMS Tracker Layout
Outer Barrel -- TOB- End Caps –TEC 1&2- Inner Barrel & Disks –TIB & TID -
Total Area : 200m2 2,4
m Channels : 9 300 000
W. Riegler/CERN Solid State Detectors 33 CMS Tracker
W. Riegler/CERN 34 Silicon Drift Detector (like gas TPC !)
bias HV divider
Collection
drift cathodes pull-up ionizing particle cathode
W. Riegler/CERN Solid State Detectors 35 Silicon Drift Detector (like gas TPC !)
Anode axis (Z)
m)
Drift time axis (R-F) Resolution ( Resolution
Drift distance (mm)
W. Riegler/CERN Solid State Detectors 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.
W. Riegler/CERN Solid State Detectors 37 Bump Bonding of each Pixel Sensor to the Readout Electronics
ATLAS: 1.4x108 pixels
W. Riegler/CERN Solid State Detectors 38 Pixel Detector Application: Hybrid Photon Detector
W. Riegler/CERN Solid State Detectors 39 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=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
W. Riegler/CERN Solid State Detectors 41 Proportional to Z2/A of the Material.
4 Proportional to Z1 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’
2 4 2 X0 M A/ ( Z1 Z )
X0: Distance where the Energy E0 of the incoming particle decreases
E0Exp(-1)=0.37E0 .
W. Riegler/CERN 42 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.
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 2 For E>>mec =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.
W. Riegler/CERN 44 Electro-Magnetic Shower of High Energy Electrons and Photons
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