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 DetectorsW. Riegler/CERN 3
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):
Gas DetectorsW. Riegler/CERN 4
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
Gas DetectorsW. Riegler/CERN 6
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.
Gas DetectorsW. Riegler/CERN 7
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.
Gas DetectorsW. Riegler/CERN 8
Detectors with Electron Multiplication
Charpak et. al. 1968, Multi Wire Proportional Chamber
Gas DetectorsW. Riegler/CERN 9
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.
Gas DetectorsW. Riegler/CERN 10
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
Gas DetectorsW. Riegler/CERN 11
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
Gas DetectorsW. Riegler/CERN 12
Drift Chambers, typical Geometries
W. Klempt, Detection of Particles with Wire Chambers, Bari 04
Electric Field 1kV/cm
Gas DetectorsW. Riegler/CERN 13
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
Gas DetectorsW. Riegler/CERN 14
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
Gas DetectorsW. Riegler/CERN 15
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
Gas DetectorsW. Riegler/CERN 16
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
Gas DetectorsW. Riegler/CERN 17
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 DetectorsW. Riegler/CERN 18
• 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
Gas DetectorsW. Riegler/CERN 19
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%)
Gas DetectorsW. Riegler/CERN 20
ALICE TPC: Pictures of the construction
Precision in z: 250m
Wire chamber: 40m
End plates 250m
Gas DetectorsW. 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 !
Gas DetectorsW. Riegler/CERN 22
ALICE TPC
Gas DetectorsW. Riegler/CERN 23
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
Solid State Detectors
• 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).
Solid State DetectorsW. Riegler/CERN 25
Solid State Detectors
Diamond Detector
Solid State DetectorsW. Riegler/CERN 26
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
Solid State DetectorsW. Riegler/CERN 27
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 !
Solid State DetectorsW. Riegler/CERN 28
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.
Solid State DetectorsW. Riegler/CERN 29
Si-Diode used as a Particle Detector !
300m
SiO2
passivation
readout capacitances
ca. 50-150 m
Silicon Detector
Solid State DetectorsW. Riegler/CERN 30
Fully depleted zone
N (e-h) = 11 000/100μm
Position Resolution down to ~ 5μm !
Solid State DetectorsW. Riegler/CERN 31
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
Solid State DetectorsW. Riegler/CERN 32
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
Solid State DetectorsW. Riegler/CERN 33
W. Riegler/CERN 34
CMS Tracker
ionizing particle
Collection
drift cathodespull-up
cathode
bias HV divider
Silicon Drift Detector (like gas TPC !)
Solid State DetectorsW. Riegler/CERN 35
Drift distance (mm)
Res
olu
tion
(
m) Anode axis (Z)
Drift time axis (R-F)
Solid State DetectorsW. Riegler/CERN 36
Silicon Drift Detector (like gas TPC !)
Pixel-Detectors
Solid State DetectorsW. Riegler/CERN 37
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
Solid State DetectorsW. Riegler/CERN 38
ATLAS: 1.4x108 pixels
Pixel Detector Application: Hybrid Photon Detector
Solid State DetectorsW. Riegler/CERN 39
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:
Solid State DetectorsW. Riegler/CERN 41
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.
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Electro-Magnetic Shower of High Energy Electrons and Photons
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