Particle Detectors
Summer Student Lectures 2007
Werner Riegler, CERN, werner.riegler@cern.ch

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
W. Riegler/CERN
•
Transport of Electrons and Holes in Solids
•
Si- Detectors
•
Diamond Detectors
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 )
–
–
–
W. Riegler/CERN
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
3
Wire Chamber: Electron Avalanche
Wire with radius (10-25m) in a tube of radius b (1-3cm):
Electric field close to a thin wire (100-300kV/cm). E.g.
V0=1000V, a=10m, 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.
ab
W. Riegler/CERN
b
Wire
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: A103-104
Semi proportional region: A104-105
(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: A109
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 100s. )
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 (v5cm/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 50m 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 s trips
0.25 mm
1.63 mm
W. Riegler/CERN
Gas Detectors
C1
C2
C1
C2
C1
C1
C2
C1
C1
C2
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, 80m 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: 80m/tube, <50m/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
• 0.5m 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= 250m 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: 250m
End plates 250m
Wire chamber: 40m
W. Riegler/CERN
Gas Detectors
21
ALICE : Simulation of Particle Tracks
W. Riegler/CERN
Gas Detectors
•
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 !
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
W. Riegler/CERN
•
Transport of Electrons and Holes in Solids
•
Si- Detectors
•
Diamond Detectors
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:
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.
W. Riegler/CERN
Solid State Detectors
26
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 300m 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
SiO2
passivation
Fully depleted zone
300m
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-
Inner Barrel & Disks
–TIB & TID -
End Caps –TEC
1&2-
2,4
m
Total Area : 200m2
W. Riegler/CERN
Channels : 9 300 000
Solid State Detectors
33
CMS Tracker
W. Riegler/CERN
34
Silicon Drift Detector (like gas TPC !)
bias HV divider
Collection
drift cathodes
ionizing particle
W. Riegler/CERN
pull-up
cathode
Solid State Detectors
35
Resolution (m)
Silicon Drift Detector (like gas TPC !)
Anode axis (Z)
Drift time axis (R-F)
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.
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
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:
Electron in Copper:
W. Riegler/CERN
p  400GeV
p  20MeV
43
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.
W. Riegler/CERN
44
Electro-Magnetic Shower of High Energy Electrons and Photons
W. Riegler/CERN
45
W. Riegler/CERN
46
W. Riegler/CERN
47
W. Riegler/CERN
48
W. Riegler/CERN
49
W. Riegler/CERN
50
W. Riegler/CERN
51
W. Riegler/CERN
52
W. Riegler/CERN
53
W. Riegler/CERN
54
W. Riegler/CERN
55
W. Riegler/CERN
56