CERN Summer Student Lectures 2003Particle Detectors
Christian Joram I/1
Introduction
Summer Student Lecture Series 2003
Christian Joram
EP / TA1
From (very) basic ideas
to
rather complex
detector systems
1 + 1 2
CERN Summer Student Lectures 2003Particle Detectors
Christian Joram I/2
Introduction
Outline + approximate timing
Introduction, basics
Tracking (gas, solid state)
Scintillation and light detection
Calorimetry
Particle Identification
Detector Systems
Discussion session I
Discussion session II
= Detector Exhibition
Thu/Fri (2x45 min)
Mon/
Tue
(2 x45 min)
Wed
(45 min)
Fri, 4 June, 11:15
Tue, 8 June, 11:00
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Introduction
Literature on particle detectorsLiterature on particle detectors
Text books C. Grupen, Particle Detectors, Cambridge University
Press, 1996 G. Knoll, Radiation Detection and Measurement, 3rd
Edition, 2000 W. R. Leo, Techniques for Nuclear and Particle
Physics Experiments, 2nd edition, Springer, 1994 R.S. Gilmore, Single particle detection and
measurement, Taylor&Francis, 1992 W. Blum, L. Rolandi, Particle Detection with Drift
Chambers, Springer, 1994 K. Kleinknecht, Detektoren für Teilchenstrahlung, 3rd
edition, Teubner, 1992
Review articles Experimental techniques in high energy physics, T.
Ferbel (editor), World Scientific, 1991. Instrumentation in High Energy Physics, F. Sauli
(editor), World Scientific, 1992. Many excellent articles can be found in Ann. Rev.
Nucl. Part. Sci.
Other sources Particle Data Book (Phys. Rev. D, Vol. 54, 1996) R. Bock, A. Vasilescu, Particle Data Briefbook
http://www.cern.ch/Physics/ParticleDetector/BriefBook/
Proceedings of detector conferences (Vienna VCI, Elba, IEEE)
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retina
“The oldest particle detector”(built many billion times)
• High sensitivity to photons• Good spatial resolution• Very large dynamic range (1:1014)
+ automatic threshold adaptation• Energy (wavelength) discrimination • Modest speed.
Data taking rate ~ 10Hz (incl. processing)
Introduction
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Use of photographic paper as detector Detection of photons / x-rays
W. C. Röntgen, 1895Discovery of the ‘X-Strahlen’
Photographic paper/film
e.g. AgBr / AgCl
AgBr + ‘energy’ metallic Ag (blackening)
+ Very good spatial resolution+ Good dynamic range- No online recording- No time resolution
Introduction
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From: J.J. Thomson: Cathode Rays. Philosophical Magazine, 44, 293 (1897).
“… The rays from the cathode C pass through a slit in the anode A, which is a metal plug fitting tightly into the tube and connected with the earth; after passing through a second slit in another earth-connected metal plug B, they travel between two parallel aluminium plates about 5 cm. long by 2 broad and at a distance of 1.5 cm. apart; they then fall on the end of the tube and produce a narrow well-defined phosphorescent patch. A scale pasted on the outside of the tube serves to measure the deflexion of this patch….”
J. Plücker 1858 J.J. Thomson 1897
accelerator manipulationBy E or B field
detector
Thomson’s cathode ray tube
Scintillation of glass
Introduction
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E. Rutherford H. Geiger1909
The Geiger counter, later further developed and then calledGeiger-Müller counter
First electrical signal from a particle
pulse
Introduction
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C. T. R. Wilson, 1912, Cloud chamber
The general procedure was to allow water to evaporate in an enclosed container to the point of saturation and then lower the pressure, producing a super-saturated volume of air. Then the passage of a charged particle would condense the vapor into tiny droplets, producing a visible trail marking the particle's path.
First trackingdetector
Introduction
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physics theories
experiments
technologies& materials
knowledge / progress
Introduction
“progress cycle”
detectors
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Introduction
A W+W- decay in ALEPH
e+e- (s=181 GeV) W+W- qq 2 hadronic jets + missing momentum
_
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Introduction
Reconstructed B-mesons in the DELPHI micro vertex detector
B 1.6 ps l = c 500 m
Primary Vertex
Primary Vertex
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Introduction
A simulated event in ATLAS (CMS)
H ZZ 4
pp collision at s = 14 TeV
inel. 70 mb
Interested in processes
with fb
23 overlapping minimum bias events / BC
1900 charged + 1600 neutral particles / BC
L = 1034 cm-2 s-1, bunch
spacing 25 ns
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tim
e
e-
e+ q
q-
Z
Introduction
Idealistic views of an elementary particle reaction
• Usually we can only ‘see’ the end products of the reaction, but not the reaction itself.
• In order to reconstruct the reaction mechanism and the properties of the involved particles, we want the maximum information about the end products !
ion)hadronizat (
0
qqZee
m
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Introduction
The ‘ideal’ particle detector should provide…
coverage of full solid angle (no cracks, fine segmentation
measurement of momentum and/or energy detect, track and identify all particles (mass, charge) fast response, no dead time
practical limitations (technology, space, budget)
Particles are detected via their interaction with matter.
Many different physical principles are involved (mainly of electromagnetic nature).
Finally we will always observe... ionizationionization and excitationexcitation of matter.
pppp,
ep,,ee
end products • charged • neutral• photons
detector
m
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Definitions and units
Some important definitions and units
energy E: measure in eV momentum p: measure in eV/c mass mo: measure in eV/c2
420
222 cmcpE
1 eV is a tiny portion of energy. 1 eV = 1.6·10-19 J
mbee = 1g = 5.8·1032 eV/c2
vbee= 1m/s Ebee = 10-3 J = 6.25·1015 eV
ELHC = 14·1012 eV
To rehabilitate LHC…
Total stored beam energy:
1014 protons * 14·1012 eV 1·108 J
this corresponds to a mtruck = 100 T
vtruck = 120 km/h
11
110
2c
v
20 cmE cmp 0
E
pc
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The concept of cross sections
Definitions and units
Cross sections or differential cross sections ddare used to express the probability of interactions between elementary particles.
Example: 2 colliding particle beams
= N1/t = N2/t
What is the interaction rate Rint. ?
beam spot area A
Rint ·t) = · L
Luminosity L [cm-2 s-1]
Example: Scattering from target
solid angle element d
incident beam
scattered beam
target
Nscat() Ninc· nA · d = dd()·Ninc·nA·
d
has dimension area !
Practical unit:
1 barn (b) = 10-24 cm2
.nA = area density
of scattering
centers in target
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Multiple scattering
Bethe-Bloch formula / Landau tails
Ionization of gases
Wire chambers
Drift and diffusion in gases
Drift chambers
Micro gas detectors
Silicon detectors strips/pixels
Momentum measurement
Silicon as a detection medium
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Momentum measurement
Momentum measurement
the sagitta s is determined by 3 measurements with error (x):
for N equidistant measurements, one obtains (R.L. Gluckstern, NIM 24 (1963) 381)
ex: pT=1 GeV/c, L=1m, B=1T, (x)=200m, N=10
L
s
B
y
x
qBpBvqmv
T )(2
2
23
23.
3.0
8)()()(
BL
px
s
x
s
s
p
p Tmeas
T
T
)4/(720
3.0
)(2
.
NBL
px
p
p Tmeas
T
T (for N 10)
%5.0
.
meas
T
T
p
p
)( 3121
2 xxxs
(s 3.75 cm)
m)(T3.0)cGeV( BpT
Tp
BLL
3.022sin
2
Tp
BLs
22
8
3.0
82cos1
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Multiple Scattering
Scattering
An incoming particle with charge z interacts with
a target of nuclear charge Z. The cross-section
for this e.m. process is
Average scattering angle Cross-section for infnite !
Multiple ScatteringSufficiently thick material layer
the particle will undergo multiple scattering.
2sin
14
4
22
p
cmzZr
d
d ee
00
Rutherford formula
d/d
L
plane
rplane
RMSplane
RMSplane space
2
120
P
plane0
Gaussian
sin-4(/2)
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Momentum measurement
Back to momentum measurements:
What is the contribution of multiple scattering to ?
0
1045.0
)(
LXBp
pMS
T
(p)/p
(p)/p
(p)/p
p
MS
meas.total error
independent of p !
%5.0)(
MS
Tp
p
Approximation
X0 is radiation length of the medium (discuss later)
00
1
X
L
p
Tp
p)(
TT
pxp
p )(
)(
px MS 1)( 0
• ex: Ar (X0=110m), L=1m, B=1T
remember
constant)(
MS
Tp
p
More precisely:
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Interaction of charged particles
Detection of charged particles
How do they loose energy in matter ?
Discrete collisions with the atomic electrons of the absorber material.
Collisions with nuclei not important (me<<mN).
If are big enough ionization.
densityelectron :
0
N
ddE
dNE
dx
dE
e-
k ,
0,mv
k ,
Instead of ionizing an atom, under certain conditions the photon can also escape from the medium.
Emission of Cherenkov and Transition radiation. (See later).
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Bethe-Bloch formula
dE/dx in [MeV g-1 cm2]
Bethe-Bloch formula only valid for “heavy” particles (mm).
dE/dx depends only on independent of m !
First approximation: medium simply characterized by
~ electron density
2
2ln
14 2max
2
222
21
2222
T
I
cm
A
ZzcmrN
dx
dE eeeA
2121 cm MeV g..dxdE
Average differential energy loss
Ionisation only Bethe - Bloch formuladxdE
A
Z
Z/A~0.5
Z/A = 1
2
1
dx
dE
22ln dx
dE
“relativistic rise”
“kinematical term” 3-4 minimum ionizing particles, MIPs
“Fermi plateau”
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Landau tails
For thin layers (and low density materials): Few collisions, some with high energy transfer.
Energy loss distributions show large fluctuations towards high losses: ”Landau tails”
For thick layers and high density materials: Many collisions. Central Limit Theorem Gaussian shape distributions.
Real detectors (limited granularity) can not measure <dE/dx> !
It measures the energy E deposited in a layer of finite thickness x.
<E>
E
e-
e-
<E>
E
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Ionization of gases
Gas detectors
Fast charged particles ionize the atoms of a gas.
Often the resulting primary electron will have enough kinetic energy to ionize other atoms.
primarytotal nn 43
Primary ionization Total ionization
10 - 40 pairs/cmE/pair ~ 20 - 40 eV
• Assume detector, 1 cm thick, filled with Ar gas:
1 cm
~ 100 e-ion pair
100 electron-ion pairs are not easy to detect!
Noise of amplifier 1000 e- (ENC) !
We need to increase the number of e-ion pairs.
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Proportional Counter
Gas amplification
Consider cylindrical field geometry (simplest case):
a
b
r
E
1/r
a
cathode
anode
gas
Ethreshold
a
rCVrV
r
CVrE
ln2
)(
1
2
0
0
0
0
C = capacitance / unit length
Electrons drift towards the anode wire ( stop and go! More
details in next lecture!).
Close to the anode wire the field is sufficiently high (some
kV/cm), so that e- gain enough energy for further ionization
exponential increase of number of e--ion pairs.
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Proportional Counter
0CVkeM
xrxE ennenn 00 or : First Townsend coefficient
(e--ion pairs/cm)
(O. Allkofer, Spark chambers, Theimig München, 1969)
1 : mean free path
Cr
a
drrn
nM exp
0Gain
(F. Sauli, CERN 77-09)
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Signal
formation
Avalanche formation within a few wire radii and within t < 1 ns!
Signal induction both on anode and cathode due to moving charges (both electrons and ions).
Proportional Counter
drdr
dV
lCV
Qdv
0
Electrons collected by anode wire, i.e. dr is small (few m). Electrons contribute only very little to detected signal (few %).
Ions have to drift back to cathode, i.e. dr is big. Signal duration limited by total ion drift time !
Need electronic signal differentiation to limit dead time.
(F. Sauli, CERN 77-09)
(F. Sauli, CERN 77-09)