CERN Summer Student Lectures 2003 Particle Detectors

1
Introduction
Particle Detectors
Summer Student Lecture Series 2003 Christian
Joram EP / TA1
From (very) basic ideas to
rather complex detector systems
1 1 ? 2
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, 1115
Tue, 8 June, 1100
3
Introduction

• Literature 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, TaylorFrancis, 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/ParticleDetec
  tor/BriefBook/

4
Introduction
The oldest particle detector (built many
billion times)

• High sensitivity to photons
• Good spatial resolution
• Very large dynamic range (11014)
• automatic threshold adaptation
• Energy (wavelength) discrimination
• Modest speed.
• Data taking rate 10Hz (incl. processing)

retina
5
Introduction
Use of photographic paper as detector ?
Detection of photons / x-rays
W. C. Röntgen, 1895 Discovery 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
6
Introduction
J. Plücker 1858 ? J.J. Thomson 1897
Thomsons cathode ray tube
accelerator
manipulation By E or B field
detector
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.
Scintillation of glass
7
Introduction
E. Rutherford
H. Geiger
1909
pulse
The Geiger counter, later further developed and
then called Geiger-Müller counter
First electrical signal from a particle
8
Introduction
C. T. R. Wilson, 1912, Cloud chamber
First tracking detector
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.
9
Introduction
progress cycle
physics theories
technologies materials
knowledge / progress
experiments
detectors
10
Introduction

• A WW- decay in ALEPH

11
Introduction

• Reconstructed B-mesons in the DELPHI micro vertex
  detector

tB ? 1.6 ps l ctg ? 500 mm?g
Primary Vertex
Primary Vertex
12
Introduction

• A simulated event in ATLAS (CMS)
• H ? ZZ ? 4m

pp collision at ?s 14 TeV sinel. ? 70
mb Interested in processes with s ? 10-100 fb
L 1034 cm-2 s-1, bunch
spacing 25 ns
m
m
m
m
? 23 overlapping minimum bias events / BC ? 1900
charged 1600 neutral particles / BC
13
Introduction
Idealistic views of an elementary particle
reaction
q
e
Z
e-
time

• 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 !

m
14
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)

detector

• end products
• charged
• neutral
• photons

m
15
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

1 eV is a tiny portion of energy. 1 eV
1.610-19 J
mbee 1g 5.81032 eV/c2 vbee 1m/s ?
Ebee 10-3 J 6.251015 eV ELHC 141012 eV
To rehabilitate LHC Total stored beam energy
1014 protons 141012 eV ? 1108 J this
corresponds to a
mtruck 100 T vtruck 120 km/h
16
Definitions and units
The concept of cross sections
Cross sections s or differential cross sections
ds/dW are used to express the probability of
interactions between elementary particles.
Example 2 colliding particle beams
beam spot area A
F1 N1/t
F2 N2/t
What is the interaction rate Rint. ?
Rint ? N1N2 / (A t) s L
s has dimension area ! Practical unit 1 barn
(b) 10-24 cm2
Luminosity L cm-2 s-1
Example Scattering from target
scattered beam
solid angle element dW
target
q
incident beam
Nscat(q) ? Ninc nA dW ds/dW (q)
NincnA dW
.nA area density of scattering centers in
target
17

Momentum measurement
Tracking
Multiple scattering
Bethe-Bloch formula / Landau tails
Ionization of gases
Wire chambers
Drift and diffusion in gases
Drift chambers
Micro gas detectors
Silicon as a detection medium
Silicon detectors strips/pixels
18
Momentum measurement

19
Momentum measurement

• Momentum measurement
• the sagitta s is determined by 3 measurements
  with error s(x)
• for N equidistant measurements, one obtains
• (R.L. Gluckstern, NIM 24 (1963) 381)
• ex pT1 GeV/c, L1m, B1T, s(x)200mm, N10

(for N ? ?10)
(s ? 3.75 cm)
20
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 Scattering
• Sufficiently thick material layer
• ? the particle will undergo multiple scattering.

Rutherford formula
21
Momentum measurement
Approximation X0 is radiation length of the
medium (discuss later)

• Back to momentum measurements
• What is the contribution of multiple scattering
  to ?

remember
independent of p !
More precisely

• ex Ar (X0110m), L1m, B1T

22
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
  (meltltmN).
• If are big enough ? ionization.

q
e
-
Instead of ionizing an atom, under certain
conditions the photon can also escape from the
medium. ? Emission of Cherenkov and
Transition radiation. (See later).
23
Bethe-Bloch formula
Average differential energy loss Ionisation
only ? Bethe - Bloch formula

• dE/dx in MeV g-1 cm2
• Bethe-Bloch formula only valid for heavy
  particles (m?mm).
• dE/dx depends only on b, independent of m !
• First approximation medium simply characterized
  by
• electron density

Z/A 1
Fermi plateau
Z/A0.5
relativistic rise
?? ? 3-4 minimum ionizing particles, MIPs
kinematical term
24
Landau tails
Real detectors (limited granularity) can not
measure ltdE/dxgt ! It measures the energy DE
deposited in a layer of finite thickness dx.

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
e-
ltDEgt
DE
For thick layers and high density materials ?
Many collisions. ? Central Limit Theorem ?
Gaussian shape distributions.
e-
ltDEgt
DE
25
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.

Primary ionization
Total ionization
10 - 40 pairs/cm DE/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.
26
Proportional Counter

• Gas amplification
• Consider cylindrical field geometry (simplest
  case)

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.
27
Proportional Counter
a First Townsend coefficient (e--ion pairs/cm)
l mean free path
Gain
(F. Sauli, CERN 77-09)
(O. Allkofer, Spark chambers, Theimig München,
1969)
e
28
Proportional Counter

• Signal
• formation
• Avalanche formation within a few wire radii and
  within t lt 1 ns!
• Signal induction both on anode and cathode due to
  moving charges (both electrons and ions).

(F. Sauli, CERN 77-09)
Electrons collected by anode wire, i.e. dr is
small (few mm). 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 !
(F. Sauli, CERN 77-09)
Need electronic signal differentiation to limit
dead time.