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Lecture 9
PARTICLE DETECTORS
Detlev Gotta
Institut für Kernphysik, Forschungszentrum Jülich / Universität zu Köln
GGSWBS'12, Batumi, Georgia
5th Georgian – German School and Workshop in Basic Science
August 16, 2012
WHAT TO DETECT ?
HOW TO DETECT?
INTERACTION OF CHARGED PARTICLES WITH MATTER
“
MASSIVE NEUTRAL PARTICLES WITH MATTER
“
RADIATION WITH MATTER
DETECTOR PRINCIPLES
EXAMPLES OF COMBINED DETECTION SYSTEMS
Folie 2
WHAT TO DETECT ?
Folie 3
PARTICLES
particle
detector
registration
Light
Heavy
Folie 4
PARTICLES
What characterizes a particle?
mass
M
charge
Q
Spin intrinsic angular momentum
S
life time
t0
shape (for extended particles)
<r2>
Folie 5
RADIATION
fluid
gas
„light“
fundamental constant: c = speed of light in vacuum ( 30 cm / ns)
Folie 6
RADIATION
What characterizes waves?
wave propagation velocity
c = ln
wave length
l
frequency
n
particle physics
usually electromagnetic radiation
wave propagation velocity in vacuum
“
“
“
in medium
index of refaction
c =ln
c‘ = l‘n < c
n = c / c‘
Folie 7
CONSTITUENTS OF MATTER I
atoms
atomic shells
electron
e
10-10 m
size
nucleus
proton
p
neutron
n
Q
1
+1
0
M
Mp / 1836
Mp
 Mp
0.8  10-15 m
< 10-18 m
life time t0
decay
> 1026 y
-
> 1029 y
-
886 s
n  p e n
Folie 8
CONSTITUENTS OF MATTER II
new particles – unstable being free
pions
kaons
p
K
Q
0,  1
M
 Mp / 7
life time t0
decay
…
2  0,  1
 Mp / 2
0.6  10-15 m
size
many more
0.6  10-15 m
p
2610-9 s
K
p0
810-17 s
K0S,L
1210-9 s
910-10 / 510-8 s
p  m  n
K  m  n, …
p0  g g
K0  p + p , p 0 p0 ,...
Folie 9
PARAMETERS
massive particles
total energy
el.-mag. radiation
Etotal  p 2c 2 + m 02c4
Etotal  pc
 hν
h Planck constant
= minimal action
 Ekin
 γ m 0c 2
Ekin  Etotal  m 0
rest mass
m0 ≠ 0
range in matter
=0
charge
Q ≠0
deflection in el.-mag fields
=0
life time
t = gt0
decay length l = v t
=
relativistic factor
γ
1
1  β2
,
β
v
c
lim γ
attenuation in matter
no deflection
  
v c
Folie 10
HOW TO DETECT ?
Folie 11
Standard Model
strength
FORCES
•
nuclear force
keeps protons and neutrons together
•
electromagnetic force
keeps electrons around the nuclei
•
weak force
makes the (free) neutron to decay
•
gravitation
keeps us on the ground
Folie 12
ELECTROMAGNETIC FORCE
a force is mediated
classical picture
quantum world
by field around a source
field quanta = particles
FCoulomb 
1 Q1Q 2

4 πε 0 r 2
„light“ particles = photons g
electromagnetic radiation = E and B fields interacts with electric charges
Folie 13
DEFLECTION OF CHARGED PARTICLES IN EL.-MAG. FIELDS
•
electric field

•



F  mx  Q  E
Q
M
magnetic field
 p



 
F  mx  Q  v  B

r
B = const.
 circular motion
B  plane of projection
ω
mv 2 / r  Q  v  B
p  QBr
Q
B
M
ω
2π
T
Folie 14
SIGNAL CREATION
•
via electric charges
•
measure the electric current I or voltage U
resistor R
I
Q
U
U
capacitor C
Folie 15
INTERACTION OF
CHARGED PARTICLES
WITH MATTER
Folie 16
CHARGED PARTICLES
interaction happens by collisions of particles type 1 and 2
before
1.
Mparticle 1 >> Mparticle 2
2.
Mparticle 1 = Mparticle 2
after collision
Folie 17
CHARGED PARTICLES I: ENERY LOSS BY COLLISIONS
collisions create electron ion pairs
1.
Bragg peak
Mparticle >> Melectron
ΔR
 1  3%
R
e.g. protons, deuterons, …
for all elements
strongly ionising
2.
Mparticle = Melectron
electrons or positrons
well defined range R!
N( x )  e µx
no defined range R!
exponential attenuation with depth x
weakly ionising
µ: material dependent attenuation coefficient
Folie 18
CHARGED PARTICLES II: ENERY LOSS BY RADIATION
Radiation if vparticle > cin medium Cerenkov 1930s
„light“ blue!
electrons „radiate“
the charge polarizes the medium
in the water above
the core of
a nuclear power plant
 ΔE 
 ΔE 
<< 



 Δx C radiation
 Δx collision
emission under specific angle C
cos C = 1 / n
n = index of refraction
(small) dispersion !
C measures the velocity of the particle
acoustics analogue: Mach‘s cone for supersonic source
Folie 19
INTERACTION OF
MASSIVE NEUTRAL PARTICLES
WITH MATTER
Folie 20
NEUTRONS
collisions create recoil particles
maximum energy transfer for Mneutral = Mrecoil
central collision
non central
all energy is transferred
all energies according to scattering angle
cloud chamber picture
detection by recoil of protons
(from hydrogen)
probability
neutrons – no defined range
MProton  MNeutron
i.e.
good shieldings are water
concrete (15% water)
paraffin ( (CH)n)
…
energy transfer DE
per collision
DE
neutrons
Folie 21
INTERACTION OF
RADIATION WITH MATTER
Folie 22
RADIATION I : PHOTO EFFECT
requires particle nature of „light“ Einstein 1905
1.
photon disappears
photo electron
2.
Ee = Ephoton - EB
refilling of hole in electron shell by
a) emission of photon or
b) Auger electron emission of
loosely bound outer electron
photo
peak
EAuger  EB
example
detected energy E
photo peak
E = Ephoton
= Ee + EB
escape peak
escape
peak
Argon
EKa = 2.95 keV
photon
EPhoton = 6.41 keV
E = Ephoton - EKa
Energy
Folie 23
RADIATION II : COMPTON EFFECT
proof of particle nature of „light“ Compton 1922
billard with photons and „quasifree“ electrons
Δλ =λ (1− cosθ )
photon does not disappear
recoil electron

detected energy
Ee = Ephoton – Ephoton‘
continuous spectrum
E = Ee
we neglegt EB of the electron
and Erecoil of the nucleus
because usually EB, Erecoil << Ee
Compton edge
= maximum energy transfer
Folie 24
RADIATION III : BREMSSTRAHLUNG
accelerated charged particles radiate Hertz 1886
electromagnetic waves
bending force by Coulomb potential
force  acceleration
a recoil partner (nucleus) is needed to fulfil
energy and momentum conservation
FCoulomb 
1 Qparticle  Qnucleus

4 πε 0
r2
 m  r
any distance r 
characteristic X-rays
refilling of holes
in inner atomic shells
continuous spectrum
Folie 25
RADIATION IV : PAIR PRODUCTION
proof of mass-energy equivalence Blackett 1948
conversion of energy into matter
Ephoton = hn > 2 melectron
in general > 2 mparticles
at very high energies
+Ze
a recoil partner (nucleus) is needed to fulfil
energy and momentum conservation
 magnetic field B
el.-mag shower
e+ e – g - cascade
pair production and Bremsstrahlung alternate
shower may start with photon or electron
radiation length x0
characteristic material dependent constant
depth, where about 2/3 of the incident energy is converted
Folie 26
CHARGED PARTICLES : SUMMARY I
 2M0
MIPs
minimum ionsing particles
stopping power
 dE  ΔE 1



 dx  Δx ρ
 ΔE 



 Δx collision
1
v2
...
T < 2M0
Fractional energy loss.
Folie 27
CHARGED PARTICLES : SUMMARY II
Fractional energy loss per radiation length in lead as a function of electron or positron energy.
Folie 28
RADIATION: SUMMARY I
cross section s  Z5
s  reaction probability
Folie 29
RADIATION: SUMMARY II
x
Lambert-Beer law
Io
attenuation
I
dx
I ( x )  I 0 e  μ ( hν ) x
I( x )
 e  μ ( hν ) x
I0
μ ( hν )   μ i ( hν )
intensity after layer thickness x
transmission
sum of linear attanuation coeff.
i
Folie 30
DETECTOR PRINCIPLES
Folie 31
not only HISTORY
(Wilson) cloud chamber
typical Open Day presentations
saturated alcohol vapor

a-particle emitting nuclide
overheated LH2
bubble chamber (D. Glaser noble prize 1960)
+
magnetic field
BEBC @ CERN 73 until 80ies
3.7 T, 35 m3 LH2
"beer" inspired !!!
among others
discovery of the
weak neutral current
Folie 32
CHARGE
capacitor
voltage
generator
ionising
particle
current or voltage detection
charge created by charged particles or by „light“ is collected
by applying a voltage by means of a curent or voltage detection
Folie 33
SCINTILLATORS produce “LIGHT”
ionisation caused by

charged particles or light
excitation and delayed light emission
usually in the UV range
anorganic
NaI(Tl), CSI, BaF2, …
inorganic
doped „plastics“
UV light is converted to charge
at a photo cathode and
multiplied by a multi stage
photo „multiplier“
Folie 34
TIME
10 ns
Folie 35
WIRE CHAMBERS I
electron multiplication
around anode (fast)
drift of ions (slow)
typical ion drift velocity:
1 - 10 cm/(µskV)
Ar CH4
multiplication  avalanche
gain 105 - 106
to control avalanche
quench gases, e.g. CO2, CH4, C2H6
wire chambers tutorial:
F. Sauli
CERN yellow report 99-07
Folie 36
WIRE CHAMBERS II
many wires: MWPC = multiwire proportional chamber
position resolution  wire distance typically 2 mm
field configuration
•
(x,y) - coordinate per pair of frames
•
trajectory from MWPC stacks
Folie 37
WIRE CHAMBERS III
tracking: cut on fiducial target volume
example: p-3He  pnn or dn
target
3He
vesssel
pion beam
MWPC 1
protons
beam defining
counters
beam defining counters
mainly p carbon reactions
deuterons
MWPC 2
good
bad
event
Folie 38
STRAW TUBES
individual counters, timing 20 ns
HV: coat, ground: sense wire (~ kV)
typical size: length 1 - 2 m, f mm - cm
"simple" mechanics
10 MHz rate
inside magnetic field
Type-2 module (520 ‘straws’)
gas filling
e.g., Ar/C2H6
ATLAS at the LHC
Monte Carlo
simulation
Ileft
Iright
resistive read out
Dz < 1 mm
z
wall: aluminised mylar foils
anode wire: f  20 µm
ZEUS - DESY wedge
Folie 39
DRIFT CHAMBERS I
time  position
external time reference,
e.g., plastic scintillator
trick: choose field configuration,
which keeps the nonlinearity of
time-to-position relation small
position resolution
20 µm
Folie 40
DRIFT CHAMBERS II
improved position resolution by nearest 3 wires method
inclined wires
The wires are arranged in layers that
pass through the cylinder at three
different angles. The set of wires that
give a signal can be used to allow
computer reconstruction of the paths (or
tracks) of all the charged particles
through the chamber.
The "drift" in the name of this chamber refers to the time it takes electrons to drift to the
nearest sense wire from the place where the high-energy particle ionized an atom. Any three
sense wires are only nearby in one place so a set of "hits" on these three fix a particle track in
this region. By measuring the drift time, the location of the original track can be determined
much more precisely than the actual spacing between the wires.
Folie 41
TPC - time projection chamber
idea:
David Nygren, 1974
avoid pile-up many MWPC planes (typical gas thickness of 1 cm)
principle: electrons produced follow the constant electric field lines to a single MPWC plane
located at one end of the volume ( x-y coordinates on this plane)
Third coordinate, z, from the drift time of the electrons to the anode plane
STAR TPC - RHIC, Brookhaven
properties:
•full 3-dimensional detector
•constant drift velocity due to the collisions
in the gas mixture (typical a few cm/µs).
•low occupancy even for high background (high rates)
•large dE/dx due to large gas thickness (particle identification)
Folie 42
Pixel Tracker
Track Cluster
• Pixel Size
• Occupancy
• Charge Sharing
• S/N
• ExB Drift
• Radiation Damage
Single Track
LHC - 1014 /cm2/yr
& Trigger
charge center of gravity

high position resolution
Charge Sharing
vertex resolution
(20-30) mm IP
Folie 43
SILICON MICRO - STRIP DETECTORS I
principle
typical x-y (front-back)
arrangements
pn diode
as almost all
semiconductor detectors
200 µm strips
layer thickness 300 µm
miniaturisation
Readout Chip
Sensor
charged
particle
arrays of soldering dots
+ 
 +
Folie 44
SILICON MICRO - STRIP DETECTORS II
silicon µ-strip module
CMS - LHC scheme
•inner tracker
ANKE - COSY
•vertex detection
•recoils
•polarisation (left-right asymmetry)
semiconductor telescope
65/300/300/5500 µm thick
double-sided Si-strip detectors
Folie 45
EXAMPLES
OF
COMBINED DETECTION SYSTEMS
Folie 46
ANKE@COSY I: SET-UP
aim: measure simultanuously positively and negatively charged particles
e.g.,
pp  pp K+ K
particle identification by dE/dx
dE
1
m m2

 
dx
T
v2
p2
1
16
focal plane
FOCAL PLANE SPECTROMETER
for positively charged particles
counter number
Folie 47
ANKE@COSY II: FOCAL PLANE DETECTOR
Folie 48
WASA@COSY I: SET-UP
aim: measure photons from neutral particle decay in coincidence with charged particles
e.g.,
dd  4He p0
gg
photon detector: calorimeter
charged particle detector: forward hodoscope
Folie 49
WASA@COSY II: CALORIMETER
Folie 50
WASA@COSY III: FORWARD HODOSCOPE
Folie 51
Todays detectors comprise ...
• Silicon Vertex Tracker (SVT) - precise position information on charged
tracks
• Drift Chamber (DCH) - the main momentum measurements for charged
particles and helps in particle identification through dE/dx
measurements
• Detector of Internally Refected Cerenkov radiation (DIRC or DRC) charged hadron identification
• Electromagnetic Calorimeter (EMC) - particle identification for electrons,
neutral electromagnetic particles, and hadrons
• Solenoid (not a subdetector) – high magnetic field for needed for
charge and momentum measurements
• Instrumented Flux Return (IFR) - muon and neutral hadron identification
• and more …
Folie 52
EXERCISES LECTURE 9: PARTICLE DETECTORS
1. Derive the nonrelativistic relation between kinetic energy and momentum from
the relativistic energy-momentum relation.
2. By which process charged particles loose kinetic energy in matter?
3. Which process dominates – depending on the energy of the radiation – the
attenuation in matter?
4. Which processes are involved in an X-ray session at your medical doctor having
an apparatus labeled
25 keV?
5. Which is the minimum velocity (in units of speed of light c) for particles in order
to produce Cerenkov light in plastic material with index of refraction n = 1.5?
6. Which kind of detector should be used to detect neutral pion decays?
7. How many planes of MWPCs are needed to measure the trajectory of a charged
particle with and without the presence of a magnetic field B.
Folie 53