Particle Detectors for Colliders
Ionization & Tracking Detectors
Robert S. Orr
University of Toronto
Generic Detector

Layers of Detector Systems around Collision Point
R.S. Orr 2009 TRIUMF Summer Institute
Tracking Detectors
• Observe particle trajectories in space with as little
disturbance as possible
2
–
–
–
–
• use a thin ( gm.cm ) detector
 cm 
Scintillators
 150 
Scintillating fibres
 150 
Gas trackers
 10 
Solid state trackers
• Gas Based Detectors
–
–
–
–
–
Multiwire proportional chamber
Drift Chamber
Time projection chamber
Gas microstrip
GEM (gas electron multiplier)
Generic Detector
R.S. Orr 2009 TRIUMF Summer Institute
Multiwire Proportional Chamber
wire spacing = resolution
cathode
anode wires
cathode
Drift Chamber – measure arrival time of charge = spatial resolution
R.S. Orr 2009 TRIUMF Summer Institute
Schematic of Wire Chamber Cell
envelope to contain gas
gas
collects signal
anode wire
should not
absorb
electrons
Repeat “n” times
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field shaping cathode
• wire mesh
• pc board
3 stages in signal generation
1) Ionization by track passing through cell
2) Ionization drifts in E field
time
3) In high E field region near wire, primary ionization
electrons gain enough energy to start ionizing the gas
- Avalanche
- More charges
7
- Charge amplification ~ 10
- Noise free amplifier
R.S. Orr 2009 TRIUMF Summer Institute
microvolt signal if
no amplification
Gas Amplification
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Behaviour as Voltage Increased
• Collection – Recombination dominated
• All charge collected
• Amplification by gas multiplication
• Still proportional – particle ident
• Saturation
• Breakdown – Geiger/Mueller
Volts
R.S. Orr 2009 TRIUMF Summer Institute
Diffusion
• Ions & electrons diffuse in space
• E field determines average direction
• Collisions limit velocity
• Maximum average velocity
=Drift velocity
R.S. Orr 2009 TRIUMF Summer Institute
Diffusion
• Ions and electrons diffuse under influence of electric field
– Maxwell velocity distribution
v
8kT
m
ve 106 cm.s 1 vI 
104 cm.s 1
• From Kinetic theory , after t, linear distribution due to diffusion
number of particles
dN

dx
RMS Spread
  x2 
N0
exp 

4
Dt
4 Dt


  x   2Dt
2-d
  r   6Dt
3-d
Diffusion coefficient
about 1mm after 1 sec in air
Mobility
• For a classical gas
2
q

3  p 0
kT u

m E
drift velocity
electric field
q, m ion charge and mass
p gas pressure
 0 ion scattering cross section
• In argon
e  40
 m ns
kV cm
I  0.1

 m ns
kV cm
• Electrons collected quickly compared to +ve ions
Diffusion and Drift Chamber Accuracy
1
D  v
3

Diffusion coefficient from kinetic theory
1 kT
2 0 p
D
In argon
Mean free path
2
1
3  0 p
 kT 
3
m
D 10  2 ns
e
Diffusion gives limit on spatial accuracy drift chamber
• To reduce D
• Lower temperature
• Raise pressure (reduce mobility)
Working Gas
• Noble gases give multiplication
at lowest electric field
– Polyatomic gases have nonionization energy loss
mechanisms
• Choose cheap noble gas with
low ionization potential
– Krypton X rare, expensive
– Xenon X
– Argon OK cheap – welding etc
Argon
• Cheap, safe, non-reactive
– remove electro-negative
contaminants O2 , CO2 , H 2O
• Pure argon limited to gain  103
• Many excited ions produced during
avalanche
Ar*  Ar    11.6 eV 
absorbed on cathode
  cathode  e  photo  emission
returns to anode - breakdown
• Absorb  - quenchers
Quenchers
Polymerization
Ar*  Ar    11.6 eV 
Absorb
  X  X * non-radiative
Rotational
Poly-atomic gas vibrational modes
e.g. Methane
80% Ar  20% CH 4
Typical gases
G 106
90% Ar  10% C3 H 8
or add electronegative gas (a bit of poison)
X   photo  electron  X 
• Organic quenchers
polymerize
• Deposits on cathodes
• high resistance
• ion buildup – discharge
• sparks, broken wires
• Add non-polymerizing agent – water
methylal
Magic Gas
75% Ar
24.5%  CH 3 2 CH CH 3
0.5% Freon
trace methylal
Typical 90% Ar  10% CO2
G 107
1% H 2O
Gas Admixtures
R.S. Orr 2009 TRIUMF Summer Institute
Signal from Gas Counter
V0
electrostatic energy
of field
1
W  lCV02
2
d ( r )
dr
dr
Q d ( r )
dV 
dr
lCV0 dr
Cathode
potential
length of counter
capacitance/unit length
• Electrons produced in avalanche
close to anode wire
• Small dr – small signal
• +ve ions drift across whole radius
• Large dr – large signal
d ( r )
dr
dr
lCV0 dV  Q
charge q moved by dr
Q d ( r )
dV 
dr
lCV0 dr
W  Q (r )
dW  Q
dW  lCV0 dV
Anode
potential energy of q
Velectron
Q

lCV0
Q
Vion  
lCV0
a 

a
CV0 r
ln
2 0 a
d  r 
Q
a
dr  
ln
dr
2 0l
a
d  r 
Q
b
dr


ln
 dr
2 0l a  
a 
b
Velectron Vion  ln
a
b
ln
a
a
Typically 1%
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 (r )  
Time Development of Signal
  CV0 

Q
t 
V t   
ln 1 
t


ln
1




4 0 
 0 a 2 
4 0  t0 
Q
• Assume
• All signal comes from ions
• Start from a
t
r (t )
0
a
V (t )   dV 
Q

lCV0

dV
Q
dr 
dr
lCV0
r (t )

a
d  r 
dr
dr
r t 
 CV0 r 
Q

l
n


l
n


2

a
2 0l
a
0

a
r
 CV0 1
dr

 E
dt
2 0 r
  CV0
a rdr  2 0
r
r t   a2 
t
 dt
Typically get 50% of signal in 103 T ~700ns
0
 CV0
t
 0

RC differentiation for fast signal
R.S. Orr 2009 TRIUMF Summer Institute
Different Realizations of Ionization Trackers
Drift Chamber
MWPC
Time Projection
Chamber
Jet Chamber
R.S. Orr 2009 TRIUMF Summer Institute
Drift Chamber Cell
potential shaping wires
drift time
sense wire
• Carefully shape potential (field lines)
• Optimize drift time – space relation
R.S. Orr 2009 TRIUMF Summer Institute
Left-Right Ambiguity Resolution
staggered anode wires
2 anode wires
ghost track
inclined anode plane
good for high magnetic field
R.S. Orr 2009 TRIUMF Summer Institute
Jet Chamber
e  e  annihilation at 30 GeV
R.S. Orr 2009 TRIUMF Summer Institute
Lorentz Angle – Drift Chamber in Magnetic Field
• Drifting electron will see
vD mean time between collisions
A t  

Electric Field
E
Magnetic Field B

qB  qE
  vD 

 
m m
vD
mv  q  E  v  B 
• Will also see stochastic force
due to collisions with gas
molecules
mv  q  E  v  B   mA  t 
solution:
(1)
(2)
(3)
E B B 2 2 

 
EB
vD 
 
 
E 
2 2 
2

1   
B
B

• Assume over time

E, B
acceleration
constant
vD  0 
stochastic
retardation
vD
qE 
qB 
  vD 
  A t 
m 
m

q
m
electron mobility

qB
m
cyclotron frequency
Lorentz Angle – Drift Chamber in Magnetic Field
solution:
(2)
(1)
(3)
E B B 2 2 

 
EB
vD 
 
 
E 
2 2 
2

1   
B
B

• Drift velocity has three components
(1) parallel to
(2) parallel to
E
B
(3) perp to plane of
• If
E, B
E , B perpendicular E   Ex , 0, 0 
B   0, 0, Bz 
vx   E x
1
1   2 2
v y    Ex
vz  0

1   2 2
tan    
tan    
vy
vx
v
qB m
 B  D B
m q
E
tan  
vD
B
E
R.S. Orr 2009 TRIUMF Summer Institute
equipotential
0
1kV
2kV
3kV
next cell
VB


field wires
VB 0
sense wires
1kV
tan  
vD
B
E
Compensate for Lorentz angle by
tilting electric field in drift cells
2kV
3kV
Structure of ZEUS DC
• Total wire tension 12 tons
• 4608 W sense wires (30 micron)
• 19584 CuBe field wires
• 120 micron space resolution
• 2.5mm 2 track resolution
• 500 ns max drift time
R.S. Orr 2009 TRIUMF Summer Institute
Tilted E Field – R-L ambiguity resolution
real track segments
reflected ghost segments
R.S. Orr 2009 TRIUMF Summer Institute
Spatial Resolution
• Small number of
primary electrons
reach sense wire
• Variation in drift time – space
relation
• Smearing
• Statistics
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Stereo Wires – 3-d Reconstruction
stereo cameras – 3-d pictures
r,x
r,x
r’,x’
r’,x’
paraxial wires
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stereo wires
R.S. Orr 2009 TRIUMF Summer Institute
R.S. Orr 2009 TRIUMF Summer Institute
R.S. Orr 2009 TRIUMF Summer Institute

Layers of Detector Systems around Collision Point
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Square Drift Cells - ARGUS
isochrones
• Precision
• High Density of Information
• Pattern recognition complex
R-L ambiguity resolved by
trying all possible combinations
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ARGUS Events
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dE/dx Particle Identification
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BaBar Drift Chamber
constructed at TRIUMF
Time Projection Chamber
• Only two drift cells
• E parallel to B , so no Lorentz angle
• measure z, from drift time  r ,
180
• measure r,Φ from pads and
 z 200
wires on endplates
• Good pattern recognition and precision
in medium multiplicity environment
space charge limitation
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wire – drift time
pad – position on the wire
R.S. Orr 2009 TRIUMF Summer Institute
Diffusion in TPC
Why does diffusion not ruin resolution?
transverse diffusion
Diffusion limits spatial resolution
drift length
2L

u
3vD
mean free path
mean electron velocity
R.S. Orr 2009 TRIUMF Summer Institute
Diffusion in TPC
Compare this to previous plot with B=0
B  0 reduces diffusion if E  B  0
particles drift along
tight helices
transverse diffusion reduced by
1
1   2
R.S. Orr 2009 TRIUMF Summer Institute
; 
2
eB
m
ATLAS Tracker
3.0 1033 cm2s1
B0d  J/ K s0
1034 cm2s1
H  ZZ  e  e e  e  (mH  130 GeV)
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ATLAS Straw Tracker
Straws
Radiator
Radiator
straw
Straws
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Straw tracker test beam module
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Assembly of straw tracker
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Inner Detector (ID)
The Inner Detector (ID) comprises
four sub-systems:
•Pixels
(0.8 108 channels)
•Silicon Tracker (SCT)
(6 106 channels)
•Transition Radiation Tracker (TRT)
(4 105 channels)
•Common ID items