Transcript Lecture_2_Draft_3 - University of Toronto, Particle Physics and

Particle Detectors for Colliders
Ionization & Tracking Detectors
Robert S. Orr
University of Toronto
Generic Detector

Layers of Detector Systems around Collision Point
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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
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Multiwire Proportional Chamber
wire spacing = resolution
cathode
anode wires
cathode
Drift Chamber – measure arrival time of charge = spatial resolution
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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
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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
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Diffusion
• Ions & electrons diffuse in space
• E field determines average direction
• Collisions limit velocity
• Maximum average velocity
=Drift velocity
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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
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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
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Different Realizations of Ionization Trackers
Drift Chamber
MWPC
Time Projection
Chamber
Jet Chamber
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Drift Chamber Cell
potential shaping wires
drift time
sense wire
• Carefully shape potential (field lines)
• Optimize drift time – space relation
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Left-Right Ambiguity Resolution
staggered anode wires
2 anode wires
ghost track
inclined anode plane
good for high magnetic field
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Jet Chamber
e  e  annihilation at 30 GeV
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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
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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
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Tilted E Field – R-L ambiguity resolution
real track segments
reflected ghost segments
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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
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
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
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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
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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
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; 
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