Making Tracks at DØ · Making Tracks at D-Zero 2 What Does a Tracker Do? It finds tracks (well,...
Transcript of Making Tracks at DØ · Making Tracks at D-Zero 2 What Does a Tracker Do? It finds tracks (well,...
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Making Tracks at DØ
Satish Desai – Fermilab
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Making Tracks at D-Zero 2
What Does a Tracker Do?
● It finds tracks (well, duh!)● Particle ID (e/ separation, b-tagging...)
● Measurements of● Momentum● Electric Charge● Impact Parameters● Position and Trajectory
really measuringcurvature/charge
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Making Tracks at D-Zero 3
Measuring MomentumB (Tesla) L (cm)
DØ 2 52
CDF 1.4 140
L
pT'pT
R
pT'
pT pT
LR==
pTpT
1pT
=
qBL
d 1pT
=
qBL2 ds
Spatial resolution of outermost tracking layer
Momentum resolution gains more from tracking
volume than magnetic field
pT=q B R
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Making Tracks at D-Zero 4
Impact Parameters
● Impact parameter measurements for b-tagging, flavor physics, lepton ID...
● IP resolution driven by hit resolution
● Get hits close to original collision
b
D
A
B
pTA
pA
b=D sin=DpTA
p A
Assume massless decay products...
sin=1D
D=DcD
Impact parameter independent of boost!
c (m)
D± 312
D0 123
B± 491
B0 457
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Making Tracks at D-Zero 5
Detector Technologies
● Bubble chambers:● Very good resolution● Way too slow for
colliders
● Scintillators● High material budget● Speed O(10 ns)● Resolution O(100 m)
● Drift chambers/tubes● Low material budget● Speed O(100 ns)● Resolution > 100 m
● Silicon● High material budget● Speed O(10 ns)● Resolution O(10 m)
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Making Tracks at D-Zero 6
Silicon Detectors
– +
++
– –
Source: http://ecee.colorado.edu/~bart/book
● Drift time ~7ns● Depends on voltage
and sensor thickness● Resolution depends
on strip spacing
● Band gap is 1.12 eV for Silicon
● Really 3.6 eV needed for ionization (heating)
● MIP deposits 79 keV● 22k electrons, 3.5 fC
MIP = Minimum Ionizing Particle
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A Problem● Signal size is 22k electrons● Charge carrier density in conduction band: 1011/cm3
● Typical sensor dimensions● 300 m thick● 6 cm long● 50 m strip spacing (more relevant than width)
● 108 background charge carriers in neighborhood of signal
● Electron-hole pairs recombine easily
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Making Tracks at D-Zero 8
A Problem● Signal size is 32k electrons● Charge carrier density in conduction band: 1011/cm3
● Typical sensor dimensions● 300 m thick● 6 cm long● 50 m strip spacing (more relevant than width)
● 108 background charge carriers in neighborhood of signal
● Electron-hole pairs recombine easily
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Making Tracks at D-Zero 9
A Problem● Signal size is 32k electrons● Charge carrier density in conduction band: 1011/cm3
● Typical sensor dimensions● 300 m thick● 6 cm long● 50 m strip spacing (more relevant than width)
● 108 background charge carriers in neighborhood of signal
● Electron-hole pairs recombine easily
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Making Tracks at D-Zero 10
Behold, the Power of Diodes
I=I 0eqV /kT
−1
O (10 nA), depends ontemperature, doping
Si Si Si Si Si
Si Si Si P+ Si
Si Si Si Si Si
Si Si Si Si Si
–
– – – –
– – – –
++
+
++
+ + + +
+ + + +
––
––
––
p-type n-type
+–
Depletion Region
n-type semiconductor
Charge fixed to lattice fights the external voltage
Reverse bias
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Making Tracks at D-Zero 11
Charge
Field
Voltage
d x
x
x
x
Depletion Voltage
● Voltages and fields from Poisson's equation
● Charge density:● Set by doping concentration (Neff)
● Zero outside depletion region
● p-side very thin, heavily doped● Need full depletion for full
efficiency
−d 2Vdx2 =
dEdx
=q N eff
0
Depletion Region
V depl=q0
2 0
∣N eff∣d2
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Making Tracks at D-Zero 12
Closer to Reality
Readout Chips
Double-sided Double-Metal
Single Sided
1st metal layer:sensor strips
2nd metal layer:bring signals to chips
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Making Tracks at D-Zero 13
Performance
Layer 0 NIM
Herb
Herb
IP resolution degradedby multiple scattering
Compare to B/Dc=O(100 m)
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Making Tracks at D-Zero 14
Radiation Damage
● Ionization effects not important
● Non-ionizing: atoms knocked out of lattice
● Effectively induces p-type doping
● Changes depletion voltage
● Charge trapping in insulating layer
● Increases in leakage current
● Large electric fields near surface
● Breakdowns at high voltage
Bulk Damage Surface Damage
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Making Tracks at D-Zero 15
Signs of Aging
V depl=q0
2 0
∣N eff∣d2 n-type semiconductor
|Neff| decreasing
type inversion
p-type semiconductor |Neff| increasing
● If applied voltage too high (~150 V):● Noise increases dramatically
(microdicharge)● Coupling capacitors
breakdown (non-recoverable)
Masato
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Making Tracks at D-Zero 16
Summary
● Tracking detectors are an important component of collider experiments
● Semiconductor devices satisfy key requirements of speed and precision
● Reverse biased diode configurations make signal to noise ratio manageable
● Lifetime of silicon detectors limited by radiation induced effects● Microdischarge● Changes in depletion voltage
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Making Tracks at D-Zero 17
For Further Information
● The Physics of Particle Detectors, Dan Green
● Semiconductor Radiation Detectors, Gerhard Lutz
● Silicon Particle Detectors - Why they are useful and how they work, William Trischuk
● Depletion Voltage for the DØ Silicon Microstrip Tracker Using the n-side Noise Method, DØ Note 4917 (S. Burdin and S. Lager)
● Radiation Damage in Silicon Particle Detectors, M Moll, PhD Thesis
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Making Tracks at D-Zero 18
Backup Slides
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Making Tracks at D-Zero 19
Measuring Depletion Voltage
Sideband region
Signal region
Determine depletion voltage by lookingat signal size vs applied voltage
Plots stolen from Masato
Can also look at noise levels
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Making Tracks at D-Zero 20
The DØ Silicon Tracker
2 H-Disks
12 F- Disks
6 Barrels
In total, 731,136 readout channels
Layer 4Layer 3Layer 2Layer 1Layer 0
r-z view
Staggered sublayers to avoid gaps
F-disk
Double sided barrel
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Making Tracks at D-Zero 21
The DØ Tracking System
With H-disks
38 cm 53 cm 121 cm
SMT 50 m
CFT 835 m
Strip spacing, sometimes larger
Fiber width
SMT NIM
Without H-disks
End View (CFT)
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Making Tracks at D-Zero 22
Lorentz Drifts
– +
++
– –
d
E = V/d
v=E=Vd
Drift velocity
E = V/d
B
h
Mobility (cm2/Vs)
electrons 1400
holes 450
e
h tan=H B
eHall Mobility (cm2/Vs) tan
electrons 1670 0.33
holes 370 0.74
Lorentz drift can bias position measurement
Same direction for electrons and holes