Drift Chambers
Drift Chambers are MWPCs where the time it takes for the ions to reach the
sense wire is recorded. This time info gives position info:
ts
x   v(t )dt t0= start time, ts=stop time=time electrons reach sense wire
t0
For some gases the drift velocity is ~constant (independent of E-field): x=v(ts-t0)
A gas with almost constant drift velocity is
50-50 Argon-Ethane, drift velocity  50mm/nsec
By using the drift time information we can improve our spatial resolution by a
factor of 10 over MWPCs (1mm 100 mm).
Hex-cell
drift chamber
drift times are
circles around the
sense wires
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Richard Kass
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Drift Chamber Spatial Resolution
The spatial resolution of a drift chamber is limited by three effects:
Statistics of primary ionization
location of the primary ionizations (a few 100mm apart)
Diffusion of the electrons as they drift to the wire
1 2 Dx

n mE
N=# of primary ions
D=gas diffusion constant
m=mobility
x=drift distance
E=electric field
magnetic field changes alters drift path:
drift path depends on “lorentz angle”, ExB
How well the electronics measures time
must measure time to < 1nsec,
must know start time (t0)
Contributions to spatial resolution
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Richard Kass
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Drift Chambers
Drift chambers come in all sizes, shapes and geometries:
planar  fixed target
cylindrical  colliding beam
Time information gives a “circle” of constant distance around the sense wire (more complicated in B field)
In almost all cases, wires in different layers are staggered to resolve the left-right ambiguity
Typical cylindrical DC:
Many wires in same gas volume.
Use small angle stereo for z.
Usually use single hit electronics.
Sense (anode) and field wires.
CLEO, CDF, BELLE, BABAR
Tube Chamber:
Single sense wire in a cylinder
Can make out of very thin wall tubes.
 very little material
Small drift cell  single hit electronics
Good cell isolation
 broken wire only affects one tube
CLEO’s PTL detector
Jet chamber: optimized to resolve two tracks in a “jet”.
Drift direction roughly perpendicular to wire plane.
Single track gives multiple hits on several wires.
Use multi-hit electronics so two tracks on a wire can be resolved.
Lorentz angle must taken into account
wires are “slanted”
880.P20 Winter 2006
Richard Kass
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A Real Life Drift Chamber-BaBar
40 layers total
10 “super layers”
100ns isochromes
spatial resolution
In B=1.5T the ions do not drift
straight to the sense wire (anode)
mom. resolution
Time to distance relationship
complicated!
7104 sense wires (20mm diameter)
30gm tension in each wire, sag~200mm
In order to measure “z” (along wire)
some wires are “slanted” at a slight angle
AR/Isobutane gas (80/20%)
HV=~1950V
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Richard Kass
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Time Projection Chamber
TPC measures all 3 space coordinates
x=y~0.1-0.2 mm (drift time), z~0.2-1mm (readout pad size)
Used at LEP, RHIC
Many hits per track (>100)  excellent dE/dx measurement
PEP4/9-TPC
Drawbacks:
Very complicated electric field shaping: E||B to reduce effects of diffusion
Long drift times  complicated gas system
Lots of electronic channels  complicated electronics
880.P20 Winter 2006
Richard Kass
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Silicon Strip Detectors
SSD’s are solid state proportional chambers
Approximately 1000X more
ionization in silicon compared
to a gas. Not necessary to have
charge multiplication to get
useable signals.
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silicon strip detector measures position to ~10mm.
silicon detector has many thin metal strips on top and (sometimes) bottom
surface of silicon wafer
charged particle ionizes the silicon as it passes through
it takes ~3.6eV to create an electron-hole pair in silicon
a minimum ionizing particle (one that passes through the silicon) deposits~390 eV/mm
in a 300mm thick Si detector (typical) there are ~ 30,000 electron hole pairs created
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electric field in silicon guides ions to top/bottom
ions are collected on one or or two (or 3) strips
knowing which strip has signal gives position of charged track relative to
silicon detector
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Richard Kass
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Silicon Strip Detectors
Dx
Resolution is mainly determined by strip pitch:  
12
Dx=3.5
 need strips every 50mm to get 15 mm resolution strips per cm
Strips can only be 5 cm long (technological limit)
Modern silicon strip detectors have 105-106 strips!
CLEO III hybrid (one of 122)
Require custom electronics
electronics must be small
electronics must be radiation hard
low power dissipation
wire bond connections (105-106)
Mechanical Structure
must be rigid/strong
must be low mass to minimize MS
mechanical tolerances ~mm
preamps
Much more engineering involved with silicon
detectors compared to drift chambers!
Digital
ADC
capacitors
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Richard Kass
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Advanced Silicon Detectors
Double sided silicon detector (CLEO, BaBar)
Put orthogonal (x,y) strips on top and bottom surface.
Allows 2 coordinate measurements per silicon wafer
minimizes amount of material  less MS
Problems in high rate environments  poor two track separation
Pixel detector (ATLAS/CMS)
Get position location (x,y) from hit pad (50mm x 50mm)
minimizes amount of material  less MS
Radiation hard(er)
Quick response time
Small detector capacitance good s/n with thin detector  less MS
Good two track resolution
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Richard Kass
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CLEO III Silicon Detector
Installation of
CLEO III silicon
detector
1.25x105 strips
Each strip has its own:
RC, preamp, ADC
Everything custom designed
for this experiment.
Readout
cables
hybrids
Silicon wafers
(layer 4)
Drift chamber
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The ATLAS Pixel Detector
~380mm
~1850mm
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Inner most charged particle tracking
Pixel size 50mm by 400 mm
~100 million pixels
Barrel layers at r = 5.1 - 12.3 cm
Disks at z = 50 - 65 cm
Dosage after 10 years:
– optical link 17 Mrad or 3.7 x 1014 1-MeV neq/cm2
disks
barrel layers
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Richard Kass
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The ATLAS Pixel Detector
OSU!
A pixel module contains:
1 sensor (2x6cm)
~40000 pixels
16 front end (FE) chips
2x8 array
bump bonded to sensor
Flex-hybrid
1 module control chip (MCC)
There are ~1700 modules
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Richard Kass
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CLEO II.V Charged Particle Tracking System
CLEO II.V had:
3 layer silicon detector
10 layer drift chamber (VD)
51 layer drift chamber (DR)
All in a 1.5T B field
DR
Si
VD
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The CLEO Vertex Detector
Designed & built at OSU
Part of the CLEO detector: 1984-1999
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PDG Summary of Tracking Detectors
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