1. 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:
t0= start time, ts=stop time=time electrons reach sense wire
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
880.P20 Winter 2006
Richard Kass

2. 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
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)
N=# of primary ions
D=gas diffusion constant
m=mobility
x=drift distance
E=electric field
Contributions to spatial resolution
880.P20 Winter 2006
Richard Kass

3. Drift Chambers
Drift chambers come in all sizes, shapes and geometries:
planar Þ fixed target cylindrical Þ 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.
Þ very little material
Small drift cell Þ single hit electronics
Good cell isolation
Þ 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
Þ wires are “slanted”
880.P20 Winter 2006
Richard Kass

4. 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
880.P20 Winter 2006
Richard Kass

5. Time Projection Chamber
TPC measures all 3 space coordinates
sx=sy~ mm (drift time), sz~0.2-1mm (readout pad size)
Many hits per track (>100) Þ excellent dE/dx measurement
Used at LEP, RHIC
PEP4/9-TPC
Drawbacks:
Very complicated electric field shaping: E||B to reduce effects of diffusion
Long drift times Þ complicated gas system
Lots of electronic channels Þ complicated electronics
880.P20 Winter 2006
Richard Kass

6. 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.
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
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
880.P20 Winter 2006
Richard Kass

7. Silicon Strip Detectors
Resolution is mainly determined by strip pitch:
Dx=3.5s
Þ need strips every 50mm to get 15 mm resolution Þ 200 strips per cm
Strips can only be »5 cm long (technological limit)
Modern silicon strip detectors have 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 ( )
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
880.P20 Winter 2006
Richard Kass

8. 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 Þ less MS
Problems in high rate environments Þ poor two track separation
Pixel detector (ATLAS/CMS)
Get position location (x,y) from hit pad (50mm x 50mm)
minimizes amount of material Þ less MS
Radiation hard(er)
Quick response time
Small detector capacitance Þ good s/n with thin detector Þ less MS
Good two track resolution
880.P20 Winter 2006
Richard Kass

9. 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
Silicon wafers
(layer 4)
hybrids
Drift chamber
880.P20 Winter 2006
Richard Kass

10. The ATLAS Pixel Detector
~380mm
~1850mm
Inner most charged particle tracking
Pixel size 50mm by 400 mm
~100 million pixels
Barrel layers at r = cm
Disks at z = cm
Dosage after 10 years:
optical link 17 Mrad or 3.7 x MeV neq/cm2
disks
barrel layers
880.P20 Winter 2006
Richard Kass

11. 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
880.P20 Winter 2006
Richard Kass

12. 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
880.P20 Winter 2006
Richard Kass

13. The CLEO Vertex Detector
Designed & built at OSU
Part of the CLEO detector:
880.P20 Winter 2006
Richard Kass

14. PDG Summary of Tracking Detectors
880.P20 Winter 2006
Richard Kass