880.P20 Winter 2006 Richard Kass 1 Drift Chambers Drift Chambers are MWPCs where the time it takes...

880.P20 Winter 2006Richard Kass 1

Drift ChambersDrift Chambers are MWPCs where the time it takes for the ions to reach the sense wire is recorded. This time info gives position info:

st

tdttvx

0

)( 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)

By using the drift time information we can improve our spatial resolution by afactor of 10 over MWPCs (1mm 100 m).

A gas with almost constant drift velocity is50-50 Argon-Ethane, drift velocity 50m/nsec

drift times arecircles around thesense wires

Hex-celldrift chamber

880.P20 Winter 2006Richard 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 100m apart) Diffusion of the electrons as they drift to the wire

magnetic field changes alters drift path: drift path depends on “lorentz angle”, ExBHow well the electronics measures time must measure time to < 1nsec, must know start time (t0)

E

Dx

n 21

N=# of primary ions D=gas diffusion constant=mobility

x=drift distanceE=electric field

Contributions to spatial resolution

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Drift Chambers Drift chambers come in all sizes, shapes and geometries:

planar fixed target cylindrical colliding beamTime 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

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 accountwires are “slanted”

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 cylinderCan make out of very thin wall tubes. very little materialSmall drift cell single hit electronicsGood cell isolation broken wire only affects one tubeCLEO’s PTL detector

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A Real Life Drift Chamber-BaBar

7104 sense wires (20m diameter)30gm tension in each wire, sag~200mIn order to measure “z” (along wire) some wires are “slanted” at a slight angleAR/Isobutane gas (80/20%)HV=~1950V

In B=1.5T the ions do not drift straight to the sense wire (anode)

Time to distance relationshipcomplicated!

spatial resolution

mom. resolution

100ns isochromes40 layers total10 “super layers”

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Time Projection ChamberTPC measures all 3 space coordinates x=y~0.1-0.2 mm (drift time), z~0.2-1mm (readout pad size)

Many hits per track (>100) excellent dE/dx measurementUsed at LEP, RHIC

Drawbacks:Very complicated electric field shaping: E||B to reduce effects of diffusionLong drift times complicated gas systemLots of electronic channels complicated electronics

PEP4/9-TPC

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Silicon Strip Detectors

• silicon strip detector measures position to ~10m.

• 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/m

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

SSD’s are solid state proportional chambers

Approximately 1000X more ionization in silicon compared to a gas. Not necessary to havecharge multiplication to getuseable signals.

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Silicon Strip Detectors

Resolution is mainly determined by strip pitch:x=3.5 need strips every 50m to get 15 m resolution strips per cmStrips can only be 5 cm long (technological limit)Modern silicon strip detectors have 105-106 strips!

12

x

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 ~m

capacitors

preamps

Digital ADC

Much more engineering involved with silicondetectors compared to drift chambers!

CLEO III hybrid (one of 122)

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Advanced Silicon Detectors

Put orthogonal (x,y) strips on top and bottom surface.Allows 2 coordinate measurements per silicon wafer minimizes amount of material less MSProblems in high rate environments poor two track separation

Get position location (x,y) from hit pad (50m x 50m) minimizes amount of material less MSRadiation hard(er)Quick response timeSmall detector capacitance good s/n with thin detector less MSGood two track resolution

Double sided silicon detector (CLEO, BaBar)

Pixel detector (ATLAS/CMS)

880.P20 Winter 2006Richard Kass 9

CLEO III Silicon Detector

Installation ofCLEO III silicondetector

hybridsSilicon wafers (layer 4) Drift chamber

Readout cables

1.25x105 stripsEach strip has its own: RC, preamp, ADC

Everything custom designedfor this experiment.

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The ATLAS Pixel Detector

• Inner most charged particle tracking• Pixel size 50m by 400 m• ~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

~380mm

~1850mm

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The ATLAS Pixel Detector

A pixel module contains:1 sensor (2x6cm) ~40000 pixels

16 front end (FE) chips 2x8 array bump bonded to sensor

Flex-hybrid1 module control chip (MCC)There are ~1700 modules

OSU!

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CLEO II.V Charged Particle Tracking SystemCLEO II.V had: 3 layer silicon detector 10 layer drift chamber (VD) 51 layer drift chamber (DR)All in a 1.5T B field

Si

VD

DR

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The CLEO Vertex Detector

Designed & built at OSUPart of the CLEO detector: 1984-1999

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PDG Summary of Tracking Detectors