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


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


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


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”


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


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.


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)


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)


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.


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


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!


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


The CLEO Vertex Detector

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


PDG Summary of Tracking Detectors

880.p20 winter 2006 richard kass 1 drift chambers drift chambers are mwpcs where the time it takes...

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