Particle Detectors for Colliders Ionization & Tracking Detectors Robert S. Orr University of...

Particle Detectors for Colliders

Ionization & Tracking Detectors

Robert S. Orr

University of Toronto

R.S. Orr 2009 TRIUMF Summer Institute

Layers of Detector Systems around Collision Point

Generic Detector

Tracking Detectors

• Observe particle trajectories in space with as little disturbance as possible

• use a thin ( ) detector– Scintillators – Scintillating fibres– Gas trackers– Solid state trackers

2.gm cm

cm 150 150

10

• Gas Based Detectors– Multiwire proportional chamber

– Drift Chamber

– Time projection chamber

– Gas microstrip

– GEM (gas electron multiplier)


Generic Detector

small – amplification?


Multiwire Proportional Chamber

wire spacing = resolution

cathode

cathode

anode wires

Drift Chamber – measure arrival time of charge = spatial resolution


Schematic of Wire Chamber Cell

anode wire

field shaping cathode• wire mesh• pc board

collects signalenvelope to contain gas

gas

should not absorb electrons

Repeat “n” times


3 stages in signal generation

1) Ionization by track passing through cell

2) Ionization drifts in E field

time

3) In high E field region near wire, primary ionization electrons gain enough energy to start ionizing the gas

- Avalanche- More charges- Charge amplification- Noise free amplifier

7~ 10 microvolt signal if no amplification


Gas Amplification


Behaviour as Voltage Increased

• Collection – Recombination dominated

• All charge collected

• Amplification by gas multiplication

• Still proportional – particle ident

• Saturation

• Breakdown – Geiger/Mueller

Volts


Diffusion

• Ions & electrons diffuse in space• E field determines average direction

• Collisions limit velocity• Maximum average velocity =Drift velocity

• Ions and electrons diffuse under influence of electric field– Maxwell velocity distribution

• From Kinetic theory , after t, linear distribution due to diffusion

Diffusion

8kTv

m

6 1 4 110 . 10 .e Iv cm s v cm s

20 exp

44

NdN x

dx DtDt

2x Dt

6r Dt

number of particles

Diffusion coefficient

RMS Spread2-d

3-d

about 1mm after 1 sec in air

• For a classical gas

ion charge and mass

p gas pressure

ion scattering cross section

• In argon

• Electrons collected quickly compared to +ve ions

Mobility

drift velocity

electric field0

2

3

q kT u

p m E

,q m

0

40e

m ns

kV cm

0.1I

m ns

kV cm

Diffusion and Drift Chamber Accuracy

13

D v

0

1

2

kT

p

210D nse

Diffusion coefficient from kinetic theory

Mean free path

3

0

2 1

3

kTD

p m

In argon

Diffusion gives limit on spatial accuracy drift chamber

• To reduce D• Lower temperature• Raise pressure (reduce mobility)

Working Gas

• Noble gases give multiplication at lowest electric field– Polyatomic gases have non-

ionization energy loss mechanisms

• Choose cheap noble gas with low ionization potential

– Krypton X

– Xenon X

– Argon OK

• Cheap, safe, non-reactive– remove electro-negative

contaminants

• Pure argon limited to gain• Many excited ions produced during

avalanche

• Absorb - quenchers

rare, expensive

cheap – welding etc

Argon

2 2 2, ,O CO H O

310

* 11.6Ar Ar eV

absorbed on cathode

cathode e photo emission

returns to anode - breakdown

Quenchers Polymerization

* 11.6Ar Ar eV

*X X Absorb

e.g. MethanePoly-atomic gas

Rotationalvibrational modes

Typical gases 4

3 8

80% 20%

90% 10%

Ar CH

Ar C H

610G

or add electronegative gas (a bit of poison)

X photo electron X

Typical 290% 10%Ar CO 710G

non-radiative

• Organic quenchers polymerize

• Deposits on cathodes

• high resistance• ion buildup – discharge• sparks, broken wires

• Add non-polymerizing agent – water methylal

Magic Gas

3 32

2

75%

24.5%

0.5%

1%

Ar

CH CH CH

Freon

tracemethylal

H O


Gas Admixtures


Signal from Gas Counter

CathodeAnode

charge q moved by dr

0V

0

( )Q d rdV dr

lCV dr

length of countercapacitance/unit length

potential

• Electrons produced in avalanche close to anode wire

• Small dr – small signal

• +ve ions drift across whole radius• Large dr – large signal

( )W Q r20

1

2W lCV

( )d rdW Q dr

dr

0dW lCV dV

0

0

( ) ln2

CV rr

a

0

( )d rlCV dV Q dr

dr

0

( )Q d rdV dr

lCV dr

0 0

ln2

a

electron

a

d rQ Q aV dr

lCV dr l a

0 0

ln2

b

ion

a

d rQ Q bV dr

lCV dr l a

ln lnelectron ion

a bV V

a a

Typically 1%

potential energy of qelectrostatic energy of field


Time Development of Signal

• Assume • All signal comes from ions• Start from a

( ) ( )

00

( )r t r tt

a a

Vd rdV Q

dV dr drdr lC

tV dr

0 0

0

0

l2

nn l2

r

a

CVQ r

lCV a

r tQ

l a

0

0

1

2

CVE

r

dr

dt

0

0

2

0

0

0

2

r t

a

CVr t

CVr r dt

t

d

a

02

0 0 0 0

ln 1 ln 14 4

CVQ QtV t

a t

t

Typically get 50% of signal in T ~700ns310

RC differentiation for fast signal


Different Realizations of Ionization Trackers

MWPC

Time Projection Chamber

Jet Chamber

Drift Chamber


Drift Chamber Cell

potential shaping wiressense wire

• Carefully shape potential (field lines)• Optimize drift time – space relation

drift

tim

e


Left-Right Ambiguity Resolution

2 anode wires staggered anode wires

ghost track

inclined anode plane

good for high magnetic field


Jet Chamber

annihilation at 30 GeVe e

Lorentz Angle – Drift Chamber in Magnetic Field

• Drifting electron will see

Electric Field

Magnetic Field

E

B

mv q E v B

• Will also see stochastic force due to collisions with gas molecules

mv q E v B mA t

• Assume over time

,E B acceleration

stochasticretardation

constant Dv

0D D

qE qBv v

m mA t

DvA t

mean time between collisions

DD

v qB qEv

m m

solution:

2 22 2 21D

E B BE Bv E

B B

(1) (2) (3)

q

m

qB

m

electron mobility

cyclotron frequency

Lorentz Angle – Drift Chamber in Magnetic Field

• Drift velocity has three components

solution:

2 22 2 21D

E B BE Bv E

B B

(1) (2) (3)

(1) parallel to

(2) parallel to

(3) perp to plane of

EB

,E B

• If perpendicular,E B

,0,0

0,0,

x

z

E E

B B

2 2

2 2

1

1

10

x x

y x

z

v E

v E

v

tan y

x

v

v

tan DqB mB

m q

vB

E


tan DvB

E

0 1kV 2kV 3kV

1kV 2kV 3kV

BV

0BV

equipotential

sense wiresfield wires

next cell

tan DvB

E

Compensate for Lorentz angle bytilting electric field in drift cells


Structure of ZEUS DC

• Total wire tension 12 tons• 4608 W sense wires (30 micron)• 19584 CuBe field wires

• 120 micron space resolution• 2.5mm 2 track resolution• 500 ns max drift time


Tilted E Field – R-L ambiguity resolution

real track segments

reflected ghost segments


Spatial Resolution

• Small number of primary electrons reach sense wire

• Statistics

• Variation in drift time – space relation

• Smearing


Stereo Wires – 3-d Reconstruction

stereo wires

stereo cameras – 3-d pictures

paraxial wires

r,x

r’,x’

r,x

r’,x’





Layers of Detector Systems around Collision Point


Square Drift Cells - ARGUS

• Precision• High Density of Information• Pattern recognition complex R-L ambiguity resolved by

trying all possible combinations

isochrones


ARGUS Events


dE/dx Particle Identification

BaBar Drift Chamber

constructed at TRIUMF


Time Projection Chamber

• Only two drift cells• parallel to , so no Lorentz angleE B

• measure z, from drift time

• measure r,Φ from pads and wires on endplates

, 180r

200z

• Good pattern recognition and precision in medium multiplicity environment

space charge limitation


wire – drift time

pad – position on the wire


Diffusion in TPC

transverse diffusion

Diffusion limits spatial resolution

2

3 D

Lu

v

drift length

mean electron velocity

mean free path

Why does diffusion not ruin resolution?


Diffusion in TPC

Compare this to previous plot with B=0

reduces diffusion if0B 0E B

particles drift along tight helices

transverse diffusion reduced by

2 2

1;

1

eB

m


ATLAS Tracker

0s

0d K J/B

33 2 13.0 10 cm s

)GeV 130( eeeeZZH H m

34 2 110 cm s


ATLAS Straw Tracker

Straws

Radiator

Radiator

Straws

End-c

apEn

d-cap

straw


Straw tracker test beam module


Assembly of straw tracker

Inner Detector (ID)The Inner Detector (ID) comprises four sub-systems:

•Pixels (0.8 108 channels)

•Silicon Tracker (SCT)(6 106 channels)

•Transition Radiation Tracker (TRT)(4 105 channels)

•Common ID items

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Particle Detectors for Colliders Ionization & Tracking Detectors Robert S. Orr University of...

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