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)

R.S. Orr 2009 TRIUMF Summer Institute

Generic Detector

small – amplification?

R.S. Orr 2009 TRIUMF Summer Institute

Multiwire Proportional Chamber

wire spacing = resolution

cathode

cathode

anode wires

Drift Chamber – measure arrival time of charge = spatial resolution

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

Gas Amplification

R.S. Orr 2009 TRIUMF Summer Institute

Behaviour as Voltage Increased

• Collection – Recombination dominated

• All charge collected

• Amplification by gas multiplication

• Still proportional – particle ident

• Saturation

• Breakdown – Geiger/Mueller

Volts

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

Gas Admixtures

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

Different Realizations of Ionization Trackers

MWPC

Time Projection Chamber

Jet Chamber

Drift Chamber

R.S. Orr 2009 TRIUMF Summer Institute

Drift Chamber Cell

potential shaping wiressense wire

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

drift

tim

e

R.S. Orr 2009 TRIUMF Summer Institute

Left-Right Ambiguity Resolution

2 anode wires staggered anode wires

ghost track

inclined anode plane

good for high magnetic field

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

Tilted E Field – R-L ambiguity resolution

real track segments

reflected ghost segments

R.S. Orr 2009 TRIUMF Summer Institute

Spatial Resolution

• Small number of primary electrons reach sense wire

• Statistics

• Variation in drift time – space relation

• Smearing

R.S. Orr 2009 TRIUMF Summer Institute

Stereo Wires – 3-d Reconstruction

stereo wires

stereo cameras – 3-d pictures

paraxial wires

r,x

r’,x’

r,x

r’,x’

R.S. Orr 2009 TRIUMF Summer Institute

R.S. Orr 2009 TRIUMF Summer Institute

R.S. Orr 2009 TRIUMF Summer Institute

R.S. Orr 2009 TRIUMF Summer Institute

Layers of Detector Systems around Collision Point

R.S. Orr 2009 TRIUMF Summer Institute

Square Drift Cells - ARGUS

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

trying all possible combinations

isochrones

R.S. Orr 2009 TRIUMF Summer Institute

ARGUS Events

R.S. Orr 2009 TRIUMF Summer Institute

dE/dx Particle Identification

BaBar Drift Chamber

constructed at TRIUMF

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

wire – drift time

pad – position on the wire

R.S. Orr 2009 TRIUMF Summer Institute

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?

R.S. Orr 2009 TRIUMF Summer Institute

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

R.S. Orr 2009 TRIUMF Summer Institute

ATLAS Tracker

0s

0d K J/B

33 2 13.0 10 cm s

)GeV 130( eeeeZZH H m

34 2 110 cm s

R.S. Orr 2009 TRIUMF Summer Institute

ATLAS Straw Tracker

Straws

Radiator

Radiator

Straws

End-c

apEn

d-cap

straw

R.S. Orr 2009 TRIUMF Summer Institute

Straw tracker test beam module

R.S. Orr 2009 TRIUMF Summer Institute

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