Gas Detectors I

Gas Detectors I

Ulrich Uwer

Physikalisches Institut

• Introduction

• Gas detector basics

• MPWC

• Drift chambers (LHCb straw detector)

• Micro pattern detectors

Ulrich Uwer ● Universität Heidelberg 2

Gas Detectors – A Frontier Technology

Advantages • Cheap large area coverage

• Good spatial resolution

• Fast and large signals

• Good dE/dx resolution

• Good double track resolution

• Many possible detector configurations

• Low material budget – low radiation length

Challenges • Extremely large area detectors needed (ATLAS 5500 m2)

• High mechanical precisions (ATLAS, better than 30 m)

• Fast readout (25 ns bunch crossing cycle at LHC)

• High rate capability (LHCb Straw Tracker 400 kHz/cm2)

• High radiation dose (charge deposition ~2 C/cm)

• Light construction (LHCb Straw Tracker 9% X0)


Example: ATLAS Muon Detector

Monitored Drift Tubes (MDT)

Sagitta s


ATLAS MDTs

Trigger on first cluster


Gaseous Detectors at LHC


V

Counting gas

Gas ionization by charged particles

Gas Z Wion [eV] nion [cm-1]

Ar 18 26 94

CO2 33 33 91

CH4 10 28 53

Energy loss dE/dx of charged particles:

- primary ionization

- secondary ionization

Total number of e/ion pairs for a particle:

ionion W

dxdEn

For comparison:

Scintillator: energy for photon ~ 100 eV

Si Detector: energy for e/hole ~ 3.5 eV

Minimum ionizing particle m.i.p

Average energy Wion to create e/ion pair

Bethe-Bloch


Drift of electrons in presence of fields

Motion of charged particles under influence of E and B fields: Langevin equation.

uKBuEedt

udm

)(

m, e = mass and charge of electron“stochastic friction force” due to collisions

Drift velocity u:

cNc

L

1

Mean free path L

Time between collisions:

instantaneous velocity

m

KOne finds:

For t>> static situation: 0dt

ud

Cyclotron frequency

Bm

e

m

e

BBEBEEE

u ˆ)ˆˆ(ˆˆˆ1

2222

Scalar mobility

0for Eu


Drift velocity

In the microscopic picture one finds for the drifting electrons (energy ):

22

mN

eEu

22

mN

eEc

(Energy received from the E field between collisions equal to energy transferred in collisions.)

)(

)( fractional energy loss

Elastic collisions: 4102

gasM

m

100222 c

ucu Drift velocity much smaller

than instantaneous velocity

drift

Instant.


Fast and slow gasesRamsauer minimum

Excitation threshold: Ar at 11.5 eV

CH4 at 0.03 eV (vibrations+rotations)

• Ramsauer minimum: v is large

• Ar: ionization >> Ramsauer

• CH4: exitation < Ramsauer

• Ar / CH4 mixture

small, i.e. slow gas

big, i.e. fast gas

Drift velocity u can be tuned


Drift velocity of ArCH4

u [

m/n

s]

50 m/ns

50 m/ns

0/100

90/10


Drift velocity of ions

• Fractional energy loss for ions large:

2

1

)(

22

gasion

gasion

Mm

Mm

• Mobility / drift velocity much smaller than for electrons.

• While for electrons =(E, Gas, p, T) one finds for ions only little dependence on E:

EvEE

EvE

~~)(

~const~)(

for small E

for large E

eioneion uv 44 1010

Gas Ion [cm2/(Vs)]

Ar Ar+ 1.5

Ne Ne+ 4.1

Xe Xe+ 0.6


Proportional Counter

Examples:

• LHCb straw tubes: a=12.5 m, b=2.5 mm

• ATLAS MDT: a=25 m, b=15 mm

Electrical field:

r

VC

rab

VrE

1

2

1

)/ln()(

0

00

)/ln(

2 0

abC

Gas amplification – avalanche:

1

L 1

dxL

dx

n

dn

e

e

Capacity/length

EC

rC


Gas amplification

For uniform field

)exp()( 0 rnrn )exp(0

rn

nG

General case of non-uniform fields

Raether limit:

810~

20

G

x Phenomenological limit:

discharges (sparks)

))(exp(cr

a

drrG

(r) = Townsend coefficient

G = gas amplification = 104… 105

)exp( VCkG

(gain)

(LHCb straws)


Pressure dependence

d

G

dG~

p

dpK

G

dG

K = gas/configuration dependent constant = 5…8 Charge signal / rel. gain with mono chromatic source:

Fe55: 6.9 keV s


Space Charge Effect

Gain drop at high particle densities: space charge around the anode.

(LHCb straws)


2nd Townsend Coefficient & Quencher

UV photons from avalanche so far neglected:

UV photons photo effect (gas molecules / cathode)

Gas amplification G including effect of UV photons:

G

GGGGGGG

1

....)()( 2

0 1 2 photo effect

= probability for photo effect

2nd Townsend coefficient

For G 1 : gas amplification becomes infinite

continuous discharges (sparks)

Use poly-atomic gas admixtures to absorb photons: Quencher


QuencherExcitation cross section for Noble gases (Ar) and poly-atomic gases (CH4)

Energy dissipation through collisions (radiation less transitions)

Quencher: CH4, C2H6, CO2, CF4


Operation modes

I) Recombination before collection

II) Ionization mode full charge collection, no charge multiplication.

III) Proportional mode detector signal proportional to primary ionization, gas amplifications 104…105, needs quencher

IV) Streamer mode strong photon emission produced secondary avalanche, strong quencher to localize streamer, large signals

Geiger mode massive photon emission, no quencher discharge over full length, needs to be stopped by HV drop


Absolute gain measurement

HERA-B Honeycomb Tracker:

Chamber current at a constant/stable irradiation for different HV (~10000 single channels contribute)

4102~


Signal development

• Avalanche starts at a few radii distance from wire (typ. 50m)

• Electrons reach anode with ~1ns: Multiplication process takes less than 1ns

• Ions will slowly drift towards cathode and induce a negative signal on anode

Induced signal of charge Q moved by dr in a system with total capacity C=l·C’

drdr

dV

VCl

Qdv

0

a

da

l

Qv

ln

2 0Electron signal

da

b

l

Qv

ln

2 0

Assumes all charge produced at distance d

Ion signal

Total signalCl

Q

a

b

l

Qvvv

ln

2 0

)%1(4.1 vv

for LHCb straws / ATLAS MDT


Signal timing

)/1ln(/)/1ln()( 0max00 ttttQtQ

ns5~)/ln(2

2

0 abV

pat

Ion signal

Signal rise timep = pressure

V = voltage

= ion mobilityMax. ion drift time s130~)/ln(

)( 22

max

abV

abpt

LHCb straws

0/QQ

][nst


Signal readout

V

2R

1R

2C1C

][nst ][nst

VI

01222 , tCRCR

“Current source”

01222 , tCRCR

“Voltage source”


Signal Shaping

Long ion tail will shadow subsequent ionizing particles:

If threshold for particle detection is used, signal stays long time above threshold.

“Current mode”

RC/CR Shaping

Signal after amplifier

Signal after shaping


Ageing Effects

In a high rate environment (e.g. LHC) wire chambers could show several “ageing effects”, nearly all of them triggered by pollutants in the gas/chamber:

• Deposits on the anode wire: gain loss

Study gain as function of totol charge deposition per length

(LHCb straw detector)


Ageing Effects II

• Etching of anode wire in case of counting gas with CF4 admixtures

• Modification of the cathode surface: Malter effect self sustaining currents

(HERA-B, Honeycomb tracker)

(LHCb straws)


Garfield is a computer program for the detailed simulation of two- and three-dimensional drift chambers

Magboltz solves the Boltzmann transport equations for electronsin gas mixtures under the influence of electric and magnetic fields.

Magboltz - Transport of electrons in gas mixtures

Heed - Interactions of particles with gases

HEED is a program that computes in detail the energy loss of fast charged particles in gases, taking delta electrons and optionally multiple scattering of the incoming particle into account. The program can also simulate the absorption of photons through photo-ionization in gaseous detectors.

Garfield - simulation of gaseous detectors

http://consult.cern.ch/writeup/garfield/

http://consult.cern.ch/writeup/magboltz/

http://consult.cern.ch/writeup/heed/

Tools for detector development


Multi Wire Proportional Chamber

Charpak, 1967/68 Nobel prize 1992

12~ s

spatial resolution

With typ. wire distance s2mm s 0.6 mm

Significantly better spatial resolution in not achievable with MWPCs

MPWC = Multiwire proportional chambers

E Field


Drift Chamber

• Drift time drift distance and intersection point of particle

• Spatial resolution of ~100 m achievable



First Drift Chamber

Physikalisches Institut, Heidelberg, 1971


LHCb Outer Tracker


Outer Tracker - Demands


Planar Tracking Stations

T1 T2 T3

OuterTracker

264 Module

6 m

5 m

1.3% area 20% tracks


Straw Tubes

pitch 5.25 mm

5mm cellsTrack

e- e

-e-

Straw tube drift chamber modules

Straw tube winding:

Lamina Dielectrics Ltd.2.5 m

Cathode


Module Construction

2


Drift time spectrum


Wire Chambers -Summary

• Technology widely used in HEP experiments

• Proven to be robust, precise and reliable devices

• Detector geometry and counting gas can be tuned and optimized to fulfill requirements of the given application

• Play an important role in all LHC detectors

• Will continue to used in future particle detector: ILC detector PANDA, CBM


Micro pattern detectors

• Micromegas

• GEM detectors


Micromegas


Gas Electron Multiplier (GEM)140 m

Single GEM

double GEM

triple GEM


Compass Triple-GEM


Novel Neutron Detector


CASCADE Neutron Detector


Detector development tools

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Gas Detectors I

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