Gas Detectors I - IRTG Heidelberg

Gas Detectors I 

Ulrich Uwer 

Physikalisches Institut 

• Introduction 

• Gas detector basics 

• MPWC 

• Drift chambers (LHCb straw detector) 

• Micro pattern detectors

Gas Detectors – A Frontier Technology 

Advantages • Cheap large area coverage 

Challenges 

• 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 

• Extremely large area detectors needed (ATLAS 5500 m 2 ) 

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

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

• High rate capability (LHCb Straw Tracker 400 kHz/cm 2 ) 

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

• Light construction (LHCb Straw Tracker 9% X 0 ) 

Ulrich Uwer Universität Heidelberg 2

Example: ATLAS Muon Detector 

Monitored Drift Tubes (MDT) 

Sagitta s 


Trigger on first cluster 

ATLAS MDTs 


Gaseous Detectors at LHC 


Gas ionization by charged particles 

Energy loss dE/dx of charged particles: 

- primary ionization 

- secondary ionization 

Total number of e/ion pairs for a particle: 

n = 

ion 

dE 

W 

dx 

ion 

+ 

+ 

+ 

− 

− 

− 

Average energy 

W ion to create 

e/ion pair 

Bethe-Bloch 

Counting gas 

+ V 

CO 2 

Ulrich Uwer Universität Heidelberg 6 

Gas 

Ar 

CH 4 

Z 

18 

33 

10 

For comparison: 

W ion [eV] 

26 

33 

28 

Minimum ionizing 

particle m.i.p 

n ion [cm -1 ] 

94 

91 

53 

Scintillator: energy for photon ~ 100 eV 

Si Detector: energy for e/hole ~ 3.5 eV

Drift of electrons in presence of fields 

Motion of charged particles under influence 

of E and B fields: Langevin equation. 

Drift velocity u: 

du 

m = e( 

E + u × B) 

− Ku 

dt 

“stochastic friction force” due to collisions 

m, e = mass and charge of electron 

du 

For t>>τ → static situation: = 0 

dt 

Cyclotron frequency Scalar mobility 

e 

ω = B 

m 

e 

µ = τ 

m 

u 

µ E 

= τ 

2 2 

1+ 

ω τ 

[ ] 

Eˆ 

Eˆ 

Bˆ 

2 2 

+ ωτ × + ω ( Eˆ 

⋅ Bˆ 

) Bˆ 

Mean free path L 

Time between collisions: 

L 1 

τ = = 

c Nσ 

⋅c 

instantaneous velocity 

K 

One finds: τ = 

m 

for ωτ → 0 = 

E 

u µ 


In the microscopic picture one finds for 

the drifting electrons (energy ε): 

drift 

Instant. 

2 

u = 

eE 

mNσ 

2 eE 

c = 

mNσ 

λ 

2 

2 

λ 

Drift velocity 

σ = σ(ε 

) 

λ = λ(ε 

) 

fractional 

energy loss 

(Energy received from the E field between 

collisions equal to energy transferred in collisions.) 

Elastic collisions: 

λ ≈ 

2m − 

≈ 10 

Mgas 

2 2 c 

u c u ≈ 

2 100 

4 

= λ Drift velocity much smaller 

than instantaneous velocity 


Fast and slow gases 

Ramsauer 

minimum 

Excitation threshold: Ar at 11.5 eV 

• Ramsauer minimum: v is large 

• Ar: ε ionization >> ε Ramsauer 

• CH 4: 

ε exitation < ε Ramsauer 

• Ar / CH 4 mixture 

CH4 at 0.03 eV 

(vibrations+rotations) 


λ 

σ 

λ 

σ 

small, i.e. slow gas 

big, i.e. fast gas 

Drift velocity u can be tuned

Drift velocity of ArCH 4 

u [µm/ns] 

0/100 

50 µm/ns 

90/10 

50 µm/ns 


• Fractional energy loss for ions large: 

2mionM 

λ ≈ 

( m + M 

ion 

gas 

gas 

Drift velocity of ions 

) 

2 ≈ 

• Mobility / drift velocity much smaller than 

for electrons. 

µ 

ion 

• While for electrons µ=µ(E, Gas, p, T) one 

finds for ions only little dependence on E: 

µ ( E) 

~ const v ~ E for small E 

µ ( E) 

~ 

E 

−4 

≈ 10 µ 

e 

v 

~ 

v 

1 

2 

ion 

E 

≈ 10 

−4 

u 

e 

for large E 

Gas 


Ar 

Ne 

Xe 

Ion 

Ar + 

Ne + 

Xe + 

µ [cm 2 /(Vs)] 

1.5 

4.1 

0.6

Examples: 

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

• ATLAS MDT: a=25 µm, b=15 mm 

Proportional Counter 

E C 

r C 

Electrical field: 

V0 

1 C′ 

V0 

1 

E( 

r ) = = 

ln( b / a) 

r 2πε 

r 

2πε 0 C′ 

= 

ln( b / a) 

Gas amplification – avalanche: 

1 

1 

L = α 

α 


dn 

n 

e 

dx 

L 

e = = α 

dx 

0 

Capacity/length

General case of non-uniform fields 

G = exp( α( 

r ) dr ) 

Raether limit: 

αx 

≈ 

G 

20 

8 

~ 10 

rc 

a 

Gas amplification 

α(r) = Townsend coefficient 

G = k exp( C′ 

V) 

Phenomenological limit: 

→ discharges (sparks) 

For uniform field 

n( r ) = n0 

exp( α r ) 

n 

G = 

n 

= exp( α r ) 

G = gas amplification = 10 4 … 10 5 

(gain) 

(LHCb straws) 


0

dG 

~ − 

G 

dG 

= −K 

G 

dρ 

ρ 

dp 

p 

Pressure dependence 

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

mono chromatic γ source: 

Fe55: 6.9 keV γs 


Gain drop at high particle 

densities: space charge 

around the anode. 

Space Charge Effect 

(LHCb straws) 


2 nd Townsend Coefficient & Quencher 

UV photons from avalanche so far neglected: 

UV photons → photo effect (gas molecules / cathode) 

Gas amplification G γ 

G 

γ 

= G + G( 

γG) 

+ G( 

γG) 

including effect of UV photons: 

G 

+ .... = 

1− 

γG 

0× 1× 2× photo effect 

For γG → 1 : gas amplification becomes infinite 

2 

continuous discharges (sparks) 

Use poly-atomic gas admixtures to absorb photons: Quencher 

γ = probability for photo effect 

2 nd Townsend coefficient 


Quencher 

Excitation cross section for Noble gases (Ar) and poly-atomic gases (CH 4 ) 

Energy dissipation through collisions 

(radiation less transitions) 

Quencher: CH 4 , C 2 H 6 , CO 2 , CF 4 


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 10 4 …10 5 , 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) 

~ 2 × 

10 


4

Induced signal of charge Q moved by 

dr in a system with total capacity C=l·C’ 

Electron signal 

Ion signal 

Total signal 

Signal development 

v 

v 

− 

+ 

= 

= 

v = v 

− 

− 

+ 

• Avalanche starts at a few radii distance 

from wire (typ. 50µm) 

• Electrons reach anode with ~1ns: 

Multiplication process takes less than 1ns 

• Ions will slowly drift towards cathode and 

induce a negative signal on anode 


dv 

Q a + d 

ln 

πε l a 

2 0 

Q b 

ln 

πε l a + d 

2 0 

+ v 

− 

= 

− 

Q 

ln 

πε l 

2 0 

Q 

= 

lC′ 

V 

b 

a 

0 

dV 

dr 

dr 

Assumes all charge produced 

at distance d 

Q 

= − 

lC′ 

− + 

v v 

≈ 1. 

4 ( 1)% 

for LHCb straws 

/ ATLAS MDT

Ion signal 

Signal rise time 

Signal timing 

Q ( t) 

= Q0 

ln( 1+ 

t / t0) 

/ ln( 1+ 

tmax 

/ t0) 

t 

0 

= 

2 

pa 

2µ 

V 

ln( b / a) 

~ 5ns 

p( 

b − a ) 

Max. ion drift time tmax 

= ln( b / a) 

~ 130µ 

s 

µ V 

Q /Q0 

2 

p = pressure 

V = voltage 

µ = ion mobility 


2 

t 

[ns] 

LHCb straws

∆V 

Signal readout 

t 

C1 

R >> 

2 C2, 

R2C1 

t0 

“Voltage source” 

[ns] 

+ V 


I 

R1 

C2 

R2 

R

Long ion tail will shadow 

subsequent ionizing particles: 

If threshold for particle detection is 

used, signal stays long time above 

threshold. 

RC/CR Shaping 

Signal Shaping 

“Current mode” 

Signal after shaping 

Signal after amplifier 


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) 


• Etching of anode wire in case of 

counting gas with CF 4 admixtures 

(LHCb straws) 

Ageing Effects II 

• Modification of the cathode surface: 

Malter effect → self sustaining currents 

(HERA-B, Honeycomb tracker) 


Tools for detector development 

Garfield - simulation of gaseous detectors 

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

Garfield is a computer program for the detailed simulation of twoand 

three-dimensional drift chambers 

Magboltz - Transport of electrons in gas mixtures 

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

Magboltz solves the Boltzmann transport equations for electrons 

in gas mixtures under the influence of electric and magnetic fields. 

Heed - Interactions of particles with gases 

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

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. 


Multi Wire Proportional Chamber 

Charpak, 1967/68 

Nobel prize 1992 

spatial resolution 

~ s 

12 

With typ. wire distance 

s≈2mm δs ≈ 0.6 mm 

Significantly better spatial 

resolution in not achievable 

with MWPCs 

E Field 

MPWC = Multiwire proportional chambers 


Drift Chamber 

• Drift time drift distance and intersection point of particle 

• Spatial resolution of ~100 µm achievable 



Physikalisches 

Institut, 

Heidelberg, 1971 

First Drift Chamber 


LHCb Outer Tracker 


Outer Tracker - Demands 


Planar Tracking Stations 

5 m 

T1 T2 T3 

264 Module 

6 m 

1.3% area 

20% tracks 

Outer 

Tracker 


Straw tube drift chamber modules 

5mm cells 

pitch 5.25 mm 

Straw Tubes 

e 

- e - 

e 

- 

Track 

2.5 m 

Cathode 

Straw tube winding: 

Lamina Dielectrics Ltd. 


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) 

triple GEM 

double GEM 

Single GEM 

140 µm 


Compass Triple-GEM 


Novel Neutron Detector 


CASCADE Neutron Detector 


Detector development tools

Gas Detectors I - IRTG Heidelberg

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