Transcript e - 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
• 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)
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Example: ATLAS Muon Detector
Monitored Drift Tubes (MDT)
Sagitta s
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ATLAS MDTs
Trigger on first cluster
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Gaseous Detectors at LHC
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Gas ionization by charged particles




Counting gas


V
Minimum ionizing
particle m.i.p
Energy loss dE/dx of charged particles:
- primary ionization
- secondary ionization
Average energy
Wion to create
e/ion pair
Gas
Z
Wion [eV]
nion [cm-1]
Ar
18
26
94
CO2
33
33
91
CH4
10
28
53
Total number of e/ion pairs for a particle:
nion 
dE dx
For comparison:
Bethe-Bloch
Wion
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Scintillator: energy for photon ~ 100 eV
Si Detector: energy for e/hole ~ 3.5 eV
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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
Mean free path L
Time between collisions:
“stochastic friction force” due to collisions
m, e = mass and charge of electron

du
For t>>  static situation:
0
dt
Cyclotron frequency
e
 B
m

u

instantaneous velocity
Scalar mobility


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One finds:
e

m
E
ˆ   Eˆ  Bˆ   2 2 (Eˆ  Bˆ )Bˆ
E
1   2 2
L
1

c N  c

for   0

K
m


u  E
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Drift velocity
In the microscopic picture one finds for
the drifting electrons (energy ):
drift
u2 
eE
mN

Instant.
eE
c 
mN
2
2
   ( )
2

   ( )
fractional
energy loss
(Energy received from the E field between
collisions equal to energy transferred in collisions.)
Elastic collisions:
2m
 104
Mgas

c
u2  c2  u 
2
100

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Drift velocity much smaller
than instantaneous velocity
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Fast and slow gases
Ramsauer
minimum
• Ramsauer minimum: v is large
• Ar:
ionization >> Ramsauer


• CH4:
small, i.e. slow gas
exitation < Ramsauer


big, i.e. fast gas
• Ar / CH4 mixture
Drift velocity u can be tuned
Excitation threshold: Ar at 11.5 eV
CH4 at 0.03 eV
(vibrations+rotations)
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Drift velocity of ArCH4
0/100
u [m/ns]
50 m/ns
90/10
50 m/ns
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Drift velocity of ions
• Fractional energy loss for ions large:

2mionMgas
(mion  Mgas )2

1
2
• Mobility / drift velocity much smaller than
for electrons.
ion  104 e
 vion  104 ue
• While for electrons =(E, Gas, p, T) one
finds for ions only little dependence on E:
 (E ) ~ const  v ~ E
for small E
 (E ) ~ E
for large E
v ~ E
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Gas
Ion
 [cm2/(Vs)]
Ar
Ar+
1.5
Ne
Ne+
4.1
Xe
Xe+
0.6
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Proportional Counter
Electrical field:
EC
E(r ) 
C 
rC
20
ln( b / a)
Capacity/length
Gas amplification – avalanche:
L
Examples:
V0 1 C V0 1

ln(b / a) r 20 r
1

1

• LHCb straw tubes: a=12.5 m, b=2.5 mm
• ATLAS MDT:
a=25 m, b=15 mm
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dne dx

  dx
ne
L
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Gas amplification
For uniform field
n(r )  n0 exp(  r )
G
n
 exp( r )
n0
G = gas amplification = 104… 105
General case of non-uniform fields
(gain)
rc

G  exp(  (r )dr )
a
(r) = Townsend coefficient
G  k exp( C V )
Raether limit:
x  20
Phenomenological limit:
G ~ 108
 discharges (sparks)
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(LHCb straws)
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Pressure dependence
dG
d
~
G

dG
dp
 K
G
p
K = gas/configuration dependent constant = 5…8
Charge signal / rel. gain with
mono chromatic  source:
Fe55: 6.9 keV s
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Space Charge Effect
Gain drop at high particle
densities: space charge
around the anode.
(LHCb straws)
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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  G  G(G)  G(G)2  .... 
0
1
G
1  G
 = probability for photo effect
2nd Townsend coefficient
2 photo effect
For G  1 : gas amplification becomes infinite
continuous discharges (sparks)
Use poly-atomic gas admixtures to absorb photons: Quencher
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Quencher
Excitation cross section for Noble gases (Ar) and poly-atomic gases (CH4)
Energy dissipation through collisions
(radiation less transitions)
Quencher: CH4, C2H6, CO2, CF4
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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
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Absolute gain measurement
HERA-B Honeycomb Tracker:
Chamber current at a
constant/stable irradiation for
different HV (~10000 single
channels contribute)
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~ 2  10 4
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Signal development
• Avalanche starts at a few radii distance
from wire (typ. 50m)
• 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 
Induced signal of charge Q moved by
dr in a system with total capacity C=l·C’
Q
ln
ad
a
ln
b
ad
Electron signal
v  
Ion signal
v  
Total signal
v  v  v  
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20l
Q
20l
Q
20l
Q dV
dr
lC V0 dr
Assumes all charge produced
at distance d
v  v   1.4 (1)%
ln
b
Q

a
lC
for LHCb straws
/ ATLAS MDT
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Signal timing
Ion signal
Q(t )  Q0 ln(1  t / t0 ) / ln(1  tmax / t0 )
Signal rise time
pa2
t0 
ln(b / a) ~ 5ns
2V
Max. ion drift time tmax
p(b 2  a 2 )

ln(b / a) ~ 130 s
V
p = pressure
V = voltage
 = ion mobility
LHCb straws
Q / Q0
t [ns ]
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Signal readout
C2
C1
R2
R1
V
R2C2 , R2C1  t0
R2C2 , R2C1  t0
“Current source”
“Voltage source”
I
V
t [ns ]
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t [ns ]
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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”
Signal after shaping
Signal after amplifier
RC/CR Shaping
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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)
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Ageing Effects II
• Etching of anode wire in case of
counting gas with CF4 admixtures
(LHCb straws)
• Modification of the cathode surface:
Malter effect  self sustaining currents
(HERA-B, Honeycomb tracker)
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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.
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Multi Wire Proportional Chamber
Charpak, 1967/68
Nobel prize 1992
spatial resolution
~s
12
E Field
With typ. wire distance
s2mm  s  0.6 mm
Significantly better spatial
resolution in not achievable
with MWPCs
MPWC = Multiwire proportional chambers
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Drift Chamber
• Drift time  drift distance and intersection point of particle
• Spatial resolution of ~100 m achievable
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First Drift Chamber
Physikalisches
Institut,
Heidelberg, 1971
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LHCb Outer Tracker
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Outer Tracker - Demands
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Planar Tracking Stations
T1 T2 T3
6m
1.3% area
20% tracks
5m
Outer
Tracker
264 Module
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Straw Tubes
Straw tube drift chamber modules
e
5mm cells
e
-
Cathode
Track
e
-
-
pitch 5.25 mm
Straw tube winding:
2.5 m
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Lamina Dielectrics Ltd.
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Module Construction
2
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
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Drift time spectrum
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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
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Micro pattern detectors
• Micromegas
• GEM detectors
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Micromegas
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Gas Electron Multiplier (GEM)
140 m
double GEM
triple GEM
Single GEM
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Compass Triple-GEM
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Novel Neutron Detector
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CASCADE Neutron Detector
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Detector development tools
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