IONIZATION 1

1
TITLE
GAS-FILLED DETECTORS
Fabio SAULI
CERN, Geneva, Switzerland
RADIATION DETECTION AND MEASUREMENT Prof. Glenn
Knoll, organizer Short Courses November
10-11 2002 IEEE NSS/MIC Norfolk, November 10-16,
2002
2
INTRODUCTION
GASEOUS DETECTORS FAMILY TREE
TIME PROJECTION CHAMBER
CHERENKOV RING IMAGING
MULTIWIRE PROPORTIONAL CHAMBER
TRANSITION RADIATION TRACKER
DRIFT CHAMBERS
STREAMER TUBES
GAS ELECTRON MULTIPLIER
COMPTEUR A TROUS
STRAWS
PROPORTIONAL COUNTER
MICROWELL
MICROMEGAS
MICROSTRIP CHAMBERS
MICROGAP
PARALLEL PLATE COUTER
AVALANCHE CHAMBERS
RESISTIVE PLATE CHAMBERS
PESTOV COUNTER
3
PART 1
PART 1- FUNDAMENTS
IONIZATION DRIFT AND DIFFUSION CAPTURE
LOSSES AVALANCHE MULTIPLICATION
4
IONIZATION
COULOMB INTERACTIONS OF CHARGED PARTICLES WITH
MOLECULES
PRIMARY IONIZATION ELECTRON-ION PAIRS
Minimum ionizing particles
Helium
Argon
Xenon
DME
CH
GAS (STP)
4
3.9
1.5
dE/
dx
(
keV
/
cm
)
2.4
6.7
0.32
6
n
(ion pairs/
cm
)
55
16
44
25

Statistics of primary ionization
n average k actual number
Poisson
(Maximum) detection efficiency
thickness
?????
GAS (STP)
1
mm
45
Helium
70
2
mm
1
mm
91.8
Argon
99.3
2
mm
5
IONIZATION
SECONDARY AND TOTAL IONIZATION
CLUSTERS AND DELTA ELECTRONS
N total ion-electron pairs
CLUSTER SIZE DISTRIBUTION
H. Fischle et al, Nucl. Instr. and Meth.
A301(1991)202
6
IONIZATION
CONSEQUENCES OF ENERGY LOSS STATISTICS
LANDAU DISTRIBUTION OF ENERGY LOSS
4 cm Ar-CH4 (95-5) 5 bars
PARTICLE IDENTIFICATION Requires statistical
analysis of hundreds of samples
N 460 i.p.
FWHM250 i.p.
0
For a Gaussian distribution
sN 21 i.p. FWHM 50 i.p.
I. Lehraus et al, Phys. Scripta 23(1981)727
7
IONIZATION
LOCALIZATION ACCURACY IN DRIFT CHAMBERS WORSENED
BY LONG-RANGE ELECTRONS
Drift Time
5 of events!
F. Sauli, Nucl. Instr. and Meth. 156(1978)147
8
IONIZATION
CENTER OF GRAVITY OF INDUCED CHARGE READOUT
STRONG ANGULAR DEPENDENCE OF POSITION ACCURACY
Position accuracy as a function of the track
angle to the normal to the chamber
G. Charpak et al, Nucl. Instr. and Meth. 167
(1979) 455
9
IONIZATION
ANGULAR DEPENDENCE OF POSITION ACCURACY IN
MICRO-STRIP CHAMBERS
F. Van den Berg et al, Nucl. Instr. and Meth.
A349 (1994) 438
10
IONIZATION
DECLUSTERING EFFECT IN TIME PROJECTION CHAMBERS
B1.5 T
B offset
Data D. Decamp et al, Nucl. Instr. and Meth.
A269(1990)121 Simulation A. Sharma, CERN
11
IONIZATION
LIMITED TIME RESOLUTION OF WIRE AND MICROPATTERN
CHAMBERS
Space distribution of the cluster closer to an
electrode
Time distribution of the cluster closer to an
electrode
w drift velocity
w 5 cm/µs
12
IONIZATION
PARALLEL PLATE CHAMBERS SUB-NANOSECOND RESOLUTION
FAST SIGNAL INDUCTION DURING AVALANCHE
DEVELOPMENT
R. Arnaldi et al, Nucl. Phys. B 78(1999)84
13
DRIFT
DRIFT AND DIFFUSION OF CHARGES IN GASES
ELECTRIC FIELD E 0 THERMAL DIFFUSION
ELECTRIC FIELD E gt 0 CHARGE TRANSPORT AND
DIFFUSION
ELECTRONS
IONS
E
14
DRIFT
DRIFT AND DIFFUSION OF IONS (CLASSIC KINETIC
THEORY OF GASES)
Ions remain thermal up to very high
fields Maxwell energy distribution
Average (thermal) energy
Diffusion equation Fraction of ions at distance x
after time t
D diffusion coefficient
RMS of linear diffusion
Molecules diffuse rapidly in the available
volume (leaks!)
15
DRIFT
IONS DRIFT VELOCITY
(Almost) linear function of field
Mobility
constant for a given gas (at fixed P and T)
MWPC 1 cm gap, Ar-CH4, 5 kV/cm
Total ions drift time T 120 µs
TPC 1 m drift, Ar-CH4, 200 V/cm
Total ions drift time T 300 ms
IONS DIFFUSION (Einsteins law)
Same for all ions!
E. McDaniel and E. Mason The mobility and
diffusion of ions in gases (Wiley 1973)
16
DRIFT
DRIFT AND DIFFUSION OF ELECTRONS IN GASES
Electric Field
Electron Swarm Drift
Ds, Dt
s
Drift velocity
Space diffusion rms
? mean collision time
Townsend expression
P pressure
Drift velocity and diffusion are gas and field
dependent
17
DRIFT
LARGE RANGE OF DRIFT VELOCITIES AND DIFFUSIONS
DRIFT VELOCITY
DIFFUSION
18
DRIFT
ELECTRON TRANSPORT THEORY
BALANCE BETWEEN ENERGY ACQUIRED FROM THE FIELD
AND COLLISION LOSSES
Energy distribution probability
Mean free path between collisions
??? electron-molecule cross section)
Fractional energy loss in collisions
Drift velocity
Diffusion coefficient
Frost and Phelps, Phys. Rev. 127(1962)1621 V.
Palladino and B. Sadoulet, Nucl. Instr. and Meth.
128(1975)323 G. Shultz and J. Gresser, Nucl.
Instr. and Meth. 151(1978)413 S. Biagi, Nucl.
Instr. and Meth. A283(1989)716
19
DRIFT
CHARGE TRANSPORT DETERMINED BY ELECTRON-MOLECULE
CROSS SECTION
MAGBOLTZ
S. Biagi, Nucl. Instr. and Meth. A421 (1999) 234
http//consult.cern.ch/writeup/magboltz/cross/
http//cpa94.ups-tlse.fr/operations/operation_03/P
OSTERS/BOLSIG/
20
DRIFT
COMPUTED DRIFT VELOCITY IN MIXTURES
http//consult.cern.ch/writeup/garfield/examples/g
as/trans2000.htmlelec
21
DRIFT
LONGITUDINAL DIFFUSION (// E)
SMALLER THAN TRANSVERSE DIFFUSION
LONGITUDINAL DIFFUSION
TRANSVERSE DIFFUSION
Transverse diffusion ( µm for 1 cm drift)
Longitudinal diffusion ( µm for 1 cm drift)
http//consult.cern.ch/writeup/garfield/examples/g
as/Welcome.html
22
DRIFT
DRIFT TIME ACCURACY DEPENDS ON IONIZATION
DENSITY
sL
Several electrons
Many electrons
Single electron
Detection threshold
Error on first electron electron
N100 s1 0.4 sL
RESOLUTION LIMITS OF DRIFT TUBES
G. Scherberger et al, Nucl. Instr. and Meth.
A424(1999)495 W. Riegler et al, Nucl. Instr. and
Meth. A443(2000)156
23
DRIFT
EFFECTS OF MAGNETIC FIELD
THE SWARM IS ROTATED BY AN ANGLE qB IN THE PLANE
PERPENDICULAR TO E AND B THE MAGNETIC DRIFT
VELOCITY IS wB w0 THE TRANSVERSE DIFFUSION IS
REDUCED
? mean collision time
Larmor frequency
//
24
DRIFT
DRIFT IN MAGNETIC FIELD SIMPLE MODEL
25
DRIFT
TRANSVERSE DIFFUSION IN SEVERAL GASES
REDUCTION IN MAGNETIC FIELD // E
26
DRIFT
COMPUTED FROM TRANSPORT THEORY (MAGBOLTZ)
27
DRIFT
MAGNETIC FIELD EFFECTS DISTORSIONS IN DRIFT
CHAMBERS
W. de Boer et al, Nucl. Instr. and Meth.
156(1978)249
28
DRIFT
MAGNETIC FIELD EFFECT COORDINATE DISTORSIONS IN
MICRO-STRIP CHAMBERS
F. Angelini et al, Nucl. Instr. and Meth.
A347(1994)441
29
DRIFT
TRANSVERSE DIFFUSION SUBSTANTIALLY REDUCED IN
SOME GASES
TIME PROJECTION CHAMBER Center-of-gravity of
cathode signal
B0
Bgt0
D. Nygren, TPC proposal (PEP4, 1976)
30
DRIFT
STABILITY OF OPERATION VOLTAGE AND PRESSURE
THE DRIFT VELOCITY IS A FUNCTION OF REDUCED FIELD
E/P
DRIFT VELOCITY SATURATION INSENSITIVE TO
VARIATIONS OF E AND P
31
DRIFT
STABILITY OF OPERATION TEMPERATURE
AT LOW FIELDS (THERMAL ELECTRONS)
At high fields, the thermal coefficient in some
gases decreases and even becomes negative
G. Shultz and J. Gresser, Nucl. Instr. and Meth.
151(1978)413
32
CAPTURE
ELECTRON CAPTURE LOSSES ON ELECTRONEGATIVE GASES
Attachmant coefficient of oxygen
Electrons surviving after 20 cm drift (E 200
V/cm)
The attachment cross section is energy-dependent,
therefore strongly depends on the gas
composition and electric field
33
CAPTURE
ELECTRON CAPTURE - VERY SENSITIVITE TO GAS MIXTURE
Energy resolution of a proportional counter with
two gas fillings (and some leaks!)
5.9 keV X-rays
Hot gas
ARGON-ETHANE 50-50
Cold gas
DIMETHYLETHER
R. Openshaw, TRIUMF (private, 2000)
34
DRIFT
USE OF CF4 AS QUENCHER REPLACING CH4 IN TPCs

• FAST DRIFT VELOCITY
• - SMALL DIFFUSION
• - NO HYDROGEN (REDUCED NEUTRON SENSITIVITY)
• - NON-FLAMMABLE

L. G. Christophorou et al, Nucl. Instr. and
Meth.163(1979)141
35
CAPTURE
ELECTRON CROSS SECTIONS IN CF4
http//consult.cern.ch/writeup/magboltz/cross/
36
MULTIPLICATION
INCREASING THE FIELD TOWARDS CHARGE
MULTIPLICATION
Electrons energy distribution at increasing
fields
IONIZATION 15.7 eV
EXCITATION 11.6 eV
37
MULTIPLICATION
IONIZATION CROSS SECTION AND TOWNSEND COEFFICIENT
Mean free path for ionization
N molecules/cm3
Townsend coefficient
Ionizing collisions/cm
S.C. Brown, basic data of plasma physics (MIT
press, 1959)
38
MULTIPLICATION
AVALANCHE MULTIPLICATION IN UNIFORM FIELD
Combined cloud chamber-avalanche chamber
Ions
Multiplication factor or Gain
Electrons
H. Raether Electron avalanches and breakdown in
gases (Butterworth 1964)
39
MULTIPLICATION
MEASUREMENT OF THE TOWNSEND COEFFICIENT
Current vs voltage for constant charge injection
in a parallel plate counter
Radiation
M
1
s
V
I
40
MULTIPLICATION
TOWNSEND COEFFICIENT IN GAS MIXTURES ARGON-CH4
A. Sharma and F. Sauli, Nucl. Instr. and Meth.
A334(1993)420
41
MULTIPLICATION
SIGNAL DEVELOPMENT
PARALLEL PLATE COUNTERS
A charge Q between two conductors induces two
negative charge profiles (image charge)
Moving the charge modifies the induced charge
profile on the conductors and generates
detectable signals Q towards an electrode
positive induced signal
Induced signals are equal and opposite on anode
and cathode
42
MULTIPLICATION
PARALLEL PLATE COUNTERS SIGNAL DEVELOPMENT
(CHARGE COLLECTION ONLY)
Single charge Q
Charge induced on each electrode by Q moving
through the difference of potential dV
CATHODE
V -V0
s
Q
s0
Integrating over s (or time t)
w drift velocity
V0
ANODE
Electrons- ion pair (-Q and Q) released at the
same distance s from the cathode
w- (w ) electron (ion) drift velocity
T- (T ) total electron (ion) drift time
Total signal
(Q on cathode , -Q on anode)
43
MULTIPLICATION
PARALLEL PLATE COUNTERS SIGNAL DEVELOPMENT
(CHARGE MULTIPLICATION)
During the avalanche development, the increase in
the number of charges after a path ds is
and the total after a path s
The incremental charge induction due to electrons
after a path s
Integrating over s
and the corresponding current
The current signal iduced by the ions is instead
given by
44
MULTIPLICATION
PARALLEL PLATE COUNTERS SIGNAL DEVELOPMENT
(CHARGE MULTIPLICATION)
Fas electron signal
Slow ion tail
45
MULTIPLICATION
SIGNAL DEVELOPMENT
WIRE PROPORTIONAL COUNTERS
Thin anode wire coaxial with cathode
Cathode radius b
Electric field
Anode radius a
Avalanche development around a thin wire
46
MULTIPLICATION
PROPORTIONAL COUNTERS GAIN CHARACTERISTICS
ln M
Streamer
Breakdown
Saturation
Multiplication
Collection
Attachment
n1
n2
IONIZATION CHAMBER
PROPORTIONAL COUNTER
Voltage
47
MULTIPLICATION
PROPORTIONAL COUNTERS SIGNAL DEVELOPMENT
Incremental charge induced by Q moving through dV
Assuming that the total charge of the avalanche Q
is produced at a (small) distance l from the
anode, the electron and ion contributions to the
induced charge are
and
The total induced signal is
on the anode ( on the cathode)
The ratio of electron and ion contributions
For a counter with a10µm, b10 m q-/q 1
The electron-induced signal is negligible
Neglecting electrons, and assuming all ions leave
from the wire surface
Total ions drift time
48
MULTIPLICATION
CHARGE SIGNAL
AMPLIFIER TIME CONSTANT
CURRENT SIGNAL
t (ns)