1. CERN, Geneva, Switzerland
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. GASEOUS DETECTORS’ FAMILY TREE INTRODUCTION 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- 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
GAS (STP)
Argon
Xenon CH 4
DME
dE/ dx (
keV / cm )
0.32
2.4
6.7
1.5
3.9 n
(ion pairs/ cm ) 6 25 44 16 55
Statistics of primary ionization: P k n = ! e -
n: average
k: actual number
Poisson:
(Maximum) detection efficiency: e = 1 - n
GAS (STP)
thickness

Helium 1 mm 45 2 mm 70 1 mm
91.8
Argon 2 mm
99.3

5. SECONDARY AND TOTAL IONIZATION
CLUSTERS AND DELTA ELECTRONS:
Argon
DME
n (ion pairs/cm) cm ) 25 55
GAS (STP)
Xenon 44 CH 4 16
N (ion pairs/cm) 90
160
300 53
Helium 6 8 n N
~ 3 _
N: total ion-electron pairs
CLUSTER SIZE DISTRIBUTION: P ( m ) ~ W 2
H. Fischle et al,
Nucl. Instr. and Meth.
A301(1991)202

6. CONSEQUENCES OF ENERGY LOSS STATISTICS
IONIZATION
CONSEQUENCES OF ENERGY LOSS STATISTICS
LANDAU DISTRIBUTION OF ENERGY LOSS:
500
1000
6000
4000
2000
N (i.p.)
Counts
4 cm Ar-CH4 (95-5)
5 bars
PARTICLE IDENTIFICATION
Requires statistical analysis of hundreds of samples
N = 460 i.p.
FWHM~250 i.p.
500
1000
6000
4000
2000
N (i.p)
Counts
protons
electrons
15 GeV/c
For a Gaussian distribution:
sN ~ 21 i.p.
FWHM ~ 50 i.p.
I. Lehraus et al, Phys. Scripta 23(1981)727

7. LOCALIZATION ACCURACY IN DRIFT CHAMBERS
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. CENTER OF GRAVITY OF INDUCED CHARGE READOUT
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 (1979) 455

9. ANGULAR DEPENDENCE OF POSITION ACCURACY IN MICRO-STRIP CHAMBERS:
IONIZATION
ANGULAR DEPENDENCE OF POSITION ACCURACY IN MICRO-STRIP CHAMBERS:
F. Van den Berg et al, Nucl. Instr. and Meth. A349 (1994) 438

10. DECLUSTERING EFFECT IN TIME PROJECTION CHAMBERS:
IONIZATION
DECLUSTERING EFFECT IN TIME PROJECTION CHAMBERS:
B=1.5 T
B offset
Drift
Data: D. Decamp et al, Nucl. Instr. and Meth. A269(1990)121
Simulation: A. Sharma, CERN

11. LIMITED TIME RESOLUTION OF WIRE AND MICROPATTERN CHAMBERS:
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. PARALLEL PLATE CHAMBERS: SUB-NANOSECOND RESOLUTION
IONIZATION
PARALLEL PLATE CHAMBERS: SUB-NANOSECOND RESOLUTION
FAST SIGNAL INDUCTION DURING AVALANCHE DEVELOPMENT:
Useful gap
R. Arnaldi et al, Nucl. Phys. B 78(1999)84

13. DRIFT AND DIFFUSION OF CHARGES IN GASES
ELECTRIC FIELD E = 0: THERMAL DIFFUSION
ELECTRIC FIELD E > 0: CHARGE TRANSPORT AND DIFFUSION
IONS
ELECTRONS E

14. 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. IONS DRIFT VELOCITY (Almost) linear function of field Mobility:
~ constant for a given gas (at fixed P and T)
GAS ION µ+ (cm2 V-1
Ar Ar
CH CH
Ar-CH CH
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 (Einstein’s law):
Same for all ions!
E. McDaniel and E. Mason
The mobility and diffusion of ions in gases (Wiley 1973)

16.  : mean collision time DRIFT AND DIFFUSION OF ELECTRONS IN GASES
Electric
Field
Electron Swarm Drift
Ds, Dt s
Drift velocity:
Space diffusion rms:
Townsend expression:
 : mean collision time
Drift velocity and diffusion are gas and field dependent:
P : pressure

17. LARGE RANGE OF DRIFT VELOCITIES AND DIFFUSIONS
DRIFT VELOCITY:
DIFFUSION:

18. ELECTRON TRANSPORT THEORY
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. CHARGE TRANSPORT DETERMINED BY ELECTRON-MOLECULE CROSS SECTION:
DRIFT
CHARGE TRANSPORT DETERMINED BY ELECTRON-MOLECULE CROSS SECTION:
MAGBOLTZ
S. Biagi, Nucl. Instr. and Meth. A421 (1999) 234

20. COMPUTED DRIFT VELOCITY IN MIXTURES

21. LONGITUDINAL DIFFUSION (// E)
DRIFT
LONGITUDINAL DIFFUSION (// E)
Drift
E Field sT sL
SMALLER THAN TRANSVERSE DIFFUSION:
LONGITUDINAL DIFFUSION:
TRANSVERSE DIFFUSION:
Longitudinal diffusion ( µm for 1 cm drift)
Transverse diffusion ( µm for 1 cm drift)

22. DRIFT TIME ACCURACY: DEPENDS ON IONIZATION DENSITY
Anode Wire sL
Single electron
Several electrons
Many electrons
Detection threshold
Error on first electron electron:
N= 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. w r B ^  : mean collision time // EFFECTS OF MAGNETIC FIELD s
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 // r B s L T w

24. DRIFT IN MAGNETIC FIELD: SIMPLE MODEL:

25. TRANSVERSE DIFFUSION IN SEVERAL GASES
DRIFT
TRANSVERSE DIFFUSION IN SEVERAL GASES
REDUCTION IN MAGNETIC FIELD // E

26. COMPUTED FROM TRANSPORT THEORY (MAGBOLTZ)
DRIFT
COMPUTED FROM TRANSPORT THEORY (MAGBOLTZ)

27. MAGNETIC FIELD EFFECTS: DISTORSIONS IN DRIFT CHAMBERS
W. de Boer et al, Nucl. Instr. and Meth. 156(1978)249

28. MAGNETIC FIELD EFFECT: COORDINATE DISTORSIONS IN MICRO-STRIP CHAMBERS
DRIFT
MAGNETIC FIELD EFFECT:
COORDINATE DISTORSIONS IN MICRO-STRIP CHAMBERS
F. Angelini et al, Nucl. Instr. and Meth. A347(1994)441

29. // TRANSVERSE DIFFUSION: SUBSTANTIALLY REDUCED IN SOME GASES
DRIFT
TRANSVERSE DIFFUSION: SUBSTANTIALLY REDUCED IN SOME GASES
TIME PROJECTION CHAMBER:
Center-of-gravity of cathode signal //
B=0
B>0
D. Nygren, TPC proposal (PEP4, 1976)

30. STABILITY OF OPERATION
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. STABILITY OF OPERATION
DRIFT
STABILITY OF OPERATION
TEMPERATURE
AT LOW FIELDS (THERMAL ELECTRONS):
At high fields, the thermal coefficient in some gases decreases and even becomes negative:
100
500
1000 -1 1 2 3 4
2000
E (V/cm) A
CO2
Methylal
C4H10
CH4
A-C4H10-Methylal
Dw/w/ºC
G. Shultz and J. Gresser, Nucl. Instr. and Meth. 151(1978)413

32. 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. 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. USE OF CF4 AS QUENCHER REPLACING CH4 IN TPCs
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. ELECTRON CROSS SECTIONS IN CF4
CAPTURE
ELECTRON CROSS SECTIONS IN CF4

36. INCREASING THE FIELD TOWARDS CHARGE MULTIPLICATION
Electrons energy distribution at increasing fields:
IONIZATION 15.7 eV
EXCITATION 11.6 eV

37. IONIZATION CROSS SECTION AND TOWNSEND COEFFICIENT
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. l E x AVALANCHE MULTIPLICATION IN UNIFORM FIELD
Combined cloud chamber-avalanche chamber: l E x
Ions
Multiplication factor or Gain
Electrons
H. Raether
Electron avalanches and breakdown in gases
(Butterworth 1964)

39. M V s 1 I MEASUREMENT OF THE TOWNSEND COEFFICIENT
MULTIPLICATION
MEASUREMENT OF THE TOWNSEND COEFFICIENT
Current vs voltage for constant charge injection in a parallel plate counter:
Radiation M V s 1 I

40. TOWNSEND COEFFICIENT IN GAS MIXTURES ARGON-CH4:
MULTIPLICATION
TOWNSEND COEFFICIENT IN GAS MIXTURES
ARGON-CH4:
in Argon
A. Sharma and F. Sauli, Nucl. Instr. and Meth. A334(1993)420

41. SIGNAL DEVELOPMENT PARALLEL PLATE COUNTERS:
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
V=0
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) s0 s
-V0
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. PARALLEL PLATE COUNTERS: SIGNAL DEVELOPMENT (CHARGE MULTIPLICATION)
Fas electron signal
Slow ion tail

45. SIGNAL DEVELOPMENT WIRE PROPORTIONAL COUNTERS:
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. ln M Streamer Breakdown Saturation Multiplication Collection
PROPORTIONAL COUNTERS: GAIN CHARACTERISTICS
ln M
Streamer
Breakdown
Saturation
Multiplication
Collection
Attachment n1 n2
IONIZATION
CHAMBER
PROPORTIONAL
COUNTER
Voltage

47. PROPORTIONAL COUNTERS: SIGNAL DEVELOPMENT
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 a=10µm, b=10 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. Q T+ CHARGE SIGNAL: 0.2 0.4 0.6 0.8 1.0 t (µs) q(t) q(t) 100 200 300
MULTIPLICATION
CHARGE SIGNAL:
0.2
0.4
0.6
0.8
1.0
t (µs)
q(t)
q(t)
100
200
300
400
500
t (µs) Q T+
AMPLIFIER TIME CONSTANT;
CURRENT SIGNAL:
100
200
300
400
500
q(t)
300 ns
100 ns
50 ns 20 40 60 80
100
t (ns)
i(t)
t (ns)