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Слайд 1 - Laboratory for High Energy Physics

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Detectors for particles and radiation
Advanced course for Master students
Spring semester 2010
S7139
Tuesday 10:15 to 12:00 - Lectures
Tuesday 16:15 to 17:00 - Exercises
5 ECTS points
Detectors for particles and radiation
February 23
Kreslo, Gornea
Introduction, History of instrumentation
March 2
Kreslo, Gornea
Particle-matter electromagnetic interactions
March 9
Kreslo, Gornea
Gas detectors : counters
March 16
Kreslo, Gornea
Gas detectors : tracking
March 23
Kreslo, Gornea
Scintillating detectors :counters
March 30
Kreslo, Gornea
Scintillating detectors : tracking
April 6
Bay, Gornea
Nuclear emulsions
April 13
Kreslo, Gornea
Semiconductor detectors : counters
April 20
Kreslo, Gornea
Semiconductor detectors : tracking
April 27
Kreslo, Gornea
Semiconductor detectors : tracking
April 27
Kreslo, Gornea
Cryogenic liquids : tracking
May 4
Kreslo, Gornea
Calorimetry
May 11
Kreslo, Gornea
Calorimetry
May 18
Kreslo, Gornea
Particle Identification
May 25
Kreslo, Gornea
Momentum measurements
June 1
Kreslo, Gornea
Discussion + Lab demonstration
Image and Logic traditions
History of �Particle Detection’
Image Tradition: Cloud Chamber
Emulsion
Bubble Chamber
Logic Tradition: Scintillating Counter
Geiger Counter
Tip Counter
Spark Counter
Electronics Image: Spark Chambers
Wire Chambers
Scintillating trackers
Silicon Detectors
Introduction, history of instrumentation
1906: Geiger Counter, H. Geiger, E. Rutherford
1910: Cloud Chamber, C.T.R. Wilson
1912: Tip Counter, H. Geiger
1928: Geiger-MГјller Counter, W. MГјller
1929: Coincidence Method, W. Bothe
1930: Emulsion, M. Blau
1940-1950: Scintillator, Photomultiplier
1952: Bubble Chamber, D. Glaser
1962: Spark Chamber
1968: Multi Wire Proportional Chamber, G. Charpak
1979: Time Projection Chamber, D. Nygren
1984: Silicon Drift Detectors, E. Gatti & P. Rehak
1997: Gas Electron Multiplier, F. Sauli
Etc. etc. etc.
History of instrumentation: Logic detectors (counters)
History of instrumentation: Logic detectors (counters)
E. Rutherford
1909
H. Geiger
pulse
The Geiger counter, later further developed and then called
Geiger-MГјller counter
First electrical signal from a particle
History of instrumentation: Logic detectors (counters)
First use of Coincidence
History of instrumentation: Logic detectors (counters)
Electronic coincidence circuit
History of instrumentation: Logic detectors (counters)
History of instrumentation: Logic detectors (counters)
History of instrumentation: Logic detectors (counters)
Ionization of Gases
Primary ionization
Total ionization
Lohse and Witzeling, Instrumentation In High
Energy Physics, World Scientific,1992
Fast charged particles ionize atoms of gas.
Often resulting primary electron will have
enough kinetic energy to ionize other atoms.
dE
n total пЂЅ
пЃ„E
Wi
пЃ„x
пЂЅ dx
Wi
n total п‚» 3пЃ‹ 4 пѓ— n primary
ntotal - number of created
electron-ion pairs
пЃ„E = total energy loss
Wi = effective <energy loss>/pair
Wi - NOT the ionisation potential !!!
Number of primary electron/ion pairs in
frequently used gases for MIP.
Ionization of Gases
Example: Ar
Density ~ 1.7 g/l
пЃ„E = 1.8 MeV/(g/cm2) ~ 3 keV/cm
Wi = 26 eV/ion
ntotal ~ 100 ions/cm
(~25 primary)
Ionization of Gases: first approximation
•
The number of primary electron/ion pairs is approximately Poisson distributed.
m пЂ­n
P (m ) пЂЅ
n e
nпЂЅ
m!
L
пЃ¬
пЂЅ LN пЃі
i
The detection efficiency is therefore limited to :
пЃҐ det пЂЅ 1 пЂ­ P ( 0 ) пЂЅ 1 пЂ­ e
пЂ­n
For thin layers пЃҐdet can be significantly lower than 1.
For example for 1 mm layer of Ar nprimary= 2.5 в†’ пЃҐdet = 0.92 .
Variation of the number of electron/pairs:
пЃіn пЂЅ
n;
NOT exactly correct!
Ionization of Gases: second approximation
•
The number of primary electron/ion pairs is NOT Poisson distributed.
пЃіn пЂЅ
F n;
F – Fano factor, related to the correlations in the
Ionization avalanche process, material dependent!
F~ 1 for scintillators
F~0.2 – 0.8 for gas detectors
F~ 0.12 for Silicon detectors
•
100 electron/ion pairs created during ionization process is not
easy to detect.
Typical noise of the amplifier ≈ 1000 e- (ENC) → gas
amplification is required !! .
Capacitor with gas at low electric field
Response to a primary ionization
Primary
ionisation Q0
Particle
Recombination
losses q0=A*Q0
Attachment
losses q=q0e -(D/О»)
E
IAr+ e-
I-
Ar+
I-
e-
e-
D, drift distance
+V
Gas amplification: capacitor with gas
Alfa-particle
Beta-particle
+V
0.2 mm
“Ionisation” mode, i.e. no amplification yet…
Gas amplification: capacitor with gas
+V
0.2 mm
“Proportional” mode, linear amplification…
Single Wire Proportional Chamber
Multiplication of ionization is described
by the first Townsend coefficient пЃЎпЂЁEпЂ©
dn = nпЃЎdx
n пЂЅ n0 e
пЃЎ пЂЅ
пЃЎ пЂЁE пЂ©x
1
пЃ¬
Ar-CH4
 – mean free path
or n пЂЅ n 0 e
пЃЎ пЂЁr пЂ©x
пЃЎпЂЁEпЂ© is determined by the excitation and
ionization cross sections of the electrons
A. Sharma and F. Sauli, NIM A334(1993)420
in the gas.
It depends also on various and complex
energy transfer mechanisms between gas molecules.
There is no fundamental expression
for пЃЎпЂЁEпЂ© в†’ it has to be measured for every
gas mixture.
Amplification factor or
Gain
Photoemission
In the avalanche process molecules of the
gas can be brought to excited states.
De-excitation of noble gases
only via emission of photons;
e.g. 11.6 eV for Ar.
This is above ionization
threshold of metals;
e.g. Cu 7.7 eV.
e11.6 eV
Ar *
Cu
cathode
When gain exceeds about 108 - new avalanches в†’ increase of the discharge current
Gas amplification: capacitor with gas
Photoemission starts…
Оі
+V
0.2 mm
“Saturated” mode, logarithmic amplification, saturation…
Gas amplification: capacitor with gas
Strong photoemission…
Оі
+V
0.2 mm
“Geiger” mode, only counting is possible, info about primary ionization is lost!
Gas amplification: capacitor with gas
Strong photoemission, ion impact ionisation…
Оі
+V
0.2 mm
“Saturated” mode, logarithmic amplification, saturation…
Gas ionization chamber – Operation Modes
•
ionization mode – full charge collection, but no
charge multiplication;
gain ~ 1
•
proportional mode – multiplication of ionization
starts; detected signal proportional to original
ionization в†’ possible energy measurement (dE/dx);
secondary avalanches have to be quenched;
gain ~ 104 – 105
•
limited proportional mode (saturated, streamer) –
strong photoemission; secondary avalanches
merging with original avalanche; requires strong
quenchers or pulsed HV; large signals в†’ simple
electronics;
gain ~ 1010
•
Geiger mode – massive photoemission; full length
of the anode wire affected; discharge stopped by
HV cut; strong quenchers needed as well
Geiger counter: coaxial geometry
Electrons liberated by ionization drift towards
the anode wire.
Electrical field close to the wire (typical wire Г�
~few tens of mm) is sufficiently high for Geiger
mode discharge.
E пЂЁr пЂ© пЂЅ
V (r ) пЂЅ
R~1-10
MOhm
pulse
CV 0
пѓ—
1
C – capacitance/unit length
2пЃ°пЃҐ 0 r
CV 0
2пЃ°пЃҐ 0
пѓ— ln
r
a
Discharge is quenched
by the current-limiting
resistor
Geiger counter: coaxial geometry
Single Wire Proportional Chamber
Electrons liberated by ionization drift towards
the anode wire.
Electrical field close to the wire (typical wire Г�
~few tens of mm) is sufficiently high for electrons
(above 10 kV/cm) to gain enough energy to
Ionize further → avalanche – exponential
increase of number of electron ion pairs
- the proportional operation mode.
anode
eprimary electron
E пЂЁr пЂ© пЂЅ
CV 0
1
пѓ—
2пЃ°пЃҐ 0 r
V (r ) пЂЅ
CV 0
пѓ— ln
2пЃ°пЃҐ 0
пѓ© rC
пѓ№
M пЂЅ
пЂЅ exp пѓЄ пѓІ пЃЎ пЂЁ r пЂ©dr пѓє
n0
пѓЄпѓ« a
пѓєпѓ»
n
r
a
C – capacitance/unit length
Cylindrical geometry is not the only one able to generate strong electric field:
parallel plate
strip
hole
groove
SWPC – Choice of Gas
In the avalanche process molecules of the
gas can be brought to excited states.
S. Biagi, NIM A421 (1999) 234
ELASTIC
Solution: addition of polyatomic gas as a
quencher
Absorption of photons in a large energy
range (many vibrational and rotational
energy levels).
IONIZATION
Energy dissipation by collisions or
dissociation into smaller molecules.
SUM OF EXCITATION
S. Biagi, NIM A421 (1999) 234
ELASTIC
De-excitation of noble gases
only via emission of photons;
e.g. 11.6 eV for Ar.
*
This is above ionization A r
threshold of metals;
e.g. Cu 7.7 eV.
IONIZATION
e11.6 eV
Cu
vibrational levels
cathode
new avalanches в†’ permanent discharges
excitation levels
SWPC – Signal Formation
+
Avalanche formation within a few
wire radii and within t < 1 ns.
Signal induction both on anode and
cathode due to moving charges
(both electrons and ions).
+
-
+
-
dv пЂЅ
Q
dV
dr
lCV 0 dr
50 ns
v(t)
Electrons collected by the anode wire i.e. dr is
very small (few mm) – almost no induction signal
100 ns
300 ns
Ions have to drift back to cathode i.e. dr is large
(few mm). Signal duration limited by total ion drift
time.
0
100
200
300
400
Need electronic signal differentiation to limit dead time.
t (ns)
500
Multiwire Proportional Chamber
Simple idea to multiply SWPC cell : Nobel Prize 1992
First electronic device allowing high statistics experiments !!
Typical geometry
5mm, 1mm, 20 mm
Normally digital readout :
spatial resolution limited to
пЃі
x
п‚»
d
12
for d = 1 mm пЃіx = 300 mm
G. Charpak, F. Sauli and J.C. Santiard
CSC – Cathode Strip Chamber
Precise measurement of the second
coordinate
by interpolation of the signal induced on
pads.
Closely spaced wires makes CSC fast
detector.
пЃі = 64 mm
Center of gravity of induced
signal method.
Space
resolution
CMS
RPC – Resistive Plate Chamber
useful gap
readout strips
HV
resistive electrode
Rate capability strong function of the resistivity
of electrodes in streamer mode.
c luste rs
gas gap
E
2 mm
A. Akindinov et al., NIM A456(2000)16
GND
resistive electrode
readout strips
пЃі = 77 ps
MRPC
HV
GND
Multigap RPC - exceptional time resolution
suited for the trigger applications
Time resolution
Limitations of Gas Detectors
Classical ageing
Avalanche region в†’
plasma formation
(complicated plasma chemistry)
•Dissociation of detector gas and pollutants
•Highly active radicals formation
•Polymerization (organic quenchers)
•Insulating deposits on anodes and cathodes
Anode: increase of the wire
diameter, reduced and variable
field, variable gain and energy
resolution.
Cathode: formation of
strong
dipoles, field emmision and
microdischarges (Malter
effect).
Limitations of Gas Detectors
Solutions: carefull material selection for the detector construction and gas system,
detector type (GEM is resitant to classical ageing), working point,
non-polymerizing gases, additives supressing polymerization (alkohols, methylal),
additives increasing surface conductivity (H2O vapour), clening additives (CF4).
Discharges
Field and charge density dependent effect.
Solution: multistep amplification
Space charge limiting rate capability
Solution: reduction of the lenght of the positive
ion path
Insulator charging up resulting in gain variable with time
and rate
Solution: slightly conductive materials
Computer Simulations
MAXWELL (Ansoft)
electrical field maps in 2D& 3D, finite element calculation for arbitrary electrodes &
dielectrics
HEED (I.Smirnov)
energy loss, ionization
MAGBOLTZ (S.Biagi)
electron transport properties: drift, diffusion, multiplication, attachment
Garfield (R.Veenhof)
fields, drift properties, signals (interfaced to programs above)
PSpice (Cadence D.S.) electronic signal
In the next lecture
Spark chamber
Gas Electron Multiplier
Drift tubes
Time Projection Chamber
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