Transcript ICFA_2013_L2_Joram

XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Christian Joram / CERN
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-1
Outline
Lecture 2 – Momentum measurement, intro. to gaseous detectors
• Detector resolution
• Concept of p-measurement
• Ionization, drift, avalanche, choice of gas, operation modes
• B x E effects
• Det. types (MWPC, drift chambers, TPC, RPC, MPGD)
Lecture 3 – More interactions: electrons, photons, neutrons,
neutrinos. Photon Detectors.
Lecture 4 – Organic Scintillators, detector testing, detector
systems
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-2
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Lecture 1 – Interaction of charged particles
Detector resolution – some general observations
Even in the case of a well defined and constant amount of energy deposited in a detector ,
the achievable resolution in terms of energy, spatial coordinates or time is constrained by
the statistical fluctuations in the number of charge carriers (electron-hole pairs, electron-ion
pairs, scintillation photons) produced in the detector.
In most cases, the number of charge carriers nc is well described by a Poisson distribution
with mean m = <nc>
P(nc , m ) 
m n em
c
nc !
P(nc , m )
m
0.2
0.18
0.16
The variance of the Poisson
distribution is equal to its
mean value.
Poisson distribution
for m = 5.0
0.14
0.12
0.1
0.08
0.06
  n  nc  nc  m
2
nc
2
c
n  m
c
0.04
0.02
0
standard deviation
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
nc
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Particle Interactions – Detector Design Principles
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Energy deposition in detectors happens in small discrete and independent steps.
Detector resolution
G (nc , m ) 
 n  m 
1
exp   c 2
2
2 

with   m
2
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
For large m values, (e.g. m > 10) the Poisson distribution becomes reasonably well
approximated by the symmetric and continuous Gauss distribution.
m
0.14
 0.12


 0.1
Poisson
Gauss

0.08
0.06
FWHM
Often used to characterize
detector resolution: FWHM
 nc  m 2  1

exp  
2
 2
2


 nc  m   2 ln 0.5  
FWHM  2(nc  m )  2.35  
0.04
0.02
0
0
2
4
6
8
10
12
14
16
18
20
22
24
nc
The energy resolution of many detectors is found to
scale like
E 
nc  m

E
E

m
1

m
m
Often, also time and spatial resolution  x ,t
1

improve with increasing <nc>
x, t
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Particle Interactions – Detector Design Principles
m
L2-4
Energy resolution – Fano factor
The measured energy deposition E is derived from the
number of charge carriers (e.g. e-h or e-ion pairs)
nc  m  a 
x
m
dE
E
1
x


dx
E
m
m
(Poisson limit)
! We ignore here many effects which, in a real detector, will
degrade the resolution beyond the Poisson limit !
E
E

1
m
In particular, depending on the thickness x and density, there may also be strong Landau tails.
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Particle Interactions – Detector Design Principles
L2-5
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Case 1: particle traversing the detector
Energy resolution – Fano factor
E
nc  m  a  E p ,
E
E

F
F = Fano factor
m
Ep
Ugo Fano,
Phys. Rev. 72 (1947),
26–29
For a formal derivation: See e.g. book by C. Grupen, pages 15-18.
For a physics motivated derivation: See e.g. H. Spieler, Heidelberg Lecture notes, ch. II, p25…
(1912 - 2001)
The fluctuation in the number of produced charge carriers nc is constrained by energy
conservation. The discrete steps are no longer fully independent. Their fluctuation can
be smaller than the Poisson limit, i.e. F < 1.
FSi = 0.12,
Fdiamond = 0.08.
The detector resolution is (can be?) improved by a factor
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F
Particle Interactions – Detector Design Principles
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Case 2: Particle stops in the detector, or (x-ray) photon is fully absorbed
 the energy deposition in the detector is fixed (but may vary from event to event).
Detector spatial resolution
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Resolution of a discrete detector
x
0
wx
wx
Assume a detector consisting of strips of
width wx, exposed to a beam of particles.
The detector produces a (binary) signal if
one of the strips was hit.
What is its resolution x for the
measurement of the x-coordinate?
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x
Consider one strip only (for
simplicity at x = xd = 0):
For every recorded hit, we know
that the strip at x = 0 was hit, i.e.
the particle was in the interval

wx
w
x x
2
2
x  ( x  xd )  ( x  0)  x
Particle Interactions – Detector Design Principles
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Detector resolution
Resolution of a discrete detector (cont.)
2
  ( x  0)
2
x
2
i

2
x


wx 2
 wx 2
i
x 
( x  0) 2  dx

wx 2

dx
 wx 2
1
3
x
3 wx 2
 wx 2
wx
wx2

12
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
 x  0

1
wx
12
We have tacitly assumed that the particles are uniformly distributed over the strip width.
More generally, a distribution function needs to be taken into account

2
x


 wx 2
 wx 2

( x  0) D( x)  dx
 wx 2
 wx 2
D(x)
1
1
D( x)  dx
During the next days, you will see such
expressions many times
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D(x)
2
-wx/2
x 
wx/2
wx
12
x
-wx/2
x 
wx/2
x
wx
24
Particle Interactions – Detector Design Principles
L2-8
Momentum measurement
x
B
q
pT  p sin q
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
z
y
B
pL
x
y
B
B=0
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B
B>0
B>0
Particle Interactions – Detector Design Principles
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Momentum measurement
We measure only p-component transverse to B field !
pT (GeV c)  0.3B

L
 sin  2   2
2
s   1  cos 2   

 
2
8
(T  m)
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
pT  qB
0.3L  B
pT
0.3 L2 B

8 pT
the sagitta s is determined by 3 measurements with error (x):
x x
s  x2  1 3
2
  pT 
pT
meas.

 (s)
s

3
 ( x)
2
s
for N equidistant measurements, one obtains
  pT 
pT
meas.

 ( x)  pT
0.3  BL
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2
720 /( N  4)

3
 ( x)  8 pT
2
2
0.3  BL
  pT 
pT
meas.

 ( x)  pT
BL2
(R.L. Gluckstern, NIM 24 (1963) 381)
(for N ≥ ~10)
Particle Interactions – Detector Design Principles
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What is the contribution of multiple scattering to
remember
 ( p)
pT
 ( x)
  ( x)  pT
MS
More precisely:
 q0 
 ( p)
pT
 ( p)
pT
1
p
 ( p)
?
pT
MS
 constant , i.e. independent of p !
Example:
MS
 0.045
1
B LX 0
pt = 1 GeV/c, L = 1m, B = 1 T, N = 10
(x) = 200 mm:
(p)/p
total error
(p)/p
meas.
pT
 0.5%
Assume detector (L = 1m) to be filled with 1
atm. Argon gas (X0 = 110m),
meas.
(p)/p
  pT 
 ( p)
MS
pT
MS
 0.5%
Optimistic, since a gas
detector consists of
more than just gas!
p
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Particle Interactions – Detector Design Principles
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Momentum measurement
A more realistic example: CMS Silicon Tracker
•
•
B=3.8T, L=1.25m, average N ≈ 10 layers,
Average resolution per layer ≈ 25mm,
  pT 
meas.

pT
 ( x)  pT
0.3  BL2
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
r ≈ 1.25m
720 /( N  4)
 p/p = 0.1 % momentum resolution (at 1 GeV)
 p/p = 10 % momentum resolution (at 1 TeV)
Material budget (Si, cables, cooling pipes, support structure…)
•
B=3.8T, L=1.25m, t/X0 ≈ 0.4-0.5 @ h < 1
 ( p)
pT
MS
 0.045
1
1
 0.045
BL
B LX 0
t
X0
 p/p = 0.7% from multiple scattering
q
(h = pseudo rapidity: h   ln(tan 2 ) )
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Particle Interactions – Detector Design Principles
L2-12
Primary ionization
Total ionization
Fast charged particles ionize atoms of gas.
Often, the resulting primary electrons will have
enough kinetic energy to ionize other atoms.
ntotal
Number of primary electron/ion pairs in
frequently used gases.
Lohse and Witzeling, Instrumentation In High
Energy Physics, World Scientific,1992
dE
x
E dx


Wi
Wi
ntotal  3 4  n primary
ntotal - number of created
electron-ion pairs
E = total energy loss
Wi = effective <energy loss>/pair
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Particle Interactions – Detector Design Principles
L2-13
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Ionization of Gases
Visualization of charge clusters and d electrons
another event
d electron
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Cluster counting with a hybrid
gas detector: pixel readout
chip + micromegas
15 mm
He / isobutane 80/20
50 mm
~ 14 x 14 mm2
micromegas
foil
Medipix chip
256 x 256 pixels,
55 x 55 mm2, each
M. Campbell et al., NIM A 540 (2005) 295
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track by cosmic particle (mip): 0.52 clusters / mm, ~3 e-/cluster
Particle Interactions – Detector Design Principles
4-14
Ionization of Gases
The actual number of primary electron/ion pairs is Poisson distributed.
P ( m) 
0.3
m m
m e
m!
0.2
The detection efficiency is therefore limited to :
 det  1  P(0)  1  e
Poisson distribution
for m = 2.5
0.25
0.15
0.1
m
0.05
For thin layers det can be significantly lower than 1.
For example for 1 mm layer of Ar
0
0
1
2
3
4
5
6
7
8
9
10
nprimary= 2.5 → det = 0.92 .
Consider a 10 mm thick Ar layer
 nprimary= 25 → det = 1
 ntotal= 80-100
100 electron/ion pairs created during ionization process are not easy to detect.
Typical noise of the amplifier ≈ 1000 e- (ENC)
→ we will increase the number of charge carriers by gas amplification .
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Particle Interactions – Detector Design Principles
4-15
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•
Single Wire Proportional Chamber
Electrons liberated by ionization drift towards
Electrical field close to the wire (typical wire Ø
~few tens of mm) is sufficiently high for electrons
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
the anode wire.
(above 10 kV/cm) to gain enough energy to ionize
further → avalanche – exponential increase of
number of electron ion pairs.
E r  
anode
e-
V (r ) 
primary electron
CV0 1

2 0 r
C – capacitance/unit length
CV0
r
 ln
2 0
a
Cylindrical geometry is not the only one able to generate strong electric field:
parallel plate
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strip
hole
groove
Particle Interactions – Detector Design Principles
L2-16
Single Wire Proportional Chamber
Multiplication of ionization is described by
dn = n· dx  
1

XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Ar-CH4
the first Townsend coefficient E
 – mean free path
n  n0e  E x or n  n0e r x
E is determined by the excitation and
ionization cross sections of the electrons
in the gas.
A. Sharma and F. Sauli, NIM A334(1993)420
It depends also on various and complex
energy transfer mechanisms between gas molecules.
There is no fundamental expression
for E → it has to be measured for every
gas mixture.
rC

n
M
 exp    r dr 
n0
 a

Amplification factor or
(E/p = reduced electric field)
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Gain
S.C. Brown, Basic data of plasma physics (MIT Press, 1959)
Particle Interactions – Detector Design Principles
L2-17
SWPC – Choice of Gas
In noble gases, ionization is the dominant process, but there are also excited states.
De-excitation of noble gases
only via emission of photons;
ELASTIC
IONIZATION
SUM OF EXCITATION
e11.6 eV
e.g. 11.6 eV for Ar.
Ar *
Cu
This is above ionization
cathode
threshold of metals, e.g. Cu 7.7 eV.
 new avalanches  permanent discharges !
ELASTIC
Solution: addition of polyatomic gas as a quencher
Absorption of photons in a large energy range
(many vibrational and rotational energy levels).
Energy dissipation by collisions with gas
molecules or dissociation into smaller molecules.
IONIZATION
vibrational levels
excitation levels
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Particle Interactions – Detector Design Principles
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
S. Biagi, NIM A421 (1999) 234
SWPC – Operation Modes
•
ionization mode – full charge collection, but no
charge multiplication;
•
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
gain ~ 1
proportional mode – multiplication of ionization
starts; detected signal proportional to original
ionization → 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 → 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
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Particle Interactions – Detector Design Principles
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SWPC – Signal Formation
Avalanche formation within a few
+
Signal induction both on anode and
+
-
+
cathode due to moving charges
-
(both electrons and ions).
dv 
Q dV
dr
lCV0 dr
50 ns
Electrons collected by the anode wire i.e. dr is
baseline
v(t)
very small (few mm). Electrons contribute only
100 ns
very little to detected signal (few %).
300 ns
Ions have to drift back to cathode i.e. dr is large
(few mm). Signal duration limited by total ion drift
time.
Need electronic signal differentiation to limit dead time. 0
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100
200
300
400
Particle Interactions – Detector Design Principles
500
t (ns)
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
wire radii and within t < 1 ns.
Multiwire Proportional Chamber
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
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
x 
d
12
for d = 1 mm x = 300 mm
G. Charpak, F. Sauli and J.C. Santiard
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Particle Interactions – Detector Design Principles
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CSC – Cathode Strip Chamber
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Precise measurement of the second coordinate
by interpolation of the signal induced on pads.
Closely spaced wires makes CSC fast detector.
 = 64 mm
Center of gravity of induced
signal method.
Space resolution
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CMS
Particle Interactions – Detector Design Principles
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RPC – Resistive Plate Chamber
useful gap
HV
resistive electrode
Rate capability strong function of the resistivity
of electrodes.
clusters
gas gap
E
2 mm
A. Akindinov et al., NIM A456(2000)16
GND
resistive electrode
readout strips
 = 77 ps
MRPC
HV
floating
electrodes
GND
Multigap RPC - exceptional time resolution
Time resolution
suited for TOF and trigger applications
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Particle Interactions – Detector Design Principles
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Operation at high E-field  streamer mode.
readout strips
Diffusion of Free Charges
F. Sauli, IEEE Short Course on Radiation Detection and Measurement,
Norfolk (Virginia) November 10-11, 2002
Free ionization charges lose energy in collisions
with gas atoms and molecules (thermalization).
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
They tend towards a Maxwell - Boltzmann
energy distribution:
F ( )    e


kT
Average (thermal) energy:
3
2
 T  kT  0.040eV
Diffusion equation:
ions in air
Fraction of free charges at distance x after time t.
2
x

dN
1
4 Dt

e dt
N
4Dt
D: diffusion coefficient
RMS of linear diffusion:
 x  2 Dt
L.B. Loeb, Basic processes of gaseous electronics
Univ. of California Press, Berkeley, 1961
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Particle Interactions – Detector Design Principles
L2-24
Drift and Diffusion in Presence of E field
A+
E>0 charge transport and
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
v t 0
E=0 thermal diffusion
e-
v t  vD
diffusion
Electric Field
vD 
s
t
Electron swarm drift
Drift velocity
s, t
 x  2 Dt  2 D
s
vD
Diffusion
s, t
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Particle Interactions – Detector Design Principles
L2-25
Simplified Electron Transport Theory
energy balance:
x
vD
vD  a 
x
vD
number of collisions;
eE
  mE 1
m
 = time between collisions  
   E  eEx 3
  
Drift is only possible
if () > 0 !
2
N  v
collision losses = energy gained in E-field
fractional energy loss per collision
 E  12 mv 2 4
 E equilibrium energy (excl. thermal motion)
Insert 2 in 1 and then use 3 and 4 
1
vD2 
v instantaneous velocity
eE
  
mN   2
time between collisions; v


 large  slow gas
 small  fast gas
 and  are both
functions of energy!
 Parameters must be measured
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

B. Schmidt, thesis, unpublished, 1986
Particle Interactions – Detector Design Principles
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Townsend expression:
Drift and Diffusion of Electrons in Gases
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Large range of drift velocity and diffusion:
F. Sauli, IEEE Short Course on Radiation Detection and Measurement, Norfolk (Virginia) November 10-11, 2002
Rule of thumb: vD (electrons) ~ 5 cm/ms = 50 mm / ns. Ions drift ~1000 times slower.
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
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Diffusion Electric Anisotropy
T
Drift
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
L
Transverse diffusion ( µm for 1 cm drift)
Longitudinal diffusion ( µm for 1 cm drift)
E Field
E (V/cm)
E(V/cm)
S. Biagi http://consult.cern.ch/writeup/magboltz/
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Particle Interactions – Detector Design Principles
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Drift Chambers
Spatial information obtained by measuring
Measure arrival time of electrons at sense
wire relative to a time t0.
Need a trigger (bunch crossing or scintillator).
Drift velocity independent from E.
F. Sauli, NIM 156(1978)147
Advantages: smaller number of wires 
less electronics channels.
Resolution determined by diffusion,
primary ionization statistics, path
fluctuations and electronics.
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Particle Interactions – Detector Design Principles
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XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
time of drift of electrons
Drift Chambers
Planar drift chamber designs
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Essential: linear space-time relation; constant E-field; little dpendence of vD on E.
U. Becker in Instrumentation in High Energy Physics, World Scientific
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-30
Equation of motion of free charge carriers in presence of E and B fields:




dv
 
Q
(t ) stochastic force resulting from collisions
m
 eE  e(v  B)  Q(t ) where
dt

m



Q
(
t
)

vD friction force
;
v

v

const
.
Time averaged solutions with assumptions: D


Eˆ  Bˆ

 m
dv

y
 0  eE  e(vD  B)  vD  mean time between collisions
dt


mE

ˆ   ( Eˆ  Bˆ )   2 2 ( Eˆ  Bˆ ) Bˆ
vD 
E
=1
1   2 2


e
eB
mobility
cyclotron frequency

m
m
   
In general drift velocity has 3 components: || E;|| B;|| E  B
m

B=0 → vDB  vD0  mE
 
B
0
E || B → vD  vD
 
E

B
E  B → vD 
B 1   2 2
Ex
vD
B
x
Ez
z
=0


EB
y
Eˆ  Bˆ
E
  1 particles follow E-field
  1 particles follow B-field
vD
L
B
C. Joram CERN – PH/DT
=oo
tan  L  
Lorentz angle
E
x
Particle Interactions – Detector Design Principles
L2-31
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Drift in Presence of E and B Fields
Diffusion Magnetic Anisotropy
T
E
L

v
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
 
E || B
B
D

B
L   0
T 
0
1  2 2
F. Sauli, IEEE Short Course on Radiation Detection and Measurement, Norfolk (Virginia) November 10-11, 2002
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-32
Time Projection Chamber
neg. high voltage plane
full 3D track reconstruction:
particle track
B
Z (e-drift time)
E
liberated e-
x-y from wires and segmented
cathode of MWPC (or GEM)
z from drift time
•
momentum resolution
space resolution + B field
gating plane
cathode plane
anode plane
pads
(multiple scattering)
•
energy resolution
measure of primary ionization
Y
Induced charge on the plane
X
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-33
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
TPC – Time Projection Chamber
TPC – Time Projection Chamber
E
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
E
E E
Alice TPC
HV central electrode at –100 kV
Drift length 250 cm at E = 400 V/cm
Gas Ne-CO2 90-10
Space point resolution ~500 mm
dp/p = 2%@1GeV/c; 10%@10 GeV/c
Events from STAR TPC at RHIC
Au-Au collisions at CM energy of 130 GeV/n
Typically ~2000 tracks/event
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-34
Micropattern Gas Detectors (MPGD)
General advantages of gas detectors:
• low mass (in terms of radiation length)
• flexible geometry
• spatial, energy resolution …
Main limitation:
• rate capability limited by space charge defined by
the time of evacuation of positive ions
MWPC
scale factor
Solution:
1
• reduction of the size of the detecting cell (limitation
MSGC
of the length of the ion path) using chemical
etching techniques developed for microelectronics
5
MGC
and keeping at same time similar field shape.
10
R. Bellazzini et al.
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-35
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
• large areas at low price
Micromegas – Micromesh Gaseous Structure
Metal micromesh mounted above readout
E field similar to parallel plate detector.
Ea/Ei ~ 50 to ensure electron transparency
and positive ion flowback supression.
100 mm
micromesh
 = 70 mm
Ei
Ea
Space resolution
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-36
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
structure (typically strips).
GEM – Gas Electron Multiplier
I+
e-
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Ions
5 µm
50 µm
Induction gap
55 µm
70 µm
ee-
Thin, metal coated polyimide foil perforated
with high density holes.
Electrons are collected on patterned readout board.
A fast signal can be detected on the lower GEM electrode
for triggering or energy discrimination.
All readout electrodes are at ground potential.
Positive ions partially collected on the GEM electrodes.
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-37
GEM – Gas Electron Multiplier
Full decupling of the charge ampification
readout structure.
Cartesian
Both structures can be optimized
Compass, LHCb
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
structure from the charge collection and
independently !
A. Bressan et al, Nucl. Instr. and Meth. A425(1999)254
Small angle
33 cm
Hexaboard, pads
MICE
Compass
Totem
Both detectors use three GEM foils in cascade for amplification
Mixed
to reduce discharge probability by reducing field strenght.
Totem
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-38
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Backup slides
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
4-39
MSGC – Microstrip Gas Chamber
200 mm
Thin metal anodes and cathodes on
Problems:
High discharge probability under exposure
to highly ionizing particles caused by the
regions of very high E field on the border
between conductor and insulator.
Charging up of the insulator and modification
of the E field → time evolution of the gain.
insulating support
IN PRESENCE OF
 PARTICLES
slightly conductive support
Solutions:
R. Bellazzini et al.
• slightly conductive support
• multistage amplification
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-40
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
insulating support (glass, flexible polyimide ..)
Drift and Diffusion of Ions in Presence of E Field
Drift velocity of ions
is almost linear function of E vDion  m ion E
e
m ion 
Mobility:
is
m
constant for given gas at fixed p and T,
Ar
up to very high E fields.
Diffusion of ions
E/p (V/cm/torr)

D
m
ion

kT
e
→
x (mm)
from microscopic picture, it can be shown:
3 De
2 m
 xion 
E. McDaniel and E. Mason
average energy of ion is unchanged
The mobility and diffusion of ions in gases, Wiley 1973
direct consequence of the fact that
Drift velocity of ions
Ne
2kT x
e E
thermal limit
the same for all gases !!
E (V/cm)
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-41
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
He
TPC – Time Projection Chamber
Positive ion backflow modifies electric field resulting in track distortion.
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Solution : gating
Prevents electrons to enter amplification region in case of uninteresting event;
Prevents ions created in avalanches to flow back to drift region.
gate open
gate closed
gating plane
cathode plane
anode wires
readout pads
ALEPH coll., NIM A294(1990)121
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-42
GEM – Gas Electron Multiplier
2x105 Hz/mm2
Rate capability
4.5 ns
5.3 ns
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
9.7 ns
4.8 ns
Time resolution
 = 69.6 µm
Space resolution
C. Joram CERN – PH/DT
Charge corellation (cartesian readout)
Particle Interactions – Detector Design Principles
L2-43
Limitations of Gas Detectors
Classical ageing
Avalanche region → plasma formation
(complicated plasma chemistry)
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
•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).
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-44
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),
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
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
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-45
Computer Simulations
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
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-46
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
MAXWELL (Ansoft)
Computer Simulations
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Input: detector geometry, materials and elctrodes potentials, gas cross sections.
Magboltz
Maxwell
GEM
Field Strenght
Townsend coefficient
P. Cwetanski, http://pcwetans.home.cern.ch/pcwetans/
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-47
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Computer Simulations
Magboltz
Magboltz
Longitudinal, transverse diffusion
Drift velocity
P. Cwetanski, http://pcwetans.home.cern.ch/pcwetans/
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-48
Computer Simulations
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
P. Cwetanski, http://pcwetans.home.cern.ch/pcwetans/
Garfield
Garfield
Micromegas
GEM
Electrons paths and multiplication
Positive ion backflow
Conclusion: we don’t need to built detector to know its performance
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-49
Other (than tracking) Applications
Radiography with GEM (X-rays)
UV light detection with GEM
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
UV transparent Quartz window
200 µm
Trigger from the bottom electrode of GEM.
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-50
ALICE:
TPC (tracker), TRD (transition rad.), TOF (MRPC), HMPID (RICH-pad chamber),
Muon tracking (pad chamber), Muon trigger (RPC)
ATLAS:
TRD (straw tubes), MDT (muon drift tubes), Muon trigger (RPC, thin gap chambers)
CMS:
Muon detector (drift tubes, CSC), RPC (muon trigger)
LHCb:
Tracker (straw tubes), Muon detector (MWPC, GEM)
TOTEM: Tracker & trigger (CSC , GEM)
C. Joram CERN – PH/DT
Particle Interactions – Detector Design Principles
L2-51
XII ICFA School on Instrumentation in Elementary Particle Physics, Bogota, 2013
Gas Detectors in LHC Experiments