Transcript Drift chambers - Indico

Multi wire proportional chambers
Multi wire proportional chamber (MWPC)
(G. Charpak et al. 1968, Nobel prize 1992)
field lines and equipotentials around anode wires
Capacitive coupling of non-screened parallel wires?
Negative signals on all wires? Compensated by
positive signal induction from ion avalanche.
Typical parameters:
L=5mm, d=1mm,
awire=20mm.
Normally digital readout:
d


spatial resolution limited to
x
12
( d=1mm,
x=300 mm )
Address of fired wire(s) give only 1-dimensional
information. Secondary coordinate ….
CERN Summer Student Lectures 2003
Particle Detectors
Christian Joram
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Multi wire proportional chambers
Secondary coordinate


Crossed wire planes. Ghost hits.
Restricted to low multiplicities. Also
stereo planes (crossing under small
angle).
Charge division. Resistive wires (Carbon,2k/m).
y
track
QB
QA
y
QB

L QA  QB
 y
 L
   up to 0.4%
L

Timing difference (DELPHI Outer detector, OPAL
vertex detector)
 (T )  100 ps
  ( y)  4cm
L
y
CFD

track
T
(OPAL )
CFD
1 wire plane
+ 2 segmented
cathode planes
Analog readout of
cathode planes.
   100 mm
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Derivatives of proportional chambers
Some ‘derivatives’

Thin gap chambers (TGC)
cathode pads
ground plane
graphite
3.2 mm
G10 (support)
50 mm
4kV
2 mm
Gas:
CO2/n-pentane
( 50/50)
Operation in saturated mode. Signal amplitude
limited by by the resistivity of the graphite layer
( 40k/).
Fast (2 ns risetime), large signals (gain 106), robust
Application: OPAL pole tip hadron calorimeter.
G. Mikenberg, NIM A 265 (1988) 223
ATLAS muon endcap trigger, Y.Arai et al. NIM A 367 (1995) 398
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Christian Joram
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Derivatives of proportional chambers
Resistive plate chambers (RPC)

No wires !
spacer
2 mm
10 kV
bakelite
(melamine
phenolic laminate)
pickup strips
Gas: C2F4H2, (C2F5H) + few % isobutane
(ATLAS, A. Di Ciaccio, NIM A 384 (1996) 222)
Time dispersion  1..2 ns  suited as trigger chamber
Rate capability  1 kHz / cm2
Double and
multigap
geometries 
improve timing
and efficiency
15 kV
Problem: Operation close to streamer mode.
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Particle Detectors
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Drift chambers
Drift chambers
(First studies: T. Bressani, G. Charpak, D. Rahm, C. Zupancic, 1969
First operation drift chamber: A.H. Walenta, J. Heintze, B. Schürlein, NIM 92 (1971) 373)
DELAY
Stop
TDC
Start
scintillator
x
drift
low field region
drift
anode
Measure arrival time of
electrons at sense wire
relative to a time t0.
high field region
gas amplification
x   v D (t ) dt
What happens during the drift towards the anode wire ?


Diffusion ?
Drift velocity ?
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Drift and diffusion in gases
Drift and diffusion in gases
No external fields:
Electrons and ions will lose their energy due to collisions with
the gas atoms  thermalization
3
2
Undergoing multiple collisions, an originally localized
ensemble of charges will diffuse
  kT  40 meV
2
dN
1

e ( x
N
4Dt
 x (t )  2 Dt
4 Dt )
D: diffusion coefficient
dx
or D 
 x2 (t )
2t
dN


x
t
External electric field:
“stop and go” traffic due to
scattering from gas atoms
 drift


vD  mE
m

e-
e
(mobility)
m
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Drift and diffusion in gases

in the equilibrium ...
x
vD
   eEx
 : fractional energy loss / collision
1
v: instantaneous velocity

Nv
2
vD

eE
mN
e-

2
  () !
  () !
 [eV]
(B. Schmidt, thesis, unpublished, 1986)
 [eV]
Typical electron drift velocity: 5 cm/ms
Ion drift velocities: ca. 1000 times smaller
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Drift and diffusion in gases
In the presence of electric and magnetic fields,
 
drift and diffusion are driven by E  B effects
Look at 2 special cases:
Special case:
 
EB
y
L:
Lorentz angle
B

eB
cyclotron frequency

m
Special case:
 
vD || E
vD
tan  L  
L
E
x
Transverse diffusion  (mm) for a
drift of 15 cm in different
Ar/CH4 mixtures
 
E || B
(A. Clark et al.,
PEP-4 proposal, 1976)
The longitudinal diffusion (along
B-field) is unchanged.
In the transverse projection the
electrons are forced on circle
segments with the radius vT/.
The transverse diffusion coefficient
appears reduced
D0
DT ( B) 
1   2 2
Very useful… see later !
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Particle Detectors
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Drift chambers
Some planar drift chamber designs
Optimize geometry  constant E-field
Choose drift gases with little dependence vD(E)
 linear space - time relation r(t)
(U. Becker, in: Instrumentation in High Energy Physics, World Scientific)
The spatial resolution is not limited by the cell size
 less wires, less electronics,
less support structure than in MWPC.
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Drift chambers
Resolution determined by
• diffusion,
• path fluctuations,
• electronics
• primary ionization
statistics
(N. Filatova et al., NIM 143 (1977) 17)
Various geometries
of cylindrical drift
chambers
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Drift Chambers
Time Projection Chamber  full 3-D track reconstruction



x-y from wires and segmented cathode of MWPC
z from drift time
in addition dE/dx information
PEP-4 TPC
Diffusion significantly
reduced by B-field.
Requires precise
knowledge of vD 
LASER calibration +
p,T corrections
Drift over long distances  very good gas quality required
Space charge problem from positive ions, drifting back to
midwall  gating
ALEPH TPC
Gate open
Gate closed
(ALEPH coll., NIM A 294 (1990) 121,
W. Atwood et. Al, NIM A 306 (1991) 446)
Ø 3.6M, L=4.4 m
Rf  173 mm
z  740 mm
(isolated leptons)
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Vg = 150 V
Christian Joram
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Micro gaseous detectors
Faster and more precision ?  smaller structures

Microstrip gas chambers
(A. Oed, NIM A 263 (1988) 352)
drift electrode (ca. -3.5 kV)
geometry and typical dimensions
(former CMS standard)
Gold strips
+ Cr underlayer
C (-700V)
10 mm
100 mm
A
substrate
300 mm
80 mm
3 mm
gas volume
backplane
Glass DESAG AF45 + S8900
semiconducting glass coating,
r=1016 /
Field geometry
ions
A
C
Fast ion evacuation  high rate capability
 106 /(mm2s)
Gas: Ar-DME, Ne-DME (1:2), Lorentz angle 14º at 4T.
Gain 104
CMS
Passivation: non-conductive protection of cathode edges
Resolution:  30..40 mm
Aging: Seems to be under control.
10 years LHC operation  100 mC/cm
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Micro gaseous detectors
 GEM: The Gas Electron Multiplier
(R. Bouclier et al., NIM A 396 (1997) 50)
140  00 mm
0  10 mm
0 mm Kapton
+ 2 x 5-18 mm Copper
Micro photo of a GEM foil
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Micro gaseous detectors
Single GEM
+ readout pads
Double GEM
+ readout pads
 Same gain
at lower voltage
 Less discharges
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Silicon detectors
Silicon detectors
Solid state detectors have a long tradition for energy
measurements (Si, Ge, Ge(Li)).
Si sensor
Here we are interested in
their use as precision trackers !
ATLAS
SCT
Some characteristic numbers for silicon
 Band gap: Eg =1.12 V.
 E(e--hole pair) = 3.6 eV, ( 30 eV for gas detectors).
 High specific density (2.33 g/cm3)  E/track length for
M.I.P.’s.: 390 eV/mm  108 e-h/ mm (average)
 High mobility: me =1450 cm2/Vs, mh = 450 cm2/Vs
 Detector production by microelectronic techniques  small
dimensions  fast charge collection (<10 ns).
 Rigidity of silicon allows thin self supporting structures.
Typical thickness 300 mm   3.2 104 e-h (average)
 But: No charge multiplication mechanism!
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Particle Detectors
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Silicon detectors
How to obtain a signal ?
E
conductance band
e
In a pure intrinsic
(undoped) material the
electron density n and
hole density p are
equal. n = p = ni
Ef
h
valence band
For Silicon: ni  1.451010 cm-3
In this volume
we have 4.5 108 free charge
carriers, but only 3.2 104 e-h
pairs produced by a M.I.P.
300 mm
1 cm
1 cm
 Reduce number of free charge carriers,
i.e. deplete the detector
Most detectors make use of reverse biased p-n junctions
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Silicon detectors
Doping
E
E
E
CB
e
CB
f
Ef
VB
h
VB
n-type: Add elements from p-type: Add elements from IIIrd
group, acceptors, e.g. B.
Vth group, donors, e.g. As.
Holes are the majority carriers.
Electrons are the majority
carriers.
detector grade
electronics grade
doping concentration
resistivity
E
p
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 5 k·cm
1 ·cm
pn junction
e.V
VB
1017(18) cm-3
n
CB
Ef
1012 cm-3 (n) 1015 cm-3 (p+)
There must be a single
Fermi level !
Deformation of band
structure  potential
difference.
Christian Joram
II/17
Silicon detectors
diffusion of e- into pzone, h+ into n-zone
 potential difference
stopping diffusion
thin depletion zone
no free charge carriers
in depletion zone
(A. Peisert, Instrumentation In High Energy
Physics, World Scientific)
• Application of a reverse bias voltage (about 100V)  the thin
depletion zone gets extended over the full junction  fully
depleted detector.
• Energy deposition in the depleted zone, due to traversing
charged particles or photons (X-rays), creates free e--hole
pairs.
• Under the influence of the E-field, the electrons drift towards
the n-side, the holes towards the p-side  detectable current.
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Particle Detectors
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Silicon detectors
Spatial information by segmenting
the p doped layer 
single sided microstrip detector.
Schematically !
ca. 50-150 mm
readout capacitances
SiO2
passivation
300mm
(A. Peisert, Instrumentation
In High Energy Physics,
World Scientific)
defines end of depletion zone
+ good ohmic contact
ALICE: Single sided
micro strip prototype
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Silicon detectors

Silicon pixel detectors



Segment silicon to diode matrix
also readout electronic with same geometry
connection by bump bonding techniques
Flip-chip technique
detector
electronics
bump bonds
RD 19, E. Heijne et al., NIM A 384 (1994) 399



Requires sophisticated readout architecture
First experiment WA94 (1991), WA97
OMEGA 3 / LHC1 chip (2048 pixels, 50x500 mm2) (CERN
ECP/96-03)

Pixel detectors will be used also in LHC experiments
(ATLAS, ALICE, CMS)
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Silicon Detectors

The DELPHI micro vertex detector (since 1996)
50 mm Rf
50-150 mm z
200 mm SS
50 mm Rf
44-176 mm z
50 mm Rf
50-100 mm z
330 x 330 mm2
readout channels
ca. 174 k strips, 1.2 M pixels
total readout time: 1.6 ms
Total dissipated power 400 W
 water cooling system
Hit resolution in barrel
part  10 mm
Impact parameter
resolution (rf)
3
28mm  71 /  p sin 2  


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