particle detectors for colliders ionization & tracking detectors robert s. orr university of...
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Particle Detectors for Colliders
Ionization & Tracking Detectors
Robert S. Orr
University of Toronto
R.S. Orr 2009 TRIUMF Summer Institute
Layers of Detector Systems around Collision Point
Generic Detector
Tracking Detectors
• Observe particle trajectories in space with as little disturbance as possible
• use a thin ( ) detector– Scintillators – Scintillating fibres– Gas trackers– Solid state trackers
2.gm cm
cm 150 150
10
• Gas Based Detectors– Multiwire proportional chamber
– Drift Chamber
– Time projection chamber
– Gas microstrip
– GEM (gas electron multiplier)
R.S. Orr 2009 TRIUMF Summer Institute
Generic Detector
small – amplification?
R.S. Orr 2009 TRIUMF Summer Institute
Multiwire Proportional Chamber
wire spacing = resolution
cathode
cathode
anode wires
Drift Chamber – measure arrival time of charge = spatial resolution
R.S. Orr 2009 TRIUMF Summer Institute
Schematic of Wire Chamber Cell
anode wire
field shaping cathode• wire mesh• pc board
collects signalenvelope to contain gas
gas
should not absorb electrons
Repeat “n” times
R.S. Orr 2009 TRIUMF Summer Institute
3 stages in signal generation
1) Ionization by track passing through cell
2) Ionization drifts in E field
time
3) In high E field region near wire, primary ionization electrons gain enough energy to start ionizing the gas
- Avalanche- More charges- Charge amplification- Noise free amplifier
7~ 10 microvolt signal if no amplification
R.S. Orr 2009 TRIUMF Summer Institute
Gas Amplification
R.S. Orr 2009 TRIUMF Summer Institute
Behaviour as Voltage Increased
• Collection – Recombination dominated
• All charge collected
• Amplification by gas multiplication
• Still proportional – particle ident
• Saturation
• Breakdown – Geiger/Mueller
Volts
R.S. Orr 2009 TRIUMF Summer Institute
Diffusion
• Ions & electrons diffuse in space• E field determines average direction
• Collisions limit velocity• Maximum average velocity =Drift velocity
• Ions and electrons diffuse under influence of electric field– Maxwell velocity distribution
• From Kinetic theory , after t, linear distribution due to diffusion
Diffusion
8kTv
m
6 1 4 110 . 10 .e Iv cm s v cm s
20 exp
44
NdN x
dx DtDt
2x Dt
6r Dt
number of particles
Diffusion coefficient
RMS Spread2-d
3-d
about 1mm after 1 sec in air
• For a classical gas
ion charge and mass
p gas pressure
ion scattering cross section
• In argon
• Electrons collected quickly compared to +ve ions
Mobility
drift velocity
electric field0
2
3
q kT u
p m E
,q m
0
40e
m ns
kV cm
0.1I
m ns
kV cm
Diffusion and Drift Chamber Accuracy
13
D v
0
1
2
kT
p
210D nse
Diffusion coefficient from kinetic theory
Mean free path
3
0
2 1
3
kTD
p m
In argon
Diffusion gives limit on spatial accuracy drift chamber
• To reduce D• Lower temperature• Raise pressure (reduce mobility)
Working Gas
• Noble gases give multiplication at lowest electric field– Polyatomic gases have non-
ionization energy loss mechanisms
• Choose cheap noble gas with low ionization potential
– Krypton X
– Xenon X
– Argon OK
• Cheap, safe, non-reactive– remove electro-negative
contaminants
• Pure argon limited to gain• Many excited ions produced during
avalanche
• Absorb - quenchers
rare, expensive
cheap – welding etc
Argon
2 2 2, ,O CO H O
310
* 11.6Ar Ar eV
absorbed on cathode
cathode e photo emission
returns to anode - breakdown
Quenchers Polymerization
* 11.6Ar Ar eV
*X X Absorb
e.g. MethanePoly-atomic gas
Rotationalvibrational modes
Typical gases 4
3 8
80% 20%
90% 10%
Ar CH
Ar C H
610G
or add electronegative gas (a bit of poison)
X photo electron X
Typical 290% 10%Ar CO 710G
non-radiative
• Organic quenchers polymerize
• Deposits on cathodes
• high resistance• ion buildup – discharge• sparks, broken wires
• Add non-polymerizing agent – water methylal
Magic Gas
3 32
2
75%
24.5%
0.5%
1%
Ar
CH CH CH
Freon
tracemethylal
H O
R.S. Orr 2009 TRIUMF Summer Institute
Gas Admixtures
R.S. Orr 2009 TRIUMF Summer Institute
Signal from Gas Counter
CathodeAnode
charge q moved by dr
0V
0
( )Q d rdV dr
lCV dr
length of countercapacitance/unit length
potential
• Electrons produced in avalanche close to anode wire
• Small dr – small signal
• +ve ions drift across whole radius• Large dr – large signal
( )W Q r20
1
2W lCV
( )d rdW Q dr
dr
0dW lCV dV
0
0
( ) ln2
CV rr
a
0
( )d rlCV dV Q dr
dr
0
( )Q d rdV dr
lCV dr
0 0
ln2
a
electron
a
d rQ Q aV dr
lCV dr l a
0 0
ln2
b
ion
a
d rQ Q bV dr
lCV dr l a
ln lnelectron ion
a bV V
a a
Typically 1%
potential energy of qelectrostatic energy of field
R.S. Orr 2009 TRIUMF Summer Institute
Time Development of Signal
• Assume • All signal comes from ions• Start from a
( ) ( )
00
( )r t r tt
a a
Vd rdV Q
dV dr drdr lC
tV dr
0 0
0
0
l2
nn l2
r
a
CVQ r
lCV a
r tQ
l a
0
0
1
2
CVE
r
dr
dt
0
0
2
0
0
0
2
r t
a
CVr t
CVr r dt
t
d
a
02
0 0 0 0
ln 1 ln 14 4
CVQ QtV t
a t
t
Typically get 50% of signal in T ~700ns310
RC differentiation for fast signal
R.S. Orr 2009 TRIUMF Summer Institute
Different Realizations of Ionization Trackers
MWPC
Time Projection Chamber
Jet Chamber
Drift Chamber
R.S. Orr 2009 TRIUMF Summer Institute
Drift Chamber Cell
potential shaping wiressense wire
• Carefully shape potential (field lines)• Optimize drift time – space relation
drift
tim
e
R.S. Orr 2009 TRIUMF Summer Institute
Left-Right Ambiguity Resolution
2 anode wires staggered anode wires
ghost track
inclined anode plane
good for high magnetic field
R.S. Orr 2009 TRIUMF Summer Institute
Jet Chamber
annihilation at 30 GeVe e
Lorentz Angle – Drift Chamber in Magnetic Field
• Drifting electron will see
Electric Field
Magnetic Field
E
B
mv q E v B
• Will also see stochastic force due to collisions with gas molecules
mv q E v B mA t
• Assume over time
,E B acceleration
stochasticretardation
constant Dv
0D D
qE qBv v
m mA t
DvA t
mean time between collisions
DD
v qB qEv
m m
solution:
2 22 2 21D
E B BE Bv E
B B
(1) (2) (3)
q
m
qB
m
electron mobility
cyclotron frequency
Lorentz Angle – Drift Chamber in Magnetic Field
• Drift velocity has three components
solution:
2 22 2 21D
E B BE Bv E
B B
(1) (2) (3)
(1) parallel to
(2) parallel to
(3) perp to plane of
EB
,E B
• If perpendicular,E B
,0,0
0,0,
x
z
E E
B B
2 2
2 2
1
1
10
x x
y x
z
v E
v E
v
tan y
x
v
v
tan DqB mB
m q
vB
E
R.S. Orr 2009 TRIUMF Summer Institute
tan DvB
E
0 1kV 2kV 3kV
1kV 2kV 3kV
BV
0BV
equipotential
sense wiresfield wires
next cell
tan DvB
E
Compensate for Lorentz angle bytilting electric field in drift cells
R.S. Orr 2009 TRIUMF Summer Institute
Structure of ZEUS DC
• Total wire tension 12 tons• 4608 W sense wires (30 micron)• 19584 CuBe field wires
• 120 micron space resolution• 2.5mm 2 track resolution• 500 ns max drift time
R.S. Orr 2009 TRIUMF Summer Institute
Tilted E Field – R-L ambiguity resolution
real track segments
reflected ghost segments
R.S. Orr 2009 TRIUMF Summer Institute
Spatial Resolution
• Small number of primary electrons reach sense wire
• Statistics
• Variation in drift time – space relation
• Smearing
R.S. Orr 2009 TRIUMF Summer Institute
Stereo Wires – 3-d Reconstruction
stereo wires
stereo cameras – 3-d pictures
paraxial wires
r,x
r’,x’
r,x
r’,x’
R.S. Orr 2009 TRIUMF Summer Institute
R.S. Orr 2009 TRIUMF Summer Institute
R.S. Orr 2009 TRIUMF Summer Institute
R.S. Orr 2009 TRIUMF Summer Institute
Layers of Detector Systems around Collision Point
R.S. Orr 2009 TRIUMF Summer Institute
Square Drift Cells - ARGUS
• Precision• High Density of Information• Pattern recognition complex R-L ambiguity resolved by
trying all possible combinations
isochrones
R.S. Orr 2009 TRIUMF Summer Institute
ARGUS Events
R.S. Orr 2009 TRIUMF Summer Institute
dE/dx Particle Identification
BaBar Drift Chamber
constructed at TRIUMF
R.S. Orr 2009 TRIUMF Summer Institute
Time Projection Chamber
• Only two drift cells• parallel to , so no Lorentz angleE B
• measure z, from drift time
• measure r,Φ from pads and wires on endplates
, 180r
200z
• Good pattern recognition and precision in medium multiplicity environment
space charge limitation
R.S. Orr 2009 TRIUMF Summer Institute
wire – drift time
pad – position on the wire
R.S. Orr 2009 TRIUMF Summer Institute
Diffusion in TPC
transverse diffusion
Diffusion limits spatial resolution
2
3 D
Lu
v
drift length
mean electron velocity
mean free path
Why does diffusion not ruin resolution?
R.S. Orr 2009 TRIUMF Summer Institute
Diffusion in TPC
Compare this to previous plot with B=0
reduces diffusion if0B 0E B
particles drift along tight helices
transverse diffusion reduced by
2 2
1;
1
eB
m
R.S. Orr 2009 TRIUMF Summer Institute
ATLAS Tracker
0s
0d K J/B
33 2 13.0 10 cm s
)GeV 130( eeeeZZH H m
34 2 110 cm s
R.S. Orr 2009 TRIUMF Summer Institute
ATLAS Straw Tracker
Straws
Radiator
Radiator
Straws
End-c
apEn
d-cap
straw
R.S. Orr 2009 TRIUMF Summer Institute
Straw tracker test beam module
R.S. Orr 2009 TRIUMF Summer Institute
Assembly of straw tracker
Inner Detector (ID)The Inner Detector (ID) comprises four sub-systems:
•Pixels (0.8 108 channels)
•Silicon Tracker (SCT)(6 106 channels)
•Transition Radiation Tracker (TRT)(4 105 channels)
•Common ID items