PDF_1 - Infn

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PDF_1 - Infn

Dottorato in Fisica
Maggio 2005
“Fisica dei rivelatori”
Identificazione delle particelle
• Basic definitions and introductory remarks
• Ionization energy loss
• Time of Flight
• Cherenkov radiation
• Transition radiation
Advised textbooks:
R. Fernow, Introduction to Experimental Particle Physics, Cambridge University Press
R.S. Gilmore, Single particle detection and measurement, Taylor&Francis, 1992
G. F. Knoll, Radiation Detection and Measurement, John Wiley and Sons, New York
W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer, 1994
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Complete event analysis (based on the reconstruction of conservation
laws):
4-momenta of secondary particles
(p,E)
Deflection in a
magnetic field
(+ sign of particle’s charge)
Calorimetry
(destructive measurement,
effective for neutral particles only)
PID measurement
2
E = m c + p c2
2 4
“m” uniquely identifies the internal quantum numbers of the particle
Example:
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Very useful for neutral particles and leptons because of their peculiar interactions
with media: electron quickly produces an em shower, µ travels through the entire detector
Hadronic showers from π, K, p all look alike and calorimeter energy resolution is not
enough to allow measuring mass from m2=E2-p2
example: p=2 GeV/c, Eπ= 2.005 GeV, EK= 2.060 GeV
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Hadronic shower
Various complex processes involved:
hadronic and electromagnetic
components
charged pions, protons, kaons ….
The lateral spread of the shower is
mainly governed by the multiple
scattering of the electrons (Moliere
radius RM ).
95 % of the shower is contained inside
a cone of size 2RM
neutral pions → 2γ →
Breaking up of nuclei
electromagnetic cascade
(binding energy),
n π 0 ≈ ln E(GeV) − 4.6
( )
neutrons, neutrinos, soft γ’s
muons …. → invisible energy
Hadronic showers are much longer and
broader than electromagnetic ones !
Large energy fluctuations → limited energy resolution
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Identification method: calculate the invariant mass with all
possible daughter candidates

1
2
M = invariant mass =
Decay vertex may
be reconstructed if
it is far from
interaction
point
and daughters are
charged
Combinatorial
background is
often critical
PID mandatory
No PID
two Ks identified
c
2
(∑ E )
i
i
−



∑j p jc 

2
one K identified
φ
Κ+Κ−
mφ=1020 MeV/c2
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Branching ratios: Bd→π+π− = 0.7×10−5, →K± πm = 1.5×10−5
Bs→K+K− = 1.5×10−5, →K± πm = 0.7×10−5
PID
LHCb
Purity=13%
PID
Purity=84%
Efficiency=79%
LHCb
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Bs → Ds K
Major background: Bs → Ds π (No CP violation)
PID
LHCb
PID
LHCb
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70’s: Hydrogen bubble
chamber
1978: BEBC
A
A Look
Look at
at the
the Past
Past
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Silicon trackers +TPC (PID with energy loss)
Ring Imaging Cherenkov detector
A
A “Modern”
“Modern”
Approach
Approach to
to PID
PID
ALICE at
LHC
TOF
TRD
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magnetic field
|p|,charge
RICH
TOF and TRD
e.m. calorimeter
Basic Layout
Layers of silicon detectors with excellent
position (0(10 µm)) and double track
(0(100 µm)) resolution near the primary
collision region
•detection of secondary vertices
(short-lived strange and heavy flavor
particles)
• impact parameter resolution σ(rφ) ~ 50 µm
for pt ~ 1 GeV/c
• primary vertex resolution: ~ 10 µm
• momentum resolution improvement
• PID with energy loss
TPC, away from the interaction region, at
more moderate particle densities
• tracking (δp/p at the level of 1% for low
momenta)
• PID with energy loss
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Measuring
Measuring the
the Particle
Particle Velocity
Velocity
p = mcγβ ; γ =
2
E
(Lorentz factor)
2
mc
 dm   2 dβ 


 =  γ
β 
 m  
2
 dp 
+  
 p
2


 ∆β (m12 − m22 )c 2
≅

2 p2
2
2
2 ∆β (β1 + β 2 )  β
m1 − m2 = p 2
2
c (β1 ∗ β 2 ) 
dp
≅0
p
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PID techniques are based on the detection of particles via their interaction with matter:
ionization and excitation (Cherenkov light & Transition Radiation)
The applicable methods depend strongly on the
particle momentum (velocity) domain of interest
π-K identification ranges
TR+dE/dx
electron identification
Cherenkov
dE/dx
TOF
150 ps
FWHM
0,1
1
10
p (GeV/c)
100
1000
Particle Identification Techniques
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(example: ALICE-ITS simulation)
efficiency = ε A→ A = N A− identified N A− total
contamination = ε B → A =
∑N
B,B≠ A
higher efficiency -> larger contamination
B
A− identified
/ N A− identified
purity= 1-contamination
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3σ separation for π/K
10
ToF (100ps@FWHM)TR+dE/dx
dE/dx
1
d
-Soli
d
i
u
Liq
10-1
ses
RICH
Ga
Detector length (m)
el
g
ro
Ae
10-2
10-1
1
101
102
Momentum (GeV/c)
103
N.B. in case of samples with different population:
S − SB
at a given separation power, the resulting contamination
nσ = separation power = A
of the largest populated sample of particles in the other
σ AB
species will be larger by a factor equal to the ratio between the relative populations
Separation Power
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Energy
Energy Loss
Loss Mechanisms
Mechanisms
δ-ray
Basic processes occurring when a
charged particle traverses a medium
being surrounded by a cloud of virtual photons
that interacts with atoms in the medium
• ionization and excitation of the atoms
of the medium (secondarily produced
electrons could further ionize the medium)
• radiative phenomena
¾ Cherenkov radiation
¾ Transition radiation
Overall effect: the particle loses energy
Detection of the energy lost is the physical
basis of many of the techniques used in
charged particle detectors
Ionization trail: particle’s trajectory and velocity information
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Modern approach (unitary description in terms of matter properties): Allison and Cobb (1980)
• Charged particle moves in a dielectric medium through which virtual photons propagate
• The particle loses energy by doing work against the field created by the medium polarization
particle
time
p, m 0
virtual photon
hω , hk
fast
charged
particle
θ
e-
B
A
atom
atom
(photon)
∞
∞
dE
d 2σ
= −n ∫ dE ∫ dpE
0
ω/ v
dx
dEdp
p = hk
E = hω
density of atoms: n=ρNA/A
rel. probability
space
Schematically !
ionization by
distant collision
excitation
below excitation
threshold
(after Gilmore)
ionization by close collisions
δ-electrons
photons are virtual, their energy and momentum
are independent E≠pc
• the integral must be performed over both energy
and momentum separately
• virtual photon behaviour approximated with a
combination of cross sections for the interactions
of real photons allowing to perform the momentum
integration for virtual photons
energy transfer/photon exchange
Charged
Charged Particle-Matter
Particle-Matter Interactions
Interactions
∞
dE
dσ
= −n∫ E
dE
0
dx
dE
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Classical approach : Bethe-Bloch equation modified to include the Fermi effect
Average specific energy loss:
dE
Z
z 2  1 2me c 2 β 2γ 2 Emax β 2 δ C 
= −0 .3071 ρ ⋅ t 2  ln
−
− − 
2
β 2
I
dx
A
2 2 Z
Z=atomic number of the medium;
I~Z•12 eV=effective ionization potential;
Emax=max energy transfer (I ≤ dE ≤ Emax)
Valid only for particles with m>me
•
•
•
dE/dx does not depend on m but on the charge z
Non relativistic region: dE/dx ∝1/β2
(more precisely as β-5/3)
Minimum: at βγ = 3÷4 (Minimum Ionizing Particle)
•
•
At high βγ: dE/dx ∝lnγ2 (relativistic rise)
Density effect: δ(βγ)
dE
mip ≈ 1 ÷ 2 MeV g −1cm 2
dx
(medium polarization reduces long range effects)
230 Ar
¾ saturates at βγsat:
Ionization
Ionization Energy
Energy Loss
Loss
68.4
55.3
42.4
5.6
CH4
C2H6
C4H10
Si
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Maggio 2005
p = m βγ
(
(dE/dx)/(dE/dx)min
dE
1
∝ 2 ln β 2γ 2
dx β
)
m from simultaneous measurement of p and dE/dx
nσ =
dE A / dx − dE B / dx
σ (dE B / dx )
Fermi plateau is a few percents
above the minimum in solid and
liquid media, 50-70% in high Z
noble gases at STP -> PID in the
relativistic rise region only possible
in gases!
π/K separation (2σ) requires a
dE/dx resolution of few percents
:”cross-over” regions (as wide as + 100 MeV/c)
ambiguites-> complementary PID mandatory
Average energy loss in
80/20 Ar/CH4 (NTP)
(J.N. Marx, Physics today, Oct.78)
Particle ID using the specific energy loss dE/dx
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Fluctuations in the energy loss dE/dx
<dE/dx> is practically measured by evaluating ∆E in a short interval δx
this is not necessarily the average energy lost in the given slice of material-> the
distribution shows large fluctuations and Landau tail
Most interactions involve little energy exchange -> the total energy
loss from these interactions is a Gaussian (central limit theorem).
Few interactions involve large energy exchange-> Landau tail
Because of the high energy tail, increasing the thickness
of the detector or choosing high Z material does not
improve σ(dE/dx). Indeed the relative width of Gaussian
peak reduces but probability of high energy interaction
rises -> tail gets bigger
L: most likely energy loss
A: average energy loss
1 wire
4 wires
(B. Adeva et al., NIM A 290 (1990) 115)
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Improve dE/dx resolution and fight Landau tails
Thick absorber: large chance of high energy δ ray production cancels the reduction
of fluctuations -> (dE/dx)A – (dE/dx)B < Landau fluctuations
Usual method of measuring dE/dx is:
•
•
•
•
•
Choose material with high specific ionization
Divide detector length L in N gaps of thickness T.
Sample dE/dx N times
Calculate truncated mean, i.e. ignore samples with (e.g. 40%) highest values
Also pressure increase can improve resolution. Drawback: reduced relativistic
rise due to density effect !
Samples must not be too many:
for each total detector length L, there exists
an optimal N
Rule of thumb: at least N=100 for a total track
length of 3-5 m/atm
(M. Aderholz, NIM A 118 (1974), 419)
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dE/dx & Separation Power
Particle Separation
N S . D . ( A; B ) =
dE / dx ( A) − dE / dx ( B )
σ (dE / dx )
– dE/dx resolution (A.H. Walenta et al. Nucl. Instr. and Meth. 161 (1979) 45)
σ (dE / dx ) ∝ n −0.43÷ −0.47 ( t ⋅ p ) −0.32÷ −0.36
n: number of sampling layers,
t: thickness of the sampling layer (cm)
p: pressure of the gas (atm)
Remarks:
• σ does not follow the n-0.5 dependence owing to the Landau
fluctuations;
• if the total lever arm (nt) is fixed, it is better to increase n;
so long as the number of produced ion-pairs is enough in each layer.
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TPC: basic principle
Time Projection Chamber → full 3-D track reconstruction
• x-y from wires and segmented cathode of MWPC
• z from drift time -> precise knowledge of vD (LASER calibration + p,T corrections)
• dE/dx
Drift over long distances → very good
gas quality required
Gate open
Gate closed
∆Vg = 150 V
Space charge problem from positive ions,
drifting back to medial membrane → gating
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Tracker evolution
80’s: 6.4 TeV Sulphur - Gold event (NA35)
2000: STAR
STREAMER CHAMBER
TPC
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STAR TPC
420 cm
•
•
Gas: P10 ( Ar-CH4 90%-10% ) @ 1 atm, 50,000 Liters
Voltage : - 31 kV at the central membrane
148 V/cm over 210 cm drift path
Self supporting Inner Field Cage:
Al on Kapton using Nomex
honeycomb; 0.5% rad length
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STAR TPC
A Central Event
Typically 1000 to 2000 tracks
per event into the TPC
Two-track separation 2.5 cm
Momentum Resolution < 2%
Space point resolution ~ 500 µm
Rapidity coverage –1.5 < η < 1.5
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PID via dE/dx with the STAR TPC
dE/dx PID range:
dE/dx (keV/cm)
12
8
4
K
p
d
~ 0.7 GeV/c for K/π
~ 1.0 GeV/c for K/p
π
µ
e
Anti - 3He
0
Gas: P10 ( Ar-CH4 90%-10% ) @ 1 atm
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Pb+Pb @ 158 GeV/nucleon
NA49 TPCs
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gas volume
ALICE TPC
88 m3
drift gas
90% Ne - 10%CO2
m
560 c
6x105 channels, corresponding
to 6x108 pixels in space
Field Cage Inner Vessel
Central membrane frame
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Field Strips
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TPC Assembly
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Challenge: Track Density in Pb-Pb
Projection of the entire drift volume
into the pad plane; dNch/dy = 8000
Projection of a slice (2o in θ)
(~ 2 x 104 charged particle tracks)
Nr. of Pixels:
570,132 pads x 500 time bins
Nr. of hits = 19,431,047
slice: 2o in θ
Nch(-0.5<η<0.5) = 8000
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TPC
TPC dE/dx
dE/dx performance
performance
At dN/dy = 8000
σ dE/dx = 10 %
At dN/dy = 4000
σ dE/dx = 7 %
σ dE/dx =10%
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mechanical tolerances (gain and electrical field)
stability of high voltage power (gain)
space charge effects (track distortion)
gating efficiency (background)
temperature, pressure (drift velocity)
Drift velocity (cm/µs)
•
•
•
•
•
5.45
examples
from STAR
5.44
1010
1020
Pressure (mbar)
Drift Velocity Control:
• Lasers for coarse value
• Fine adjustment from tracking
TPC: experimental issues
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σ (dE / dx)calc = 0.41n
−0.43
( t ⋅ p)
−0.32
dE/dx Detector Performance
Belle
Type
n
Drift ch. 52
t (cm) p (bar) Gas
He/C2H6=50/50
1.5
1
Calc.(%) Meas.(%)
5.1
6.6
Babar
Drift ch. 40
1.4
1
He/C4H10=80/20
7.5
7.2
CLEO II
Drift ch. 51
1.4
1
Ar/C2H6=50/50
6.4
5.7
ALEPH*
TPC
338 0.4
1
Ar/CH4=90/ 10
4.6
4.5
PEP*
TPC
183 0.4
8.5
Ar/CH4=80/ 20
2.8
3.0
OPAL*
Jet ch.
159 1.0
4
Ar/CH4 /iC4H10 =88.2/9.8/2
3.0
2.8
1
Ar/CO2 /CH4 =89/10/1
6.9
7.0
MKII/SLC* Drift ch. 72
0.83
* Data from M. Hauschild (NIM A 379(1996) 436)
• Higher pressure gives better resolution, however, the relativistic rise saturate at
lower βγ.
4 – 5 bar seems to be the optimal pressure
• Higher content of hydro-carbons gives better resolution (Belle and CLEO II).
Landau distribution (FWMH); 60 % for noble gas, 45% for CH4,33% for C3H6
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L=particle’s path
between
two counters
t=time to traverse L
Time
Time of
of flight:
flight: basics
basics
p
c 2t 2
m=
= p 2 −1
L
γβc
L
v = particle speed =
t
For two particles:
 1 1  L 1
1 
∆t = t 2 − t1 = L −  = 
− 
 v 2 v1  c  β 2 β 1 
For known momentum p:
[(
L  E 2 E1 
L
2 4
2 2

 =
m
c
p
c
−
+
2
2
c  pc pc  pc
In the non-relativistic limit (β~0.1):
∆t =
∆t =
L
(m 2 − m1 ) = L ∆m
p
p
m 2 ≅ m1 = m
v 2 ≅ v1 = v = β c
⇒
∆t =
)
1/ 2
(
− m12 c 4 + p 2 c 2
)
1/ 2
]
L
(m 2 − m1 ) = t ∆m
m βc
m
Consequently, for a time resolution of ∆t=200 ps and a flight path L=1 m, it is possible to
discriminate between low-energy particles to better than 1% level of accuracy
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Time of flight for relativistic particles
TOF difference of two particles at a given momentum
∆t1− 2 =

L 1
1  L
Lc  2

 =  1 + m12c 2 / p 2 − 1 + m 22c 2 / p 2  ≈
−
 m1 − m 22 



c  β1 β 2  c 
 2 p2 
Combine TOF with momentum measurement
m= p
2 2
c t
2
L
2
−1
Mass resolution
 dp 
dm
 dt dL 
=   + γ 4  +

m
L 
 t
 p 
2
For momenta above some GeV/c the resolution in mass discrimination is
almost lost

 m c2
L 
∆t AB
 A
Nσ t =
=
 1+ 
σt
cσ t 
p


2
2
2
 m c  
 B 

 − 1+




p
 
 

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Momentum
Momentum limit
limit at
at 33 σσ
L=4 m
In ALICE, the
time resolution
of TOF is
100 ps
3σ separation
equivalent to
300 ps
difference
π/K up to 2.2 GeV/c
K/π up to 3.7 GeV/c
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From
From Theory
Theory
to
to Practice
Practice
TOF PID as
envisaged in
ALICE
for Pb-Pb
collisions
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TOF:
TOF: experimental
experimental issues
issues
particle
start counter

stop counter
Start and stop counters
fast detectors:
plastic scintillators (well assessed technology)
¾ gaseous detectors (old technology, new advances)
¾

Specific signal processing (timing+charge measurements)
pulse height analysis->digital conversion to stop a fast digital clock
discriminators (specifically designed for slewing correction)
TDCs

Calibrations – corrections for cable lengths, counters delay time….
Continuous stability monitoring

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Scintillation
Scintillation Counters
Counters
particle
Photocathode
∼ 106 secondary electrons
Dynodes
Anode
photon
scintillator
production of
scintillation light
(luminescence)
dE / dx ~ 2MeV / cm
photoelectron
light guide
photon detector
Matches the scintillator
shape to the PMT’s round
face and transports
photons (total internal
reflection & external
reflector)
1 photon/100 eV
Nphoton~ 2 •104/cm
fish-tail
e le c tric a l
p uls e
convert photons to
electrons
thus providing an
electrical signal
QE = Np.e./Nphotons
dynode gain = 3-20
10 dynodes with gain=4
M = 410 ≈ 106
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Scintillators
Scintillators
Two types:

Inorganic crystals (high density and Z materials: NaI, CsI,…)
good light yield, too slow for TOF application (OK for e.m. calorimeter)
Organic scintillators (low Z material: polystyrene doped with
fluorescent molecules to shift light from UV to visible &
monocrystals: naphtalene, anthracene, p-terphenyl….)

Excitation at molecular level
The light yield is lower than for inorganic scintillators because of recombination and
quenching effects of the excited molecules. Fast, suited for TOF application
photons / MeV
representative
scintillators Pilot U
11000
CsI(Tl)
50000
Decay time
1.4 ns
800 ns
Density (g/cm3)
1.03
4.5
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Organic
Organic Scintillators
Scintillators
•
•
•
•
Excitation: A0->A1 (∆E=EA1-EA0=absorption spectrum)
Vibrational energy transfer to other molecules nearby: A1->B1
Scintillation: B1->B0 (∆E=EB1-EB0=emission spectrum)
Decay from the vibrationally excited ground state to energetic minima: B0->A0
Because of the energy lost by vibrational quanta:
emission and absorption spectra are shifted in wavelength
Æ scintillator is transparent to the light it produces E. Nappi
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Design Issues
¾ Number of photo-electrons
N pe = N ph ⋅
∫
e
− L l ph (λ )
⋅ D ( λ ) ⋅ QE ( λ ) ⋅ dλ
Nph∼104/cm
L=scintillator length, lph(λ)=photon attenuation length
D=photon collection efficiency (including geometry factors)
< QE > ~ 0.25
¾ Photon detector transit time spread limits the TOF
performance:
• Line-focus type PMT : 250 ps (Philips XP2020)
• Fine-mesh type PMT : 150 ps ( Hamamatsu R2490-05)
• Micro-channel Plate : 55 ps (Hamamatsu R2809U)
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Overall Time Resolution
Expected timing resolution for long counters
From W. B. Atwood (SLAC) 1980:
σ t ~ 87 ( ps ⋅ cm
−1 / 2
)⋅
L ( cm )
N pe
Exp. application
Counter size (cm)
(T x W x L)
Scintillator
PMT
λatt (cm)
σt(meas)
σt(exp)
G.D.Agostini
3x 15 x 100
NE114
XP2020
200
120
60
T. Tanimori
3 x 20 x 150
SCSN38
R1332
180
140
110
T. Sugitate
4 x 3.5 x 100
SCSN23
R1828
200
50
53
E. Nappi
5 x 10 x 280
BC408
XP2020
270
110
125
TOPAZ
4.2 x 13 x 400
BC412
R1828
300
210
240
R. Stroynowski
2 x 3 x 300
SCSN38
XP2020
180
180
420
Belle
4 x 6 x 255
BC408
R6680
250
90
143
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EXAMPLES:Scintillator based TOF
Flight path=15 m
grid:
long scintillators (48 and 130 cm),
read out on both sides
Small, but thick scintillators
8 x 3.3 x 2.3 cm
From γ conversion
in scintillators
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PID with TOF only
Trel. = T / Tπ
NA49:TOF + dE/dx
Combined PID:
TOF + dE/dx (TPC)
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PHENIX
PHENIX TOF
TOF
Central Arm Detectors
start timing
TOF
• Consists of
960 plastic
scintillators
• Flight
path= 5 m
• PMT
readout at
both ends of
scint. (1920
385cm
200cm
200cm
ch.)
Finely segmented high resolution TOF at mid-rapidty
Keep the occupancy level < 10 %
dN ch
≅ 1500
≅ 1000 segments
dy
∆φ = 45 deg. , ∆η = 0.7
~ 100 cm2/segment
•Scintillator: Bicron BC404
• decay constant : 1.8 ns
• attenuation length : 160cm
•PMT : Hamamatsu R3478S
• Rise time : 1.3 ns
• Transit time : 14 + - 0.36 ns
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PHENIX
PHENIX TOF
TOF PERFORMANCE
PERFORMANCE
Prism light guide
to reduce dead space
PMT
Scintillator slat
π+
PHENIX Preliminary
PHENIX Preliminary
K+
p
(a.u.)
e+
π+
K+
PID cut
p
w/o PID cut
p
Ke-
π-
m2 [GeV/c2]
TOF intrinsic timing resolution ~120 ps has
been achieved without slewing correction
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TOF with fast gaseous detectors
1949: J. Keuffel (Caltech) planar spark counters
1970: Y. Pestov (Novosibirsk):
1st example of resistive plate chamber: glass electrode (Pestov glass)+ metal electrode
Excellent time resolution ~ 50 ps or better!
ALICE R&D
Many drawbacks:
test beam: σt ≈ 40 ps !
• long tail of late events
• mechanical constraints (high pressure)
pressure vessel
• non-commercial glass
• nasty gas composition (contains butadiene)
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Gas
Gas Amplification
Amplification in
in Parallel
Parallel Plate
Plate Chambers
Chambers
cathode
anode
Uniform and high electric field
Electron avalanche according to Townsend: N = No eαx
Only avalanches initiated close to anode produce
detectable signal on pickup electrodes
If set minimum gas gain at 106 (10 fC signal) and
maximum gain as 108 (streamers/sparks produced above
this limit), then sensitive region first 25% of gap
A parallel plate chamber cannot perform as a fast gas detector:
• time jitter ≈ time to cross gap ≈ gap size/drift velocity
• electron drift velocity ∼cm/µs -> few µm gap
• low detection efficiency (for 1 e-ion pair about 30 eV is needed) !!
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From
From Avalanche
Avalanche to
to Spark
Spark
5 mV/div
E
20 mV/div
As soon as the number of electrons
in the avalanche reaches ~10 8
(Raether’s criterium):
the space charge becomes so
relevant to balance the external
field, the subsequent
recombination of electrons and ions
generate UV photons that initiate
other avalanches (streamer) up to
the spark regime
FAST (signal formation
driven by UV light rather by
slower electrons) but high
dead time
20 mV/div
E
50 mV/div
E
Volts !
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Resistive
Resistive Plate
Plate Chambers
Chambers
Pestov idea:
use as anodic electrode
a high resistivity glass !!
Concept extended to RPCs
with both electrodes with
high resistivity
Ci
Ci
Ci
Ci
Ci
R
HV C i
R
Ci
R
Ci
R
Ci
R
Ci
R
R
R
R
R
particle
τdischarge << τrecovery= ρε ~ 10 ms
Recovery time longÆ electrodes behave as insulators while electrons reach the
anode Æ the electrical field is quenched locally (a small region of almost 0.1 cm2
will appear “dead” for ~ 10 ms)
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Designing
Designing aa Fast
Fast Gaseous
Gaseous Detector
Detector
Requirements:
(a) Small gaps to achieve a high time
resolution
(b) Very high gas gain (immediate
production of signal)
(c) Possibility to stop growth of avalanches
(otherwise streamers/sparks will occur)
C. Williams – INFN Bologna (1999):
add boundaries that stop avalanche
development. These boundaries must
be invisible to the fast induced signal external pickup electrodes sensitive to
any of the avalanches
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MULTIGAP RESISTIVE PLATE CHAMBER
Internal plates electrically floating!
Cathode -10 kV
Stack of equally-spaced resistive plates with
voltage applied to external surfaces
Pickup electrodes on external surfaces
(resistive plates transparent to fast signal)
(-8 kV)
Flow of electrons
and negative ions
(-6 kV)
Flow of positive ions
(-4 kV)
In this example: 2 kV across
each gap (same E field in each
gap) since the gaps are the
same size - on average - each
plate has same flow of positive
ions and electrons (from
opposite sides of plate) - thus
zero net charge into plate.
STABLE STATE
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(-2 kV)
Anode 0 V
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MGRPC: OPERATIONAL STABILITY
Cathode -10 kV
(-8 kV)
-6.5 kV(-6
kV)
(-4 kV)
(-2 kV)
Anode 0 V
Low E field - low gain
High E field - high gain
Decreased flow of electrons
and increased flow of
positive ions - net flow of
positive charge. This will
move the voltage on this
plate more positive than 6.5 kV (i.e. towards 6 kV)
Internal plates take correct voltage - initially due to electrostatics but kept at correct voltage
by flow of electrons and positive ions - feedback principle that dictates equal gain in all gas gaps
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MRPC PERFORMANCE
Anode electrode 3 x 3 cm2
Schott 8540
(2 mm thick)
SPRING 1999
Single cell Multigap RPC
Cathode electrode 3 x 3 cm2
5 gas gaps of 220 micron
Schott A2
(0.5 mm thick)
Schott A14
(0.5 mm thick)
5 cm
12 kV
100.0
Counts / 50 ps
1000
Tail of late signals
29 events / 17893 events
= 0.16 %
100
Efficiency [%]
Gaussian fit σ = 77 ps
140
95.0
90.0
Efficiency [%]
85.0
Resolution [ps]
80.0
10
Subtract jitter of start counters of
33 ps give time resolution of 70 ps
1
-2000
-1000
0
1000
2000
3000
time difference between start counter and MRPC [ps]
120
100
75.0
80
70.0
65.0
60.0
Resolution [ps]
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60
8
9
10
11
12
Applied HV (kV)
13
14
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ALICE TOF
Hits in inner
tracker
TPC hits
TOF with very high
granularity needed!
The red hits/track
corresponds to a
single particle
(π in this case)
Hits in TOF
array
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ALICE TOF GEOMETRY
TOF ARRAY arranged
as a barrel
with radius of 3.7 m
Divided into 18 sectors
A standard TOF system built of fast scintillators +
photomultipliers would cost >100 MCHF
Along the beam
direction
each sector divided
into 5 modules
i.e 5 x 18 = 90
modules in total
1674 MRPC strips
in total
160 m2 and 160,000
channels
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Cross section of double-stack MRPC (5x250 µm gaps per stack)
made of resistive plates ‘off-the-shelf’ soda lima glass
Standard unit
detector
there will be ~
1600 strips
130 mm
active area 70 mm
honeycomb panel
(10 mm thick)
Flat cable connector
Differential signal sent
from strip to interface card
PCB with cathode
pickup pads
external glass plates
0.55 mm thick
internal glass plates
(0.4 mm thick)
120 cm
PCB with
anode pickup pads
Mylar film
(250 micron thick)
5 gas gaps
of 250 micron
M5 nylon screw to hold
fishing-line spacer
connection to bring cathode signal
to central read-out PCB
PCB with cathode
pickup pads
Honeycomb panel
(10 mm thick)
Silicon sealing compound
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1000
Efficiency [%]
STRIP 10 H.V. +- 6 kV
Uncorrected time spectrum
800
Entries/50 ps
10 gaps of 220 micron
σ = 66 ps minus 30 ps jitter
of timing scintillator = 59 ps
600
400
0
1000
1500
2000
2500
3000
3500
Time with respect to timing scintillators [ps]
1200
Entries/50 ps
strip 12
strip 10
5.0
5.5
6.0
Applied voltage [kV]
Resolution [ps]
200
1000
100
80
60
40
20
0
time spectrum after
correction for slewing
STRIP 10 H.V. +- 6 kV
800
80
75
70
65
60
55
50
45
40
6.5
strip 12
strip 10
5.0
5.5
6.0
6.5
Applied voltage [kV]
σ = 53 ps minus 30 ps jitter
of timing scintillator = 44 ps
600
Typical performance
400
200
Typical time spectrum
0
-1000
-500
0
500
1000
Time with respect to timing scintillators [ps]
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TOF: rate capability
GAMMA IRRADIATION FACILITY AT CERN
Efficiency [%]
100
95
90
85
80
75
70
65
60
55
50
0
200
Time resolution [ps]
90
80
70
60
50
Strip 10 effective voltage 11.4 kV
40
30
Strip 10 applied voltage 11.4 kV
20
10
400
600
800
1000 1200 1400 1600
0
0
200
400
600
800
1000 1200 1400 1600
Equivalent flux of through-going charged particles [Hz/cm2]
Investigated performance with muon beam while test device irradiated with
high flux of 662 keV gammas
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Particle Identification Ranges
Efficiency and contamination at high density (dN/dy = 8000)
B = 0.2 T
B = 0.4 T
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