Transcript Introduction

Scintillation + Photo Detection

Inorganic scintillators

Organic scintillators

Geometries and readout

Fiber tracking

Photo detectors
CERN Summer Student Lectures 2003
Particle Detectors
Christian Joram
III/1
Scintillation
Scintillation
photodetector
Energy deposition by ionizing particle
 production of scintillation light (luminescense)
Scintillators are multi purpose detectors





calorimetry
time of flight measurement
tracking detector (fibers)
trigger counter
veto counter
…..
Two material types: Inorganic and organic scintillators
high light output
but slow
CERN Summer Student Lectures 2003
Particle Detectors
Christian Joram
lower light output
but fast
III/2
Inorganic scintillators
Three different scintillation mechanisms:
1a. Inorganic crystalline scintillators (NaI, CsI, BaF2...)
conduction band
exciton
band
scintillation
(200-600nm)
excitation
quenching
luminescense
activation
centres
(impurities)
electron
traps
Eg
hole
valence band
often  2 time constants:
• fast recombination (ns-ms) from activation centre
• delayed recombination due to trapping ( 100 ms)
Due to the high density and high Z inorganic scintillator
are well suited for detection of charged particles, but
also of g.
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Christian Joram
III/3
Inorganic scintillators
Light output of inorganic crystals shows strong
temperature dependence
(From Harshaw catalog)
BGO
PbWO4
1b. Liquid noble gases (LAr, LXe, LKr)
excited
molecule
excitation
de-excitation and
dissociation
A
A2*
A*
A
collision
with g.s.
atoms
A
ionization
A+
A2*
A2+
ionized
molecule
UV
130nm (Ar)
150nm (Kr)
175nm (Xe)
recombination
e-
also here one finds 2 time constants: few ns and
100-1000 ns, but same wavelength.
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Inorganic scintillators
Properties of some inorganic scintillators
Photons/
MeV
4  104
1.1  104
1.4104
6.5  103
2  103
2.8  103
PbWO4
8.28
1.82
440, 530
0.01
LAr
1.4
1.295)
120-170
0.005 / 0.860
LKr
2.41
1.405)
120-170
0.002 / 0.085
LXe
3.06
1.605)
120-170
0.003 / 0.022
5)
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100
4  104
at 170 nm
Christian Joram
III/5
Inorganic scintillators
PbWO4 ingot and final polished CMS ECAL scintillator
crystal from Bogoroditsk Techno-Chemical Plant (Russia).
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Particle Detectors
Christian Joram
III/6
Organic scintillators
2. Organic scintillators: Monocrystals or liquids or
plastic solutions
Molecular states
Scintillation is based
on the 2 p electrons
of the C-C bonds.
singlet states
S3
10-11 s
S2
S1
triplet states
nonradiative
T2
T1
fluorescence
10-8 - 10-9 s
phosohorescence
>10-4 s
Emitted light is in
the UV range.
S0
Monocrystals: naphtalene, anthracene, p-terphenyl….
Liquid and plastic scintillators
They consist normally of a solvent + secondary (and
tertiary) fluors as wavelength shifters.
Fast energy transfer via non-radiative dipole-dipole
interactions (Förster transfer).
 shift emission to longer wavelengths
 longer absorption length and efficient read-out
device
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Particle Detectors
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III/7
Organic scintillators (backup)
Schematic
representation
of wave length
shifting
principle
(C. Zorn, Instrumentation In
High Energy Physics, World
Scientific,1992)
Some widely used solvents and solutes
solvent
Liquid
Benzene
scintillators Toluene
Xylene
Plastic
Polyvinylbenzene
scintillators Polyvinyltoluene
Polystyrene
secondary
fluor
p-terphenyl
DPO
PBD
p-terphenyl
DPO
PBD
tertiary
fluor
POPOP
BBO
BPO
POPOP
TBP
BBO
DPS
After mixing the components together plastic
scintillators are produced by a complex polymerization
method.
Some inorganic scintillators are dissolved in PMMA
and polymerized (plexiglas).
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Particle Detectors
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III/8
Organic scintillators
yield/
NaI
0.5
Organic scintillators have low Z (H,C). Low g detection
efficiency (practically only Compton effect). But high
neutron detection efficiency via (n,p) reactions.
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III/9
Scintillator readout
Scintillator readout
Readout has to be adapted to geometry and emission
spectrum of scintillator.
Geometrical adaptation:

Light guides: transfer by total internal reflection
(+outer reflector)
“fish tail”

adiabatic
wavelength shifter (WLS) bars
WLS
small air gap
green
Photo detector
blue (secondary)
UV (primary)
scintillator
primary particle
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Particle Detectors
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Scintillator readout

Optical fibers
light transport by total
internal reflection
typ. 25 mm
core
polystyrene
n=1.59
typically <1 mm
  arcsin
n2
 69.6
n1
cladding
(PMMA)
n=1.49

n1
n2
d
 3.1%
4p
in one direction
minimize ncladding.
Ideal: air (n=1), but impossible due to surface imperfections
cladding
(PMMA)
n=1.49
25 mm
multi-clad fibres
for improved
aperture
d
 5.3%
4p
core
polystyrene
n=1.59
fluorinated
outer cladding
n=1.42
25 mm
and absorption
length: l>10 m for
visible light
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Particle Detectors
Christian Joram
III/11
Scintillating fiber tracking
Scintillating fiber tracking


Scintillating plastic fibers
Capillary fibers, filled with liquid scintillator
Planar geometries
(end cap)
Circular geometries
(barrel)
a) axial
b) circumferential
c) helical
(R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281)

High geometrical flexibility

Fine granularity

Low mass

Fast response (ns) (if fast read out)  first level trigger
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Christian Joram
III/12
Scintillating fiber tracking
Charged particle
passing through a
stack of
scintillating fibers
(diam. 1mm)
UA2 (?)
Hexagonal
fibers with
double cladding.
Only central
fiber illuminated.
3.4 mm
Low cross talk !
(H. Leutz, NIM A 364 (1995) 422)
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Photo Detectors
Photo Detectors
Purpose: Convert light into detectable electronics signal
Principle: Use Photoelectric Effect to convert photons
to photoelectrons
standard requirement
 high sensitivity, usually expressed as
quantum efficiency Q.E. = Np.e./ Nphotons
Main types of photodetetcors
 gas based devices (see RICH detectors)
 vacuum based devices
 solid state detectors
Threshold of some photosensitive material
GaAs ...
TMAE,CsI
UV
visible
bialkali
TEA
12.3
100
4.9
250
multialkali
E (eV)
3.1
2.24
1.76
400
550
700
l (nm)
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Particle Detectors
Christian Joram
III/14
Photo Detectors
Photoelectric effect in photocathodes
3-step process



photo ionization of molecule
Electron propagation through cathode
escape of electron back into the vacuum
Semitransparent photocathode
Opaque photocathode
g
g
substrate
glass
PC
ee-
PC
Most photocathodes are semiconductors:
band model:
Photon energy has to be
sufficient to bridge the
band gap Eg, but also to
overcome the electron
affinity EA, so that the
electron can be released
into the vacuum.
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Particle Detectors
Christian Joram
III/15
Photo Detectors
Quantum efficiencies of typical photo cathodes
Q.E.
Bialkali
SbK2Cs
SbRbCs
Multialkali
SbNa2KCs
Solar blind
CsTe
(cut by quartz
window)
Q.E.%  
sk mA / W 
124  e
l (nm)
Transmission
of various
PM windows
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NaF, MgF2, LiF, CaF2
(Philips Photonic)
Christian Joram
III/16
Vacuum Based Photo Detectors
Photo Multiplier Tube
(PMT)
photon
e-
(Philips
Photonic)
main phenomena:
• photo emission from photo
cathode.
• secondary emission from
dynodes.
dynode gain g=3-50 (f(E))
N
total gain M   g i
i 1
10 dynodes with g=4
M = 410  106
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Particle Detectors
Christian Joram
III/17
Vacuum Based Photo Detectors

Energy resolution of PMT’s
The energy resolution is determined mainly by the
fluctuation of the number of secondary electrons emitted
from the dynodes.
n me m
Poisson distribution: P(n , m) 
m!

n
1

Relative fluctuation: n 
n
n
n
Fluctuations biggest, when
n
small !  First dynode !
GaP(Cs)
Negative
electron
affinity (NEA) !
(Philips Photonic)
(Philips Photonic)
1 p.e.
(Philips Photonic)
Pulse height spectrum of a
PMT with NEA dynodes.
counts
counts
Single photons.
Pulse height spectrum of a
PMT with Cu-Be dynodes.
1 p.e.
2 p.e.
3 p.e.
(H. Houtermanns,
NIM 112 (1973) 121)
Pulse height
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Pulse height
Christian Joram
III/18
Vacuum Based Photo Detectors
Dynode configurations
traditional
New ‘micro-machined’
structures
(Philips Photonics)
position
sensitive
PMT’s
PM’s are in general very sensitive to B-fields, even to
earth field (30-60 mT). m-metal shielding required.
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Vacuum Based Photo Detectors
Multi Anode PMT
example: Hamamatsu R5900 series.
Up to 8x8 channels.
Size: 28x28 mm2.
Active area 18x18 mm2 (41%).
Bialkali PC: Q.E. = 20% at lmax =
400 nm. Gain  106.
Gain uniformity and cross-talk used to be problematic, but
recently much improved.
Very recent development:
Flat Panel PMT (Hamamatsu )
Excellent surface coverage (>90%)
8 x 8 channels (4 x 4 mm2 / channel)
Bialkali PC, eQ  20%
50 mm
CERN Summer Student Lectures 2003
Particle Detectors
Christian Joram
III/20
Vacuum Based Photo Detectors

Hybrid photo diodes (HPD)
photocathode
electron
focusing
electrodes
DV
photo cathode + p.e.
acceleration + silicon
det. (pixel, strip, pads)
silicon
sensor
Photo cathode like in PMT, DV 10-20 kV
G
eDV 20 keV

 5 103
WSi 3.6 eV
(for DV =20 kV)
Commercial HPD (DEP
PP0270K) with slow
electronic (2ms shaping
time)
(C.P. Datema et al. NIM
A 387(1997) 100
Single photon detection
with high resolution
Poisson statistics
with n =5000 !
Background from
electron backscattering
from silicon surface
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Particle Detectors
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III/21
Vacuum Based Photo Detectors
Cherenkov ring imaging with HPD’s
(CERN)
2048 pads
Pad HPD, Ø127 mm,
fountain focused
test beam data, 1 HPD
(LHCb - DEP)
3 x 61 pixels
Pixel-HPD, 80mm Ø
cross-focused
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test beam data, 3 HPDs
Christian Joram
III/22
Solid State Photo Detectors
Photo diodes

P(I)N type
p
n=i
(intrinsic)
n+
h
e
p layer must be very thin (<1 mm), as visible light is rapidly
absorbed by silicon.
High Q.E. (80% at l  700nm),
but no gain: G = 1.
Can’t be used for single photon
detection, but suitable for readout
of scintillators.
Even better…

Avalanche Photo diodes (APD)
High reverse bias
voltage  100-200V.
High internal field
 avalanche
multiplication.
G  100(0)
E
p
drift
h
e
p
n
avalanche
CERN Summer Student Lectures 2003
Particle Detectors
Christian Joram
III/23
Photo Detectors (backup)
Visible Light Photo Counter VLPC

Intrinsic
Region
Gain
Region
•e •h
Drift
Region
Substrate
Spacer
Region
•e
Photon
•+
50m
eV
•-
Hole drifts towards
highly doped drift
region and ionizes a
donor atom  free
electron.
Multiplication by
ionization of further
neutral donor atoms.
C
B
im
purityband
Si:As impurity band
conduction avalanche diode
• Operation at low bias
voltage (7V)
• High IR sensitivity
 Device requires
cooling to LHe
temperature.
• Q.E.  70% around 500
nm.
• Gain up to 50.000 !
Q.E.
V
B
1.0
0.8
VLPC
0.6
0.4
bialkali (ST)
GaAs (opaque)
0.2
Multialkali (ST)
0.0
300 400
500
600
700 800
900 1000
l (nm)
CERN Summer Student Lectures 2003
Particle Detectors
Christian Joram
III/24
Photo Detectors (backup)
High gain  real photon counting as in HPD
no light
pedestal noise
with light
0
1
2
3
4
5
ADC counts (a.u.)
Fermilab: D0 (D zero) fiber tracker (72.000 channels)
Ø1 mm
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8 pixels per chip
(vapour phase epitaxial growth)
Christian Joram
III/25