CERN Summer Student Lectures 2003 Particle Detectors

1

• Scintillation Photo Detection
• Inorganic scintillators
• Organic scintillators
• Geometries and readout
• Fiber tracking
• Photo detectors

2
Scintillation

• Scintillation
• Two material types Inorganic and organic
  scintillators

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 ..
high light output lower light output but
slow but fast
3
Inorganic scintillators

• Three different scintillation mechanisms
• 1a. Inorganic crystalline scintillators (NaI,
  CsI, BaF2...)

• 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.
4
Inorganic scintillators

• Light output of inorganic crystals shows strong
  temperature dependence
• 1b. Liquid noble gases (LAr, LXe, LKr)

(From Harshaw catalog)
BGO
PbWO4
also here one finds 2 time constants few ns and
100-1000 ns, but same wavelength.
5
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
100
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
4 ? 104

0.003 / 0.022
5) at 170 nm
6
Inorganic scintillators
PbWO4 ingot and final polished CMS ECAL
scintillator crystal from Bogoroditsk
Techno-Chemical Plant (Russia).
7
Organic scintillators

• 2. Organic scintillators Monocrystals or liquids
  or plastic solutions
• 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

Scintillation is based on the 2 p electrons of
the C-C bonds. Emitted light is in the UV
range.
8
Organic scintillators (backup)

• Some widely used solvents and solutes
• After mixing the components together plastic
  scintillators are produced by a complex
  polymerization method.

Schematic representation of wave length
shifting principle
(C. Zorn, Instrumentation In High Energy Physics,
World Scientific,1992)
9
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.
10
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)
• wavelength shifter (WLS) bars

fish tail
adiabatic
11
Scintillator readout

• Optical fibers
• minimize ncladding.
• Ideal air (n1), but impossible due to surface
  imperfections

light transport by total internal reflection
in one direction
multi-clad fibres for improved aperture and
absorption length lgt10 m for visible light
12
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

13
Scintillating fiber tracking
Charged particle passing through a stack of
scintillating fibers (diam. 1mm)
UA2 (?)
Hexagonal fibers with double cladding. Only
central fiber illuminated. Low cross talk !
60 mm
3.4 mm
(H. Leutz, NIM A 364 (1995) 422)
14
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 ...
visible
UV
multialkali
TMAE,CsI
bialkali
TEA
E (eV)
12.3 4.9 3.1
2.24 1.76
100 250 400
550 700
l (nm)
15
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
glass
PC
substrate
e-
e-
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.
16
Photo Detectors

• Quantum efficiencies of typical photo cathodes

Q.E.
Bialkali SbK2Cs SbRbCs Multialkali
SbNa2KCs Solar blind CsTe (cut by quartz
window)
(Philips Photonic)
Transmission of various PM windows
NaF, MgF2, LiF, CaF2
17
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 g3-50 (f(E))
• total gain
• 10 dynodes with g4
• M 410 ? 106

18
Vacuum Based Photo Detectors

• Energy resolution of PMTs

The energy resolution is determined mainly by the
fluctuation of the number of secondary electrons
emitted from the dynodes.
Poisson distribution
Relative fluctuation
Fluctuations biggest, when small ! ?
First dynode !
GaP(Cs)
Negative electron affinity (NEA) !
(Philips Photonic)
(Philips Photonic)
Single photons. Pulse height spectrum of a PMT
with Cu-Be dynodes.
Pulse height spectrum of a PMT with NEA dynodes.
1 p.e.
counts
counts
(Philips Photonic)
1 p.e.
2 p.e.
3 p.e.
noise
(H. Houtermanns, NIM 112 (1973) 121)
Pulse height
Pulse height
19
Vacuum Based Photo Detectors

• Dynode configurations

traditional
New micro-machined structures
(Philips Photonics)
position sensitive PMTs
PMs are in general very sensitive to B-fields,
even to earth field (30-60 mT). m-metal shielding
required.
20
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 (gt90) 8 x 8 channels
(4 x 4 mm2 / channel) Bialkali PC, eQ ? 20
50 mm
21
Vacuum Based Photo Detectors

• Hybrid photo diodes (HPD)

photo cathode p.e. acceleration silicon det.
(pixel, strip, pads)
Photo cathode like in PMT, DV 10-20 kV
(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 5000 !
Background from electron backscattering from
silicon surface
22
Vacuum Based Photo Detectors

• Cherenkov ring imaging with HPDs

(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
test beam data, 3 HPDs
23
Solid State Photo Detectors

• Photo diodes

p
h e
P(I)N type
n i (intrinsic)
n
p layer must be very thin (lt1 mm), as visible
light is rapidly absorbed by silicon.
High Q.E. (?80 at l ? 700nm), but no gain G
1. Cant 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
h e
drift
p
n
avalanche
24
Photo Detectors (backup)

• Visible Light Photo Counter VLPC

Hole drifts towards highly doped drift region and
ionizes a donor atom ? free electron.
Multiplication by ionization of further neutral
donor atoms.
Gain
Drift
Substrate
Region
Region
Intrinsic
Spacer
Region
Region

• e

• h

• e

Photon

• -

SiAs impurity band conduction avalanche diode
1.0 0.8 0.6 0.4 0.2 0.0
Q.E.

• 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 !

VLPC
bialkali (ST)
GaAs (opaque)
Multialkali (ST)
300 400 500 600 700 800 900 1000

l (nm)
25
Photo Detectors (backup)
High gain ? real photon counting as in HPD
Fermilab D0 (D zero) fiber tracker (72.000
channels)
8 pixels per chip (vapour phase epitaxial growth)
Ø1 mm