1. Scintillation + Photo Detection
Inorganic scintillators
Organic scintillators
Geometries and readout
Fiber tracking
Photo detectors
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

2. high light output lower light output
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
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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.
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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 ns, but same wavelength.
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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)
0.005 / 0.860
LKr
2.41
1.405)
0.002 / 0.085
LXe
3.06
1.605)
0.003 / 0.022
4  104
5) at 170 nm
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

6. Inorganic scintillators
PbWO4 ingot and final polished CMS ECAL scintillator crystal from Bogoroditsk Techno-Chemical Plant (Russia).
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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
 longer absorption length and efficient read-out
device
Scintillation is based on the 2 p electrons of the C-C bonds.
Emitted light is in the UV range.
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

8. Organic scintillators (backup)
Some widely used solvents and solutes
After mixing the components together plastic scintillators are produced by a complex polymerization method.
Some inorganic scintillators are dissolved in PMMA and polymerized (plexiglas).
Schematic representation
of wave length shifting
principle
(C. Zorn, Instrumentation In High Energy Physics, World Scientific,1992)
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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.
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

11. light transport by total internal reflection
Scintillator readout
Optical fibers
minimize ncladding.
Ideal: air (n=1), but impossible due to surface imperfections
light transport by total
internal reflection
in one direction
multi-clad fibres
for improved
aperture
and absorption
length: l>10 m for visible light
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

12. 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
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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 !
3.4 mm
60 mm
(H. Leutz, NIM A 364 (1995) 422)
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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 ... UV
visible
multialkali
TMAE,CsI
bialkali
TEA
E (eV)
l (nm)
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

15. Semitransparent photocathode
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.
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

16. Quantum efficiencies of typical photo cathodes
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
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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 g=3-50 (f(E))
total gain
10 dynodes with g=4
M = 410  106
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

18. 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.
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
(Philips Photonic)
counts
1 p.e.
2 p.e.
3 p.e.
noise
(H. Houtermanns,
NIM 112 (1973) 121)
Pulse height
Pulse height
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

19. 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.
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

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 (>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

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 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
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

22. 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
test beam data, 3 HPDs
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

23. Solid State Photo Detectors
Photo diodes p h e
P(I)N type
n = i
(intrinsic) n+
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  V.
High internal field  avalanche multiplication.
G  100(0) E p h e
drift p n
avalanche
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

24. Photo Detectors (backup)
Visible Light Photo Counter VLPC
Gain
Drift
Substrate
Hole drifts towards highly doped drift region and ionizes a donor atom  free electron. Multiplication by ionization of further neutral donor atoms.
Intrinsic
Region
Region
Spacer
Region
Region e h e
Photon + -
Si:As impurity band
conduction avalanche diode
1.0
0.8
0.6
0.4
0.2
0.0
Operation at low bias voltage (7V)
High IR sensitivity
 Device requires cooling to LHe temperature.
Q.E.  70% around 500 nm.
Gain up to !
Q.E.
VLPC
bialkali (ST)
GaAs (opaque)
Multialkali (ST)
l (nm)
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram

25. Photo Detectors (backup)
High gain  real photon counting as in HPD
pedestal noise
no light
with light
ADC counts (a.u.)
events
Fermilab: D0 (D zero) fiber tracker ( channels)
8 pixels per chip
(vapour phase epitaxial growth)
Ø1 mm
CERN Summer Student Lectures 2003 Particle Detectors
Christian Joram