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

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

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

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. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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 0.003 / 0.022 4  104 5) at 170 nm CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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

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

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

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

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

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

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

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

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) 12.3 4.9 3.1 2.24 1.76 100 250 400 550 700 l (nm) CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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

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

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

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

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

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

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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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

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  100-200V. 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

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 50.000 ! Q.E. VLPC bialkali (ST) GaAs (opaque) Multialkali (ST) 300 400 500 600 700 800 900 1000 l (nm) CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Photo Detectors (backup) High gain  real photon counting as in HPD pedestal noise no light with light 0 1 2 3 4 5 ADC counts (a.u.) events Fermilab: D0 (D zero) fiber tracker (72.000 channels) 8 pixels per chip (vapour phase epitaxial growth) Ø1 mm CERN Summer Student Lectures 2003 Particle Detectors Christian Joram