Inorganic scintillators. Geometries and readout

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1 Topics of this lecture K K Inorganic scintillators Organic scintillators K Geometries and readout K Fiber tracking K Photo detectors Particle Detectors Christian Joram III/1

2 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 lower light output but slow but fast Particle Detectors Christian Joram III/2

3 Inorganic scintillators Three different scintillation mechanisms: 1a. Inorganic crystalline scintillators (NaI, CsI, BaF 2...) conduction band exciton band electron activation centres (impurities) scintillation ( nm) luminescense quenching excitation traps E g hole valence band often 2 time constants: fast recombination (ns- s) 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 C. Particle Detectors Christian Joram III/3

4 Inorganic scintillators Light output of inorganic crystals shows strong temperature dependence (From Harshaw catalog) BGO PbWO 4 1b. Liquid noble gases (LAr, LXe, LKr) A excitation ionization A* A + collision with g.s. atoms excited molecule A 2 * A 2 + ionized molecule de-excitation and dissociation e - A A A 2 * recombination UV 130nm (Ar) 150nm (Kr) 175nm (Xe) also here one finds 2 time constants: few ns and ns, but same wavelength. Particle Detectors Christian Joram III/4

5 Inorganic scintillators Properties of some inorganic scintillators Photons/ MeV PbWO , LAr ) / LKr ) / LXe ) / ) at 170 nm Particle Detectors Christian Joram III/5

6 Organic scintillators 2. Organic scintillators: Monocrystals or liquids or plastic solutions S 3 S 2 S 1 singlet states s Molecular states nonradiative triplet states T 2 Scintillation is based on the 2 F electrons of the C-C bonds. fluorescence s T 1 phosohorescence >10-4 s S 0 Emitted light is in the UV range. 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 Particle Detectors Christian Joram III/6

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 Liquid scintillators Plastic scintillators solvent Benzene Toluene Xylene Polyvinylbenzene 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). Particle Detectors Christian Joram III/7

8 Organic scintillators yield/ NaI 0.5 Organic scintillators have low Z (H,C). Low C detection efficiency (practically only Compton effect). But high neutron detection efficiency via (n,p) reactions. Particle Detectors Christian Joram III/8

9 Scintillator readout Scintillator readout Readout has to be adapted to geometry and emission spectrum of scintillator. Geometrical adaptation: K Light guides: transfer by total internal reflection (+outer reflector) fish tail adiabatic K wavelength shifter (WLS) bars small air gap WLS green Photo detector blue (secondary) scintillator UV (primary) primary particle Particle Detectors Christian Joram III/9

10 Scintillator readout K Optical fibers typ. 25 m light transport by total internal reflection core polystyrene n=1.59 cladding (PMMA) n=1.49 n 1 G typically <1 mm n 2 n G arcsin d9 3.1% in one direction n1 4F minimize n cladding. Ideal: air (n=1), but impossible due to surface imperfections multi-clad fibres for improved aperture d9 5.3% 4F and absorption length: >10 m for visible light core polystyrene n=1.59 cladding (PMMA) n= m fluorinated outer cladding n= m Particle Detectors Christian Joram III/10

11 Scintillator readout readout of a scintillator with a fiber (schematically) to photo detector scintillator optical fiber in machined groove ATLAS Hadron Calorimeter: (ATLAS TDR) Scintillating tile readout via fibers and photomultipliers 1 mm fiber Each tile is read out on both outer sides 1 of 64 independent wedges ca. 2m Periodical arrangement of scintillator tiles (3 mm thick) in a steel absorber structure ca. 11m Particle Detectors Christian Joram III/11

12 Scintillating fiber tracking Scintillating fiber tracking K Scintillating plastic fibers K Capillary fibers, filled with liquid scintillator Planar geometries (end cap) (R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281) Circular geometries (barrel) a) axial b) circumferential c) helical High geometrical flexibility Fine granularity Low mass Fast response (ns) (if fast read out) first level trigger Particle Detectors Christian Joram III/12

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. 3.4 m 60 m Low cross talk! (H. Leutz, NIM A 364 (1995) 422) Particle Detectors Christian Joram III/13

14 Photo Detectors Photo Detectors Purpose: Convert light into detectable electronics signal In HEP we are usually interested in visible and UV spectrum Threshold of some photosensitive material UV visible GaAs... TEA TMAE,CsI threshold for photo effect bialkali multialkali E (ev) standard requirement high sensitivity, usually expressed as quantum efficiency Q.E. = N p.e. / N photons Main types gas based devices (see RICH detectors) vacuum based devices solid state detectors Particle Detectors Christian Joram III/14

15 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 M N g i i 1 10 dynodes with g=4 M = PM s are in general very sensitive to B- fields, even to earth field (30-60 T). metal shielding required. Particle Detectors Christian Joram III/15

16 Photo Detectors Quantum efficiencies of typical photo cathodes Q.E. Bialkali SbK 2 Cs SbRbCs Multialkali SbNa 2 KCs Solar blind CsTe (cut by quartz window) (Philips Photonic) Q. E. % ske ma/ W 124 ( nm) Transmission of various PM windows NaF, MgF 2, LiF, CaF 2 Particle Detectors Christian Joram III/16

17 Photo detectors K 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: n P( n, m) I n n n n 1 m m Fluctuations biggest, when n small! First dynode! e m! n GaP(Cs) Negative electron affinity (NEA)! (Philips Photonic) (Philips Photonic) counts Single photons. Pulse height spectrum of a PMT with Cu-Be dynodes. 1 p.e. (Philips Photonic) counts Pulse height spectrum of a PMT with NEA dynodes. 1 p.e. 2 p.e. 3 p.e. noise (H. Houtermanns, NIM 112 (1973) 121) Pulse height Pulse height Particle Detectors Christian Joram III/17

18 Photo Detectors Dynode configurations (Philips Photonics) Multi Anode PM example: Hamamatsu R5900 series. position sensitive PMT s Up to 8x8 channels. Size: 28x28 mm 2. Active area 18x18 mm 2 (41%). Bialkali PC: Q.E. = 20% at max = 400 nm. Gain Gain uniformity and cross-talk used to be problematic, but recently much improved. Particle Detectors Christian Joram III/18

19 Photo Detectors K Hybrid photo diodes (HPD) photocathode focusing electrodes electron,v photo cathode + p.e. acceleration + silicon det. (pixel, strip, pads) silicon sensor Photo cathode like in PMT,,V kv G e, V W Si 20 kev 3.6 ev (for,v =20 kv) Single photon detection with high resolution Commercial HPD (DEP PP0270K) with slow electronic (2 s shaping time) (C.P. Datema et al. NIM A 387(1997) 100 Poisson statistics with n =5000! Background from electron backscattering from silicon surface Particle Detectors Christian Joram III/19

20 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 Particle Detectors Christian Joram III/20

21 Photo Detectors K Photo diodes P(I)N type (sketches from J.P. Pansart, NIM A 387 (1997), 186) High Q.E. ( 80% at 700nm), gain G = 1. K Avalanche Photo diodes (APD) (J.P. Pansart, NIM A 387 (1997), 186) High reverse bias voltage V. High internal field avalanche multiplication. G 100(0) K Photo triodes = single stage PMT (no Silicon!) G 10. work in axial B-fields of 1T OPAL, DELPHI: readout of lead glass in endcap calorimeter G at 1T mm IEEE NS-30 No. 1 (1983) 479 photo cathode vacuum tube anode (grid) dynode Particle Detectors Christian Joram III/21

22 Photo Detectors K Visible Light Photo Counter VLPC Photon Intrinsic Region + 50 mev Gain Region e h CB VB e impurity band Drift Region Substrate Spacer Region - Hole drifts towards highly doped drift region and ionizes a donor atom free electron. Multiplication by ionization of further neutral donor atoms. 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 ! Q.E VLPC bialkali (ST) GaAs (opaque) Multialkali (ST) (nm) Particle Detectors Christian Joram III/22

23 Photo Detectors High gain real photon counting as in HPD events pedestal noise no light events with light ADC counts (a.u.) Fermilab: D0 (D zero) fiber tracker ( channels) Ø1 mm 8 pixels per chip (vapour phase epitaxial growth) Particle Detectors Christian Joram III/23