Topics of this lecture Inorganic scintillators Organic scintillators Geometries and readout Photo detectors Fiber tracking Nuclear emulsions Electronic bubble chamber 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 lower light output but slow but fast III/2
Inorganic scintillators Three different scintillation mechanisms: 1a. Inorganic crystalline scintillators (NaI, CsI, BaF 2...) conduction band exciton band electron activation centres (impurities) scintillation (200-600nm) luminescense quenching excitation traps E g hole valence band often 2 time constants: fast recombination (ns-µs) 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 γ. III/3
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 100-1000 ns, but same wavelength. III/4
Inorganic scintillators Properties of some inorganic scintillators Photons/ MeV 4 10 4 1.1 10 4 1.4 10 4 6.5 10 3 2 10 3 2.8 10 3 PbWO 4 8.28 1.82 440, 530 100 LAr 1.4 1.29 5) 120-170 0.005 / 0.860 LKr 2.41 1.40 5) 120-170 0.002 / 0.085 LXe 3.06 1.60 5) 120-170 0.003 / 0.022 4 10 4 5) at 170 nm III/5
Organic scintillators 2. Organic scintillators: Monocrystals or liquids or plastic solutions S 3 S 2 S 1 singlet states 10-11 s Molecular states nonradiative triplet states T 2 Scintillation is based on the 2 π electrons of the C-C bonds. fluorescence 10-8 - 10-9 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 III/6
Organic scintillators 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). III/7
Organic scintillators yield/ NaI 0.5 Organic scintillators have low Z (H,C). Low γ detection efficiency (practically only Compton effect). But high neutron detection efficiency via (n,p) reactions. III/8
Organic scintillators The response of plastic scintillators is not linear dl dx A de dx k de dx 1 B Birk s formula dl/dx A/k B (J.B. Birks, Proc. Phys. Soc. A64, 874 (1951)) (Also other models and parameterizations on the market) 100 MeV cm 2 g -1 The light yield is reduced by recombination and quenching effects of the excited molecules. Effect density of excited molecules de/dx de/dx Another funny effect: Light output of some plastic scintillators depends on B-field. Effect not fully understood. (J.M. Chapuis et al., ATLAS internal note, TILECAL-NO-40 (1994)) 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 green small air gap Photo detector blue (secondary) scintillator UV (primary) primary particle III/10
Scintillator readout Widely used wavelength shifters: BBQ, Y7, K27, embedded in PMMA - absorption around 400 nm - emission around 500 nm - big Stokes shift minimal overlap between absorption and emission spectrum long absorption length for emitted spectrum. Conversion efficiencies around 10-20%. Fibers typ. 25 m light transport by total internal reflection core polystyrene n=1.59 cladding (PMMA) n=1.49 n 1 typically <1 mm n 2 n2 arcsin 69. 6 d % n1 4 3.1 minimize n cladding. Ideal: air (n=1), but impossible due to surface imperfections in one direction III/11
Scintillator readout multi-clad fibres for improved aperture d 4 5.3% and absorption length: λ>10 m for visible light core polystyrene n=1.59 cladding (PMMA) n=1.49 25 m fluorinated outer cladding n=1.42 25 m to photo detector readout of a scintillator with a fiber (schematically) scintillator optical fiber in machined groove fibers exist as - clear fibers - WLS fibers - scintillating fibers (fiber tracking,see below) III/12
Scintillator readout ATLAS Hadron Calorimeter: (ATLAS TDR) Scintillating tile readout via fibers and photomultipliers 1 mm fiber 1 of 64 independent wedges ca. 2m Periodical arrangement of scintillator tiles (3 mm thick) in a steel absorber structure ca. 11m Each tile is read out on both outer sides III/13
Photo Detectors Photo Detectors Photo Multiplier basic principle: (Philips Photonic) main phenomena: photo emission from photo cathode. Q.E. = N p.e. /N photons secondary emission from dynodes. dynode gain g=3-50 (f(e)) total gain M N g i i1 10 dynodes with g=4 M = 4 10 10 6 Fast PM s require well designed input optics to limit chromatic and geometric aberrations transit time spread < 200 ps PM s are in general very sensitive to B-fields, even to earth field (30-60 µt). µ-metal shielding required. Equi-potentials and trajectories in a fast input system III/14
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. % 124 sk e ma/ W ( nm) Transmission of various PM windows (Philips Photonic) not shown: MgF 2 : cut @115 nm LiF: cut @105 nm III/15
Photo detectors Energy resolution of PM s The energy resolution is determined mainly by the fluctuation of the number of secondary electrons emitted from the dynodes. Poisson distribution: Relative fluctuation: P( n, m) n n n n 1 n m m Fluctuations biggest, when n small! First dynode! e m! n GaP(Cs) Negative electron affinity! (Philips Photonic) (Philips Photonic) Illustration of a pulse height spectrum of a PM. First dynode: δ = 25 (Gilmore) III/16
Photo Detectors Dynode configurations (Philips Photonics) Multi Anode PM example: Hamamatsu R5900 series. position sensitive PM s Up to 8x8 channels. Size: 28x28 mm 2. Active area 18x18 mm 2 (41%). Bialkali PC: Q.E. = 20% at λ max = 400 nm. Gain 10 6. Gain uniformity and cross-talk used to be problematic, but recently much improved. III/17
Photo Detectors Micro Channel plates + fast signal (transit time spread 50 ps), + less sensitive to B-field (0.1 T) + 2-dimensional readout possible - limited life time (0.5 C/cm 2 ) - limited rate capability (µa/cm 2 ) (Philips Photonics) III/18
Photo Detectors Photo diodes P(I)N type (sketches from J.P. Pansart, NIM A 387 (1997), 186) High Q.E. ( 80% at λ 700nm), gain G = 1. 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 7-10 40 mm IEEE NS-30 No. 1 (1983) 479 photo cathode vacuum tube anode (grid) dynode Avalanche Photo diodes (APD) High reverse bias voltage 100-200V. High internal field avalanche multiplication. G 100 (J.P. Pansart, NIM A 387 (1997), 186) III/19
Photo Detectors Visible Light Photo Counter VLPC Si:As impurity band conduction avalanche diode 50 mev CB VB impurity band Hole drifts towards highly doped drift region and ionizes a donor atom free electron. Multiplication by ionization of further neutral donor atoms. M. Wayne, NIM A387 (1997) 278 Operation at low bias voltage (7V) High IR sensitivity Device requires cooling to LHe temperature. Q.E. 60% around 500 nm. Gain up to 15.000! Successfully operated in Fermilab E835 fiber tracker. Under test for D0 fiber tracker (72.000 fibers) III/20
Photo Detectors Hybrid photo diodes (HPD) = photo cathode + p.e. acceleration + silicon det. (pixel, strip, pads) proximity focusing (i.e. no focussing) cross focusing V V Photo cathode like in PMT, V 10-20 kv G ev W Si 20 kev 3.6 ev 5 10 3 (for V =20 kv) Single photon detection with high resolution Commercial HPD (DEP PP0270K) with slow electronic (2µs shaping time) (C.P. Datema et al. NIM A 387(1997) 100 Poisson statistics with n =5000! Background from electron backscattering from silicon surface III/21
Photo Detectors Imaging with HPD s Imaging with Silicon Pixel Array Electronic chip bump bonded to silicon detector, inside vacuum tube. (RD7/RD19, IEEE Trans. Nucl. Sc. Vol 42, No. 6 (1995) 222) III/22
Photo Detectors Imaging with HPD s Commercial HPD s (DEP 61 hexagonal pixels) detect Cherenkov rings (C 4 F 10 and aerogel) in LHCb RICH prototype (LHCB technical proposal) Problem: active area fraction still too small! III/23
Photo Detectors Development of a 2048 pad HPD for RICH detectors (LHC-b, CERN, Bologna, CdF) Aim for active area > 80%. Fountain focusing. Demagnification 2.4 (LHCb 98-007, RICH) 5 (127 mm) Bialkali photocathode 50 mm 40 pins (vacuum feedthroughs) Silicon detector 16 x 128 = 2048 pads (1x1 mm 2 ) analogue electronics (16 VA3 chips) inside tube III/24
Scintillating fiber tracking Scintillating fiber tracking Scintillating plastic fibers 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 raed out) first level trigger III/25
Scintillating fiber tracking Charged particle passing through a stack of scintillating fibers (diam. 1mm) Hexagonal fibers with double cladding. Only central fiber illuminated. 3.4 µm 60 µm Low cross talk! (H. Leutz, NIM A 364 (1995) 422) III/26
Scintillating fiber tracking UA2: Scintillating fiber tracker + preshower 1050 550 100 lead fibers Carbon fiber cylinder fibers 60.000 fibers Ø1mm (axial + stereo) readout via 2 demagnifying + 1 proximity focused image intensifiers onto CCD s. (32 readout systems) Carbon fiber cylinder IP (H. Leutz, NIM A 364 (1995) 422) 3 Image intensifiers. Photocathodes + Phosophor anodes 2.3 p.e. / fiber / m.i.p. tracking resolution (intrinsic) 150 µm III/27
Scintillating fiber tracking CHORUS scintillating fiber tracking (PPE-97-033, PPE-97149) 63.000 fibers Ø0.5mm readout via 58 image intensifiers chains 550 x 228 pixels 3 p.e. / mm fiber σ = 140 µm 2 track resolution 500 µm III/28
Nuclear emulsions The rebirth of nuclear emulsions Emulsion: Silver halide crystals (AgBr, AgCl) embedded in gelatine substrate. Grain size: 0.1-1 µm ρ emulsion 3.8 g/cm 3, X 0 = 2.9 cm Ionization (partly) reduces AgBr/Cl to metallic silver. Emulsion development: complete reduction, preferably at places where reduction has already been started by ionization. Fixation: remove remaining AgBr/Cl de/dx(β) darkness of the tracks Multiple scattering 1/p Particle Identification Emulsions offer submicron spatial resolution. 3-D track reconstruction possible by tomographic analysis of the data. But: Manual or semi-automatic scanning of data is too slow for big surfaces and high particle multiplicities. Hybrid experiments combine emulsions for precision tracking with normal electronic tracking. Electronic tracker defines region of interest on emulsion. III/29
Nuclear emulsions Example CHORUS (PPE-97-033, PPE-97149) Fiber tracker + 2 bulk emulsion stacks (1.4 x 1.4 m 2, 2.75 cm thick) magnet calorimeter µ spectrometer 15 m emulsion III/30
Nuclear emulsions Emulsion to beam Track selector ~100 µm Good tracks appear as dots Tracking implies connection of dots in different layers automatic microscope CCD TOMOGRAPHIC IMAGE TO VIDEO PROCESSOR reconstruction by hardware video processor (1 3 sec/event) 50 focal depth 5m (one view 512 512 pixels 2 120 150m ) frame (formed by adjacent views) emulsion track ( matching grains) III/31
Nuclear emulsions The track selector (University of Nagoya, Japan) (S. Aoki et al, NIM AB 51 (1990) 466) III/32
Nuclear emulsions Calibration of µ flux associated with the CERN ν beam (very high track density) Digitised track segments in emulsion sheets New hybrid proposals, based on emulsion technique TENOR, TOSCA: NOMAD + UA1 magnet. Emulsions in B- field OPERA (long baseline, Gran Sasso) III/33
Liquid Argon Image Chamber The Liquid Argon Image Chamber - An Electronic Bubble Chamber (NIM A 332 (1993), 395; NIM A 345 (1994), 230) 3-dimensional LAr Time Projection Chamber Developed by the ICARUS collaboration 3 stage program: 3 tons prototype detector, working at CERN 600 tons prototype, under construction final goal: 5000 tons detector for proton decay search and neutrino physics at Gran Sasso laboratory Ionization signals in Liquid Argon No charge multiplication process relatively small signals to be detected: dq/dx 5000 e - /mm Small drift velocity: v drift 2mm/µs (saturated) High purity required: O 2 < 0.1 ppb for electron life times > 2.5 ms III/34
Liquid Argon Image Chamber Ionization chamber I d v - v + V dw ee( v v) dt VIdt I e v v ) / d ev / d Q( x) ( d Idt x I v dx e( d x) / d + Charges outside the chamber are invisible! - No spatial resolution! - Signal depends on distance from anode! III/35
Liquid Argon Image Chamber 3D Imaging Chamber (TPC) E = 1 kv/cm screen grid and T=0! E = 5 kv/cm 1st sense wire grid "y" induction x y 2nd sense wire grid "x" collection z x-y from wire grids z from drift time signals independent of track distance III/36
Liquid Argon Image Chamber III/37
Liquid Argon Image Chamber An electromagnetic cosmic ray shower in the ICARUS 3-tons prototype 40 cm LAr, x-wires 50 cm LAr 400 µs drift time III/38
Liquid Argon Image Chamber ICARUS 50 liter prototype in the CERN WBνB π 0 2γ 2(e + e - ) Second π 0 difficult to see! drift time 0-800 µs A ν induced reaction with 2 π 0 decays. x-wires (1-128) y-wires (1-128) stopping π p drift time 0-800 µs High multiplicity ν event with stopping π. x-wires (1-128) y-wires (1-128) III/39