Scintillation Detectors Particle Detection via Luminescence

Transcription

1 Scintillation Detectors Particle Detection via Luminescence

2 Scintillators General Characteristics Principle: de/dx converted into visible light Detection via photosensor [e.g. photomultiplier, human eye...] Main Features: Sensitivity to energy Fast time response Pulse shape discrimination Plastic Scintillator BC412 Requirements High efficiency for conversion of exciting energy to fluorescent radiation Transparency to its fluorescent radiation to allow transmission of light Emission of light in a spectral range detectable for photosensors Short decay time to allow fast response

3 Scintillators Basic Counter Setup Thin window Mu Metal Shield Iron Protective Shield Light Photomultiplier [or other photosensor] PMT Base [voltage divider network etc.] Scintillator Photosensors Scintillator Types: Organic Scintillators Inorganic Crystals Photomultipliers Gases Micro-Channel Plates Hybrid Photo Diodes Visible Light Photon Counter Silicon Photo Multipliers PMT Pulse F--? Output Signal Time [ns]

4 Inorganic Crystals Materials: Sodium iodide (NaI) Cesium iodide (CsI) Barium fluoride (BaF2)... Mechanism: Energy deposition by ionization Energy transfer to impurities Radiation of scintillation photons Time constants: exciton band impurities [activation centers] scintillation [luminescence] quenching excitations conduction band electron hole traps valence band Energy bands in impurity activated crystal showing excitation, luminescence, quenching and trapping Fast: recombination from activation centers [ns... μs] Slow: recombination due to trapping [ms... s]

5 Inorganic Crystals Crystal growth Example CMS Electromagnetic Calorimeter PbW04 ingots One of the last CMS end-cap crystals

6 Inorganic Crystals Time Constants Light Output Exponential decay of scintillation can be resolved into two components... N = Ae t/τ f + Be t/τ s τ f : decay constant of fast component τ s : decay constant of slow component Time

7 Inorganic Crystals Light Output Scintillation Spectrum for NaI and CsI Intensity [a.u.] NaI(Tl) CsI(Na) CsI(Tl) Wavelength [nm] Strong Temperature Dependence [in contrast to organic scintillators]

8 Inorganic Crystals Light Output Spectral sensitivity

9 Scintillation in Liquid Nobel Gases Materials: Helium (He) Liquid Argon (LAr) Liquid Xenon (LXe)... A Excitation A* Decay time constants: Helium : τ1 =.02 μs, τ2 = 3 μs Argon : τ1.02 μs Excited molecules Collision [with other gas atoms] A2* De-excitation and dissociation A A UV LAr : 130 nm LKr : 150 nm LXe : 175 nm Ionization A* Ionized molecules A2 + A2* e Recombination

10 Inorganic Scintillators Properties Scintillator material Density [g/cm 3 ] Refractive Index Wavelength [nm] for max. emission Decay time constant [μs] Photons/MeV NaI xxx NaI(Tl) xxx CsI(Tl) xxx Bi4Ge3O xxx CsF xxx LSO xxx PbWO xxx LHe / xxx LAr * / xxx LXe * / xxx * at 170 nm

11 Inorganic Scintillators Properties Numerical examples: NaI(Tl) PBWO4 Scintillator quality: λmax = 410 nm; hν = 3 ev photons/mev = τ = 250 ns λmax = 420 nm; hν = 3 ev photons/mev = 200 τ = 6 ns Light yield εsc fraction of energy loss going into photons e.g. NaI(Tl) : photons; 3 ev/photon εsc = ev/10 6 ev = 11.3% PBWO4: 200 photons; 3 ev/photon εsc = ev/10 6 ev = 0.06% [for 1 MeV particle]

12 Organic Scintillators Aromatic hydrocarbon compounds: e.g. Naphtalene [C10H8] Antracene [C14H10] Stilbene [C14H12]... Very fast! [Decay times of O(ns)] Naphtalene Scintillation is based on electrons of the C = C bond... Antracene π bond Scintillation light arises from delocalized electrons in π-orbitals... Transitions of 'free' electrons... Two pz orbitals

13 Organic Scintillators Molecular states: Absorption in 3-4 ev range Singlet states Triplet states Fluorescence in UV range [~ 320 nm] usage of wavelength shifters Fluorescence : S1 S0 [< 10-8 s] Phosphorescence : T0 S0 [> 10-4 s]

14 Organic Scintillators Transparency requires: Shift of absorption and emission spectra... Shift due to Franck-Condon Principle Intensity Energy Stokes-Shift Absorption Emission S1 Excited State λ Excitation into higher vibrational states De-excitation from lowest vibrational state S0 Ground State Excitation time scale : s Vibrational time scale : s S1 lifetime : 10-8 s Vibrational States Nuclear distance

15 Plastic and Liquid Scintillators In practice use... solution of organic scintillators [solved in plastic or liquid] + large concentration of primary fluor + smaller concentration of secondary fluor +... Scintillator requirements: Scintillator array with light guides Solvable in base material High fluorescence yield Absorption spectrum must overlap with emission spectrum of base material LSND experiment

16 Plastic and Liquid Scintillators A Energy deposit in base material excitation Solvent Primary fluorescent - Good light yield... - Absorption spectrum matched to excited states in base material... B Secondary fluorescent C Wave length shifter Excitations S1A γa Primary Fluor S1B γb Secondary Fluor S1C γc S0A S0B S0C

17 Plastic and Liquid Scintillators Some widely used solvents and solutes POPOP Polystyrene p-terphenyl

18 Wavelength Shifting Principle: Absorption of primary scintillation light Re-emission at longer wavelength Schematics of wavelength shifting principle Adapts light to spectral sensitivity of photosensor Requirement: Good transparency for emitted light

19 Organic Scintillators Properties Scintillator material Density [g/cm 3 ] Refractive Index Wavelength [nm] for max. emission Decay time constant [ns] Photons/MeV Naphtalene xxx Antracene xxx p-terphenyl xxx NE102* xxx NE104* xxx NE110* xxx NE111* xxx BC400** xxx BC428** xxx BC443** xxx * Nuclear Enterprises, U.K. ** Bicron Corporation, USA

20 Organic Scintillators Properties

21 Organic Scintillators Properties Light yield: [without quenching] dl dx = L de 0 dx Quenching: non-linear response due to saturation of available states Birk's law: dl dx = L 0 de dx 1+kB de dx [kb needs to be determined experimentally] Also other... parameterizations... Response different... for different particle types...

22 Scintillators Comparison Inorganic Scintillators Advantages high light yield [typical; εsc 0.13] high density [e.g. PBWO4: 8.3 g/cm 3 ] good energy resolution Disadvantages complicated crystal growth large temperature dependence Expensive Organic Scintillators Advantages very fast easily shaped small temperature dependence pulse shape discrimination possible Disadvantages lower light yield [typical; εsc 0.03] radiation damage Cheap

23 Scintillation Counters Setup Scintillator light to be guided to photosensor Light guide [Plexiglas; optical fibers] Light transfer by total internal reflection [maybe combined with wavelength shifting] Liouville's Theorem: Complete light transfer impossible as Δx Δθ = const. [limits acceptance angle] Use adiabatic light guide like 'fish tail'; appreciable energy loss 'fish tail'

24 Scintillation Counters Setup Photomultiplier WS fibre Wavelength-shifting fibre ATLAS Tile Calorimeter Scintillator Steel Source tubes

25 Photon Detection Purpose : Convert light into a detectable electronic signal Principle : Use photo-electric effect to convert photons to photo-electrons (p.e.) Requirement : High Photon Detection Efficiency (PDE) or Quantum Efficiency; Q.E. = Np.e./Nphotons Available devices [Examples]: Photomultipliers [PMT] Micro Channel Plates [MCP] Photo Diodes [PD] HybridPhoto Diodes [HPD] Visible Light Photon Counters [VLPC] Silicon Photomultipliers [SiPM]

26 Photomultipliers Principle: Electron emission from photo cathode Secondary emission from dynodes; dynode gain: 3-50 [f(e)] Typical PMT Gain: > 10 6 [PMT can see single photons...] PMT Collection

27 Photomultipliers Photocathode γ-conversion via photo effect... Bialkali: SbRbCs; SbK2Cs Photon entrance window photo cathode 4-step process: Electron Electron generation via ionization Propagation through cathode Escape of electron into vacuum Q.E % [need specifically developed alloys]

28 Photomultipliers Dynode Chain Dynodes Electron Anode UB Voltage divider Multiplication process: Electrons accelerated toward dynode Further electrons produced avalanche Secondary emission coefficient: δ = #(e produced)/#(e incoming) Typical: δ = 2 10 G = δ n = 8 15 n = Gain fluctuation: δ = kud; G = a0 (kud) n dg/g = n dud/ud = n dub/ub

29 Photomultipliers Dynode Chain Optimization of PMT gain Anode isolation Linearity Transit time B-field dependence Venetian blind Box and grid Linear focused PM s are in general very sensitive to B-fields! Even to earth field (30-60 μt). μ-metal shielding required. Circular focused

30 Photomultipliers Energy Resolution Energy resolution influenced by: Linearity of PMT: at high dynode current possibly saturation by space charge effects; IA nγ for 3 orders of magnitude possible... Photoelectron statistics: given by poisson statistics. P n (n e )= nn e e n e n! n e = de dx Photons MeV η Q.E. σ n /n =1/ n e η = 0.2; Q.E. =0.25 σ n /n =0.7% Secondary electron fluctuations: with ne given by de/dx... For NaI(Tl) and 10 MeV photon; photons/mev = 40000; light collection efficiency n e =20000 P n (δ) = δn e δ n! σ n /n =1/ δ with dynode gain δ; and with N dynodes... σn n 2 = 1 δ δ N 1 δ 1 σn/<n> dominated by first dynode stage important for single photon detection

31 CuBe dynode Photomultipliers Energy Resolution For detection of single photons δ 'standard' dynodes δ Philips photonic GaP(Cs) Philips photonic Negative electron affinity (NEA) E [ev] E [kev] counts 1 p.e. counts 1 p.e. 2p.e. Large δ!... yields better energy resolution Philips photonic CuBe dynode NAE dynode Houtermanns NIM 112 (1973) 121 pulse height pulse height

32 Micro Channel Plate "2D Photomultiplier" Gain: Fast signal [time spread ~ 50 ps] B-Field tolerant [up to 0.1T] But: limited life time/rate capability

33 Chapter 2 Light Detectors ICFA the device is covered with an anti-reflecting SIO2 lay Silicon Photomultipliers Layout Silicon Photomultiplier:SiPM Geiger Mode tracks on the surface connect all pixels to the commo Principle: + n Pixelized photo diodes Pixelsinoperated in Geiger mode operated Geiger Mode (non-linear response) Single pixel works as a binary device Si* Resistor Vbias Al - conductor Si02 + Guard ring n- Photocurrent (log scale) Features: Granularity Gain Bias Voltage Efficiency - EE,, V/cm V/cm 1E5 Energy = #photons seen by summing over all pixels no amplification linear amplification 6 1E E4 10 1E3 2 1E2 10 1E1 1 1E0 non-linear Geiger mode Doping Structure of SiPM [1] * Resistor 2 Al - conductor 103Sipixels/mm : : 106 : < 100 V : ca. 30 % E, V/cm Figure 2.13: Left: Schematic view of the SiPM topology Avalanche region material on the low resistive substrate serves as a drift reg electron generated in this region will subsequently drift i a b layer where the electrical field is high enough for avalanch Breakdown Voltage electrical field in order to avoid unwanted avalanche break 1: (a) Silicon photomultiplier microphotograph, (b) impurity levels are higher. Right: Diagram of the topol electric Works atfigure room temperature! Reverse Bias Voltage Insensitive to magnetic fields tion in epitaxy layer Gain and Single Pixel Response Alexander Tadday

34 Silicon Photomultipliers HAMAMATSU MPPC 400Pixels One of the first SiPM Pulsar, Moscow

35 Silicon Photomultipliers CALICE HCAL Prototype

36 Scintillation Counters Applications Time of flight (ToF) counters Energy measurement (calorimeters) Hodoscopes; fibre trackers Trigger systems ATLAS Minimum Bias Trigger Scintillators Particle track in scintillating fibre hodoscope

37 H1 Spaghetti Calorimeter Scintillator : BICRON BCF-12 Photosensor : Photomultipliers

38 CMS Crystal Calorimeter (ECAL)

39 CMS Crystal Calorimeter (ECAL) Scintillator : PBW04 [Lead Tungsten] Photosensor : APDs [Avalanche Photodiodes] Number of crystals: ~ Light output: 4.5 photons/mev Barrel ECAL (EB) y = = Preshower (ES) z = 2.6 = 3.0 Endcap ECAL (EE)

40 ATLAS Tile Calorimeter Photomultiplier WS fibre Wavelength-shifting fibre ATLAS Tile Calorimeter Scintillator Steel Source tubes

41 CALICE Analogue HCAL Sandwich structure: 1 m 3 - Prototype 38 layers - Scintillator Tiles+WLS+SiPMs (.5 cm) - Stainless steel absorber (1.6 cm)!"#$$%&'%()(*+,%-.//(-+(0 2006/2007 CERN Testbeam [2008/09, Fermilab] 3x3 cm 2 Tile Mounted SiPM Scintillator : Plastic Photosensor : SiPMs

42 CALICE Scintillator ECAL X-Layer [1 x 4 cm 2 ] Tungsten Scintillator layers: 2 mm Tungsten layers: 3 mm Y-Layer [1 x 4 cm 2 ] X/Y-Strips: 1 x 4 cm 2 Granularity: 1 x 1 cm 2 X-Layer [1 x 4 cm 2 ] Readout: MPPC Channels: ~ 10 7 Scintillator SiPM [1600 pixels] SiPM r/o with WLS Particle WLS Fibre