Scintillation Detectors

Transcription

1 Radiation Measurement Systems Scintillation Detectors Ho Kyung Kim Pusan National University Scintillation detector = scintillator + light sensor Scintillators Inorganic alkali halide crystals Best light output & linearity Slow in response Gamma-ray spectroscopy (high-z & ) Organic-based liquids & plastics Less light output Faster in response Beta spectroscopy Neutron detection (due to hydrogen content) Light sensors Photomultiplier tubes Photodiodes 2 1

2 Scintillation light Prompt fluorescence Governing the pulse-mode operation t τ I = I 0 e τ ~ ns Phosphorescence Longer wavelength than fluorescence Slower characteristic time Delayed fluorescence Same wavelength w/ prompt fluorescence but much longer emission time 3 Four steps in radiation detection w/ scintillation detectors Interaction of radiation w/ scintillator Emission of scintillation light by scintillator Light transport in scintillator Light detection by photodetector 4 2

3 Interaction of radiation with scintillator Incident quantum interacts in scintillator, deposits all or part of its kinetic energy Electrons and charged particles interact directly, neutrons and gamma rays indirectly Characterized by Detection Efficiency, ε d ε d = no. of interactions no. of incident radiation Emission of scintillation light by scintillator Excited species in scintillator (created by deposited energy) emit UV or visible light as they de-excite to lower energy state These scintillation photons are emitted in all directions (4 ) Time scale ~ 10-9 to 10-6 seconds (generally, the slowest step) Characterized by Scintillation Efficiency, ε s energy of visible light ε s = = N phhν radiation energy loss ΔE 3

4 Light transport in scintillator Scintillation photons are reflected and scattered until they are absorbed or escape from scintillator Absorption can be either at surface or in the bulk of the scintillator Characterized by Light Collection Efficiency, ε c ε c = escaped light to PMT generated light photons Light detection by photodetectors Scintillation photons reaching photodetector may be converted into electrical charges In PM tubes, charges are electrons emitted from photocathode In photodiodes, charges are electron-hole pairs formed in diode semiconductor volume Characterized by Quantum Efficiency, ε q ε q = no. of photoelectrons emitted no. of incident photons 4

5 Signal size Light yield, Y [#/MeV] Y = N ph = ε s = 1 E hν W sc W sc = scintillation W-value [ev] Number of signal charges (or electrons) in the photodetector N e = N ph ε c ε q = E ε hν s ε c ε q = E Y ε c ε q = W eff = 1 Y ε c ε q = W sc ε c ε q E W eff Material NaI(Tl) CsI(Tl) CWO W sc (ev) Y (#/MeV) 38,000 54,000 13,000 λ (nm) Requirements in scintillation material High detection efficiency High Z & materials (for gamma-ray detection) High scintillation efficiency Linear energy-to-light conversion (LY E) Short decay time of luminescence Fast signal pulse & small afterglow Transparent to emitted wavelengths (for good light collection) Similar refractive index w/ glass (~1.5) (for good coupling efficiency) Availability in large size Favorable material properties and cost 10 5

6 Organic scintillators Transitions in the energy level structure ( -electron structure) of a single molecule Independent of physical states (e.g. anthracene in solid, vapor & liquid) 11 From Wikipedia 12 6

7 Optical spectra 13 Types of organic scintillators Pure organic crystals Liquid organic solutions Plastic scintillators Thin film scintillators Loaded organic scintillators 14 7

8 Light output Nonlinear response to HCP s (protons, alphs) Response to fast electrons Common plastic scintillator Liquid scintillator NE Light yield per path length Birk s formula (to fit data for both electrons and HCP s) dl = dx ε de sdx 1+kB de dx dl = fluorescent energy per unit path dx ε s = scintillation efficiency de = linear energy transfer dx B = proportionality constant B de = density of damaged molecules per track length dx k = quenching (no light) rate due to damaged molecules dl = dx de ε s dx 1+kB de de +C dx dx empirically fitted parameter, C 2 An extended version of Birk s formula w/ an additional 16 8

9 Time response I = I 0 e t τ e t τ 1 τ 1 = time const. describing the population of the optical levels ( ns) τ = time const. describing their decay (2 3 ns) 17 Slow components in liquid organic scintillators Stilbene 18 9

10 Inorganic scintillators 19 Types of inorganic scintillators Alkali halide crystals NaI(Tl), CsI(Tl), CsI(Na) Most typical Zinc sulfide: ZnS(Ag) Thin screens for alpha detection Busmuth germanate (BGO) Highest Z of common scintillators Detectors for PET Barium Fluoride: BaF 2 Fast UV component Glass scintillators Lithium formulations for neutron detection 20 10

11 Thallium-activated sodium iodide, NaI(Tl) Highest known light yield (when measured with PM tube) Primary decay time, = 230 ns Hygroscopic and fragile Some long-lived phosphorescence = 3.61 g/cc 21 Linearity of light output Scintillation response per unit energy deposited by fast electrons (normalized to unity at 445 kev) Perfect proportional behaviors 22 11

12 Temperature dependence Due to charges in surface reflectivity 23 Other alkali halide crystals CsI(Tl) High gamma-ray absorption, = 4.51 g/cc 45% LY of NaI(Tl), = 1 ms Good matched emission spectrum w/ photodiode CsI(Na) High light yield, 85% LY of NaI(Tl) = 630 ns Some slow decay component ZnS(Ag) 130% LY of NaI(Tl), = 110 ns, = 4.09 g/cc CWO (CdWO 4 ) 30 50% LY of NaI(Tl), = 8.0 g/cc, = 14 ms LiI(Eu) Slow ( = 1.4 ms) decay time, low light yield Useful for neutrons ( 6 Li) 24 12

13 Relative light output Bismuth germanate (BGO) High Z-value of Bi(83) leads to large photoelectric cross section and high full-energy absorption = 7.13 g/cc Low light yield [15~20% of NaI(Tl)] leads to poorer energy resolution = 300 ns 25 Time 26 13

14 Barium fluoride (BaF 2 ) Z of Ba = 56 (good efficiency) Slow (630 ns) visible = 4.88 g/cc Total light yield = 20% of NaI(Tl) Fast 3%, slow 17% Fast (0.6 ns) UV 27 Fast inorganic scintillators Material max (nm) (ns) Relative ε s [% of NaI(Tl)] CsI (pure) CeF LaF 3 (Nd) PbSO , 26, YAlO 3 :Ce Lu 2 (SiO 4 )O:Ce

15 Lutetium oxyorthosilicate (LSO, cerium-activated) ~75% LY of NaI(Tl) should favor good energy resolution = 40 ns allows fast timing and/or high counting rates Z eff = 66 and density of = 7.4 g/cc ensure high photoelectric interaction probability for gamma rays Some inherent background due to 176 Lu Cost and availability??? 29 Glass scintillators Li formulations predominate useful for neutron detection Cerium activated, = ns Good environmental immunity Some Th-induced background Also available as < 1 mm dia. optical fibers or fiber bundles 30 15

16 Scintillation fibers Light rays arriving at scintillator/air interface with incidence angle greater than the critical angle for total internal reflection are piped down the length of the fiber For refractive index of 1.5 surrounded by air, 33% of light is piped toward either end In typical glass (Eu-doped) or plastic scintillators, reabsorption is of interest 31 Scintillation gases Primarily noble gases : Xe, Kr, Ar, He,... Fast decay time (few ns) Low light yield (often in UV) 32 16

17 Comparisons btwn organic and inorganic scintillators Type Organic Inorganic Example Plastic, liquid, polycrystalline Alkali halides, other compounds Z Low Moderate to high ε s Low (~3%) High (up to 65,000 photons/mev) Linearity Nonlinear More linear Fast (~ns) Slow (~ms) but some are fast (~ns) 33 Light collection For higher energy resolution, higher light collection efficiency is required Light loss mechanisms Self-absorption Loss at surfaces Light-Tight Envelope Glass Window Reflector Optical Coupling 34 17

18 Effects of light collection on the energy resolution The statistical broadening of the response function worsens as # of scintillation photons that contribute to the measured pulse is reduced The uniformity of light collection determines the variation in signal pulse amplitude as the position of the radiation interaction is varied throughout the scintillator Often for larger scintillators, the energy resolution or spectrum broadening is determined by the non-uniformity of light signal originated from the scintillation near the surfaces 35 For refractive indices n 0 > n 1 Tot. internal reflection if incidence angle θ > θ c Partial reflection (called Fresnel reflection) & partial transmission if θ > θ c Critical angle, θ c = sin 1 n 1 n 0 Escape (or reflection) Internal reflection Material Air Glass Anthracene Plastic NaI(Tl) CsI(Tl) BGO CWO ZnS(Ag) BaF 2 GSO LSO Refractive index

19 Surfaces of scintillators Reflection surface High reflection required n out > n in Diffusive surface is better than mirror reflection Reflectors: white paint, Teflon Escape surface High transmission required Two cases of finish: glass window, bare crystal Scintillator + (glass) + epoxy + PMT surface (glass) n glass n epoxy n scintillator 37 Light pipes To match geometry of PMT with scintillators To randomize light across the PMT surface Scintillator PM Tube PM Tube 38 19

20 Effects of reflective wrappings d. Specular reflector w/o light guide b. Tot internal reflection w/ reflective covering a. Tot internal reflection c. Surface painted w/ NE560 reflector paint e. Diffuse reflector w/o light guide 39 Internal reflection Fraction of light "piped" (or internally reflected) along the rod length in one direction in cylinder F = Ω 4π = 1 4π φ=φ c dω = 1 φ=0 4π φ c 2π sin φ dφ = cos φ 2 c = 1 1 sin θ 2 c = 1 1 n 1 2 n0 F = % for n 0 = 1.5 & n 1 = 1 In slab geometry Tot. escaping fraction: E = 1 1 n 2 1 n0 Fraction of light trapped: F = 1 E = 1 n 2 1 n0 F = 75% for n 0 = 1.5 & n 1 = 1 The other cone w/i c 40 20

21 Photomultiplier tubes 41 Photocathode Need thin material for electron to escape Metals & semiconductors About 100 ev per photoelectron due to 25 30% of quantum efficiency & typical scintillation efficiency Inferior to gas-filled or semiconductor detectors E e = hν φ hν = ~3 ev (blue light) φ = ev (work function) 42 21

22 no. of photoelectrons emitted Quantum efficiency, QE = no. of incident photons Designation Composition Peak QE (%) peak (nm) S1 S10 S11 S20 (multialkali) Bialkali Bialkali (high temp.) AgOCs BiAgOCs Cs 3 SbO Na 2 KSbCs K 2 CsSb Na 2 KSb QE & spectral response The use of silica or quartz windows is necessary to extend the response into the UV region 44 22

23 45 Secondary electron emission Multiplication factor for a single dynode δ = no.of secondary electrons emitted primary incident electron V NEA material Standard dynode material 46 23

24 Multiple stage multiplication overall gain = αδ N for N stages α = the fraction of all photoelectrons collected by the multiplier struture Typically, α 1 & δ = 5 If N = 10, gain = gain = V 6 V 9 47 Statistics of electron multiplication Conventional dynodes NEA material 48 24

25 Pulse timing properties Max voltage between the photocathode & first dynode should be applied for min t t is minimized also by max number of photoelectrons t = 1 10 ns ns 49 Electron transit from photocathode to first dynode Differences in paths are critical in the spread in transit time Curved to minimize the spread in transit time Electrodes carrying adjustable voltage that electrostatically focus the electrons for optimal performance 50 25

26 Timing resolution in scintillation detectors Timing will be best from scintillators with fastest decay time and greatest light yield. For very fast scintillators (few ns decay time or less), PM tube may also contribute to overall timing resolution. NaI(Tl) BGO Plastic BaF 2 Decay time (ns) Timing resolution (ns) Typical wiring diagrams for the base of a PM tube 52 26

27 Ancillary equipment High voltage power supply Stability Current capacity Voltage divider Magnetic shielding Mu-metal Preamplifier 53 PMT operational characteristics Gain vs. Voltage Linearity Noise and Spurious pulses Thermionic emission Photocathode non-uniformity Gain variation with rate - fatigue 54 27

28 PMT dark current Spontaneous thermionic emission at room temp. from photocathode e - 's cm -2 s -1 Can be reduced by cooling At a rate of 10 3 e - /s, avg. time btwn emitted electrons is 1 ms. In pulse mode operation, each gives rise (after multiplication) to a very small "single electron" pulse that is easily discriminated. In current mode, corresponding typical dark current at anode w/ multiplication of 10 6 : Idark = (10 3 e - /s) (10 6 ) ( C/e - ) = 160 pa 55 Scintillation pulse shape analysis PMT anode circuit (simple parallel RC circuit) i t = i 0 e λt λ = scintillator decay const. Q = i t dt 0 = i 0 e λt 0 dt = i 0 λ Therefore, i t = λqe λt KCL: i t = i C + i R = C dv(t) + V(t) dt R First-order inhomogeneous DE: dv(t) + 1 V t = dt RC W/ I.C. V 0 = 0; V t = 1 λq λ θ C e θt e λt λq C e λt θ = 1 the reciprocal of the anode time const. RC 56 28

29 Large RC (θ λ or RC τ) V t Q C e θt e λt V t Q 1 C e λt, t 1 (leading edge) θ» Rise time by V t Q C e θt, t 1 (pulse tail) λ» Decay by RC Small RC (θ λ or RC τ) V t λ Q θ C e λt e θt Very small pulse height V t λ Q 1 θ C e θt, t 1 λ (leading edge) V t λ θ Q C e λt, t 1 θ (pulse tail) 57 Effect of the "discrete" nature of the anode current due to discrete photoelectrons 58 29

30 Configurations of PM tubes Focused linear structure Venetian blind Circular grid Box-and-grid 59 Continuous channel electron multiplier Microchannel plate electron multiplier 60 30

31 Position-sensing photomultiplier tubes 61 Silicon photomultiplier (SiPM) 62 31

32 Photodiodes 63 Quantum efficiency Si PD 64 32

33 Temperature dependence of I leak for Si PD's 65 Comparison w/ PMT's Advantages High quantum efficiency Compact & rugged Low applied voltage Insensitive to magnetic fields Disadvantages Low signal amplitude (no gain) Increasing preamplifier noise w/ area & capacitance 66 33

34 Conventional photodiodes Convert scintillation photon (2 3 ev) to e-h pair Produce ~10 4 to 10 5 electronic charges per typ. scintillation pulse No internal charge gain Low signal-to-noise ratio Poorer energy resolution than PMT's for NaI Better for CsI above few hundred kev Frequently used in current mode 67 Avalanche photodiodes (APD's) Region of high E-field created in Si wafer Multiplication of e-h pairs takes place in this region thru electron collisions Analogous to avalanche formation in gas-filled detectors Signal size increased by factor of 100 or more (improving SNR) Problems w/ multiplication uniformity across entrance are have retarded commercial availability 68 34