Scintillation Detectors

Transcription

1 Scintillation Detectors J.L. Tain Instituto de Física Corpuscular C.S.I.C - Univ. Valencia

2 Scintillation detector: SCINTILLATION MATERIAL LIGHT-GUIDE / WAVELENGTH-CONVERTER LIGHT TO ELECTRIC-PULSE TRANSDUCER Simple Versatile Rugged Cheap

3 History: Crookes (1903): ZnS screen + microscope Regener, Crookes (1908): nature of α- particles. + Rutherford, Geiger (gas counter) Geiger, Marsden (1909): angular distribution of scattered α-particles Rutherford (1911): discovery of atomic nucleus Rutherford (1919): discovery of nuclear reactions, 14 N(α,p) 17 O Cockcroft, Walton (1932): coincidence experiment, 7 Li(p,α)α Krebs (1941): photo-sensitive Geiger-Muller counter Curran, Baker (1944): use of photomultiplier + scintillator(zns) Kallman (1947): first organic scintillator (naphthalene) Hofstadter (1948): NaI(Tl) several (1980 s): BGO Laval (1983): fast component of BaF 2 several (1990 s): many new scintillation materials

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5 Scintillation materials Luminescent materials: reemit part of the absorbed energy in the form of light Emissions: fluorescence (prompt), delayed fluorescence and phosphorescence (delayed, different wavelength) Scintillation material properties: transparency to its fluorescence luminous efficiency light spectral distribution light temporal distribution mechanical and chemical properties Organic materials: Crystals Liquids Plastics Inorganic crystals Glasses Gasses

6 Luminescence in organic materials Several time components Material transparent to its own light

7 Luminescence in inorganic materials Several mechanisms have been identified: luminescence of doping centers, self-activated luminescence and cross-luminescence

8 The non-radiative transfer mechanism between excited centers induces an energy-loss dependent light production Simple parameterization: dl dx = de A dx de 1+ B dx Birk s formula

9 Both effects can be used to identify particles As a consequence there is a particle type and energy dependence of scintillation pulse shape and light output

10 Light yield vs. temperature Emission spectra

11 Properties of some inorganic scintillation crystals Density (g/cm 3 ) Wavelength at max.(nm) Refractive index Decay time (ns) Light yield (ph/mev) NaI(Tl) CsI(Tl) , ,25000 Bi 4 Ge 3 O BaF , , ,9500 CeF , , YAlO 3 (Ce) Lu 2 SiO 5 (Ce) LaBr 3 (Ce) Plastic BC

12 Properties of some organic scintillation plastics wavelength shifters è

13 Light collection and transmission n Snell law: i sin θ = n i t sin θ t R para Fresnel formulae: tan ( θi θt ) ( θ + θ ) sin 2 2 =, Rperp = 2 2 tan i t sin ( θi θt ) ( θ + θ ) i t Reflector: mirror-like: θ r = θ i diffuse: θ r random Light-guide Optical contact

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15 PMT window, photo-cathode and focusing electrodes: spectral sensitivity, gain, energy resolution and time resolution window transmission quantum efficiency

16 Dynode secondary emission ratio δ kv D PMT dynode structure: gain & time resolution PMT gain G δ ΔG G n n ΔV V D D

17 PMT base - voltage divider: linearity

18 Semiconductor photo-sensors Si Photo-Diodes (PD): very small current proportional to photon intensity

19 Avalanche Photo-Diodes (APD): initial current amplified by avalanche process, still proportional to initial photon intensity

20 Very high amplification, current independent from initial photon intensity (Geiger mode) Silicon Photomultipliers (Si-PMT, SPM, MPPC, DAPD, ) single photon counting 42 µm 1 mm 20 µm 24*24=576 pixels N hits = N 1 cells ( 1 ( P ) N photons ) cell

21 Scintillation detector energy resolution: 1. Light yield variations of scintillation material: statistical (intrinsic) and non-proportionality of light production 2. Light collection variations: statistical, geometrical dependency 3. Light conversion variations: statistical (quantum efficiency), non-uniformity of photo cathode 4. Electron multiplication and collection variations (single photon response) 5. Read-out electronic noise Scintillation detector linearity: 1. Light yield linearity 2. Electron multiplication linearity 3. Read-out electronic linearity Scintillation detector noise: 1. Scintillation material activity and after-glow 2. Photo-multiplier dark current 3. Read-out electronic noise

22 Can we predict/influence the expected resolution? The energy resolution in an scintillation detector for γ-rays will be given by the statistical fluctuations in: 1. the energy of secondaries generated (non-proportionality) 2. the number of scintillation photons per energy deposited (intrinsic) 3. the fraction of photons collected at the photocathode (transport, absorption) 4. the fraction of photo-electrons collected at the 1st dynode (quantum efficiency, collection) 5. the PMT gain (multiplication, collection) 6. the electronic noise FWHM E = α 2,3 E + β 4,5 + χ 1 ( E)

23 The case of the GSI D. Cano et al. NIMA430 (1999) 333 EXP: FWHM E EXP = E %@1.33MeV NaI(Tl) Light yield non-proportionality χ: FWHM E non prop = E %@1.33MeV β: FWHM E multiplication = %@1.33MeV α: FWHM E photontransfer = E 3.1%@1.33MeV

24 Light yield non-proportionality GEANT3 MC simulation NaI(Tl) non-proportionality R=3.1% IEEE NS-43(96)1271 NIM 12(61)115 Light yield statistics assumed Poisson Y scin LaBr 3 negligible? NaI = 38 ph / kev è 1.0% + Poisson statistics R=3.2% Y scin = 63 LaBr 3 ph / kev è 0.8% R = 2.35 E Y scin

25 Simulation of light transport and collection with Geant4 Influence of geometry, reflector type, surface treatment, refractive index matching, light-guide, Snell law: n i sin θ = i n Fresnel formulae: R tan t sinθ ( θi θt ) ( θ + θ ) 2 2 =, R = 2 2 tan i t sin t sin ( θi θt ) ( θ + θ ) i t Reflector: mirror-like: θ r = θ i diffuse: θ r random Surface state: polished rough

26 Geant4 MC simulations NaI module n NaI = 1.85, n PMT =1.47, n LG = 1.60 λ NaI = 1 m? Specular reflector: ρ=1 Diffusse reflector: ρ=1 (Lambertian) Point source λ abs = 1 m Extended source λ abs = 1 m Extended source λ abs = 20 cm Extended source λ abs = 1 m Light guide n = 1.6 Specular reflector Diffuse reflector 12.5% 16.3% 12.1% 16.5% 4.4% 4.3% 13.7% 18.0% N ph = 10 6

27 Geant4 MC simulation Light transport contribution to the energy resolution NaI module γ E γ = 1.33 MeV, Y scint = 38 ph/kev No Poisson statistics, no non-proportionality LF = N E Y γ cathode scin Specular reflector Difusse reflector R=3.2% R=3.5% tail

28 Photo-electron conversion quantum efficiency Binomial distribution x : success, N : trials, p : probability P(x) = x = Np σ 2 = x 1 p N! ( x! ( N x)! px 1 p) N x ( ) R = QE ( ) N ph QE