Presentation on theme: "INFN and Sapienza-University of Rome Italy"— Presentation transcript:
1 INFN and Sapienza-University of Rome Italy 1th Workshop on“Photo Detection”June , Perugia, ItalyPhotodetector requirements for gamma ray imaging with scintillation crystalsRoberto PaniINFN and Sapienza-University of Rome Italy
3 Individual coupling technique Munich APD PET*4 x 8 APD Array (Hamamatsu Photonics)2 x 2 x 6 mm3 LSO individual coupledIntrinsic FWHM ~ 1.2 mm* Courtesy of Roger Lecomte – Université de Sherbrooke (Québec, Canada)
4 Individual coupling technique High packing fraction > 80%Spatial resolution limited by crystal pixel size ( 1mm tomography,> 1mm planar image)Electronic readout up to chains (SPET)Single photoelectron readout not neededLow noise to allow 140 keV photon energy detectionHigh gain (104 or more) not neededEnergy resolution depending on scintillation crystal / photodetector
5 Light sharing technique Scintillation light flash onphotocathodePosition:Energy:Si i niSi niX =E = Si niX & Y Position Centroid AlgorithmAnode array (Hamamatsu H8500)Charge distribution samplingby anode array12345678k… i
6 Position determination in light sharing technique one γ-ray interactionScintillation light PSF mm FWHMImage PSF 1mm FWHMMany γ-ray interactionsPosition linearityCo57 pulse height analisys
7 Light sharing technique Spatial resolution limited by crystal pixel size ( scintillation array)Spatial resolution not limited for continuous crystalLow number of electronic chainsSingle photoelectron readout neededEnergy resolution depending on scintillation crystal /photodetectorHigh gain ( >104 ) is neededTiming/rise time < 500 ps for ToF
8 Point Spread Function and critical angle c Planar crystal / PMT glass windowPixellated crystal / PMT glass windowLight output angle < 45°
9 Pixellated scintillation crystal NaI:Tl1m m x 1mm x 4 mm+H8500 MAPMTPixel Spatial resolution < 1.3 mmPoor energy resolution ~ 14%Image Spatial resolution > 1.3 mm
10 Continuous scintillator crystal 1.5 mm step scannig – 0.4 mm Ø Tc99m point sourceLaBr3:Ce49 mm x 49 mm x 4 mm+ 3 mm glass windowH8500 MAPMTBest Values:Energy resolution = 9.6 % 1000V)Overall Spatial Resolution= 1.1 mmIntrinsic Spatial Resolution= 1.0 mmVery good linearity !!!
11 Modulation Transfer Function MTF for Continuous CrystalSpatial Resolution limited to LEGPEnhancement in Contrast - increased AUC (Area Under Curve)NO restrictions in image digitization (Nyquist frequency not limited from image pixel)Continuous position responseIncreased detection efficiencyDetector assembly:MAPMT Hamamatsu H8500LEGP collimator(1.5 mm hole, 22 cm lenght)Multi-anode read-outCrystal samples:LaBr3:Ce continuous, 5mm thickNaI:Tl array, 1.1mm pixel 1.3 pitch
12 Scintillation crystal: requirements for SPECT keV)Z 40 → Photofraction greater than 70%High density (> 3 gr/cc) → Reduction of crystal thickness to obtain80-90% efficiency ( important for light collection)Refraction index close to 1.5 → To avoid light loosing due to criticalangle (continuous crystal)Decay time 1 ms→ To obtain 200 kHz max.High luminous efficiency (> at suitable wavelength) →To improve: Decoding crystal pixel in scintillation array Spatial resolution, in continuous crystal Energy resolution.Low afterglow for high counting rateThere are few predictions if energy resolution or light output dominates the intrinsic spatial resolution in light sharing
14 Scintillation crystal: requirements for PET (@ 511 keV) Z 50 → Photofraction greater than 30%High density ( >7 gr/cc) → To obtain, in 30 mm crystal length, 50%coincidence efficiency and reduction parallax error for small animalimaging.Scintillation decay time 300 ns → To allow good coincidencetime resolution. Time resolution better than 0.5 ns can reducerandom coincidences (50 % in a 3D PET) and time of flight canbe realized.High luminous efficiency > 8000 ph/MeV → To enable block detectors with a greater number of pixel (from8 8 BGO to 16 16 LaBr3(Ce) crystal pixel/module). Improvement in energy resolution reduces scatter background (25%Compton scattering / 25% “true” events in a 3D PET).Low afterglow for high counting rate
15 Scintillator crystals for PET Density(g/cm3)Emiss. max(nm)Z eff.Photo-fraction(%)Light yield(ph/MeV)Decay time(ns)Relative coinc. efficiencyCoinc. timing res.(ps)ΔE/E@511 keV(PMT)BGO7.148083439,000300100%300010%Lu2SiO5:Ce(LSO)7.4420653426,0004090%Lu2(1-x)Y2xSiO5:Ce(LYSO)54-30,00011%LaCl3:Ce3.8635049.51546,00020 (65%)36%2654%LaBr3:Ce5.0738047.41463,00016 (97%)42%2603%LuI3:Ce5.604725352990,0003073%200<15%
16 Statistical generation of the signal Energy ResolutionIntrinsic Scintillation ContributeNon homogeneitiesNon proportionality of scintillation responseElectronic noisePhotodetector and preamplifier system[Equivalent noise charge– E. Gatti, NIM Phys Res 1990 ]Statistical generation of the signalNph: number of photons in a scintillation flasha : worsening of the Poisson behaviourh : Quantum EfficiencyPMTP-I-NAPDSSDSiPM1.2512 1 at pk 30% 80%> 80% 60%M 5 105< 103ENC/M 03702025where the 1st term is the intrinsic resolution of the scintillator, function of several factors among which the inhomogeneity of the material and non-proportionality of the scintillation response.The 2nd term is the statistical spread affecting the signal generation. It depends on the statistics of the scintillation photon generation, of the electron generation in the photodetector material and, where it applies, of the electron multiplication. Nph is the number of generated photons in the scintillation flash and η is the quantum efficiency (QE) of the photodetector. α accounts for the worsening of the pure Poisson statistics, due to the nature of the electron multiplication process within the photodetector (α=1 if there is a multiplication gain M=1). The third term in the formula above is the statistical spread due to the electronic noise of the photodetector-preamplifier system. The term ENC is the electronic noise (in term of equivalent noise charge) of the photodetector-preamplifier system, as defined by E. Gatti . It depends on several contributions, among which output capacitance and leakage current of the photodetector, shaping type and time constant.
17 Intrinsic Scintillator Energy Resolution CrystalNph/MeVNel@ 662 keVER(%)ERscint.(%)ERstERnoiseLight detectorRef.NaI(Tl)4000060006.75.93.2PMTtypicalCsI(Tl)650006.65.8XP2254B PhilipsAllier (1998)260004.33.81.51.2SDDFiorini (1997)LaBr3(Ce)63000120003.62.22.5PMT XP20Y0 PhotonisMoszynski (2006)190002.72.01.70.5Fiorini (2006)LSO200053008.87.8BGO900088011.78.0YAP21000103002.32.6APD – Adv.Phot.Inc.Moszynski (2000)A – Prescott and Narayan, NIM A, 75 (1969)B – G.Bizarri, IEEE TNS, Vol 53,02 (2006)LaBr3(Ce)BNaI(Tl)ALuminosity 662keV - PMT 25% QE)W.Moses, NIM A, 487 (2002)
18 Is the QE really useful? 1° PMT HIGH QE: Hamamatsu R7600-200 Crystal Test: LaBr3:Ce Cylinder(½”Ø ½” thickness)1 inchQE max. = nmNumber of dinode = 10Gain= 2.0 HV= -700 V
19 Pulse heigh Resolution & Coincidence Resolving Time: Is the QE really useful?2° PMT HIGH QE:Hamamatsu R C12QE max. = nmNumber of dinode = 12Gain= 1.0 HV= -800 VPulse heigh Resolution & Coincidence Resolving Time:Crystal TEST: LSO 4 x 4 x 20 mm3Source : Na22 511keV)PHR (%)CRT (psec)PositionStandardType(QE=22%)HIGH QEA15.114.0460400B16.014.5500440C16.414.8520D15.8550480E15.4600510F17.1590530PMT position*Courtesy of Hamamatsu Photonics K.K. (Iwata City - Japan)
20 Critical Angle & Q.E. :MC Simulation GEANT 4 Scintillation crystal : LaBr3:Ce continuous crystal50 x 50 x 4 mm3 ( white entrance face – black edges)8 8 Photodetector array ( 6.0 mm pitch)140 keV photon energyNo glass windowQ.E = 0.22 – Phe n°=18603 mm glass windowQ.E = 0.22 – Phe n°=1153No glass windowQ.E = 0.60 – Phe n° = 5102S.R.= mmE.R. = 2.3%( 5.1 % including intrinsic energy resolution of LaBr3:Ce)S.R.=0.82 mmE.R. = 5.1 %( 6.9 % including intrinsic energy resolution of LaBr3:Ce)S.R.= 0.60 mmE.R. = 1.4 %( 4.8 % including intrinsic energyresolution of LaBr3:Ce)
21 ConclusionLaBr3:Ce seems a very promising crystal for SPET ( PET ToF) applicationLight sharing on continuous crystal requires position sensitive photodetectors with superior performancesIntrinsic energy resolution of scintillators can seriously limit the energy resolution response of a high Q.E. photodetectorsRemoving glass window( critical angle) in scintillator coupling, could strongly enhance imaging performances
Your consent to our cookies if you continue to use this website.