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Photodetector requirements for gamma ray imaging with scintillation crystals Roberto Pani

1th Workshop on “Photo Detection” June 13 - 14 , 2007 Perugia, Italy. Photodetector requirements for gamma ray imaging with scintillation crystals Roberto Pani INFN and Sapienza-University of Rome Italy. Scintillation crystal readout technique. Light Sharing. Individual Coupling.

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Photodetector requirements for gamma ray imaging with scintillation crystals Roberto Pani

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  1. 1th Workshop on “Photo Detection” June 13 - 14 , 2007 Perugia, Italy Photodetector requirements for gamma ray imaging with scintillation crystals Roberto Pani INFN and Sapienza-University of Rome Italy

  2. Scintillation crystal readout technique Light Sharing Individual Coupling Continuous crystal Pixellated crystal

  3. Individual coupling technique Munich APD PET* 4 x 8 APD Array (Hamamatsu Photonics) 2 x 2 x 6 mm3 LSO individual coupled Intrinsic 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 20000 chains (SPET) • Single photoelectron readout not needed • Low noise to allow 140 keV photon energy detection • High gain (104 or more) not needed • Energy resolution depending on scintillation crystal / photodetector

  5. X & Y Position Centroid Algorithm Si i ni Position: X = Si ni E = Si ni Energy: Light sharing technique Scintillation light flash on photocathode Anode array (Hamamatsu H8500) Charge distribution sampling by anode array 1 2 3 4 5 6 7 8 k 1 2 3 4 5 6 7 8 … i

  6. Image PSF1mm FWHM oneγ-ray interaction Manyγ-rayinteractions Scintillation light PSF 15 mm FWHM Position linearity Co57 pulse height analisys Position determination in light sharing technique

  7. Light sharing technique • Spatial resolution limited by crystal pixel size ( scintillation array) • Spatial resolution not limited for continuous crystal • Low number of electronic chains • Single photoelectron readout needed • Energy resolution depending on scintillation crystal /photodetector • High gain ( >104 ) is needed • Timing/rise time < 500 ps for ToF

  8. Point Spread Function and critical angle c Planar crystal / PMT glass window Pixellated crystal / PMT glass window Light output angle < 45°

  9. Pixellated scintillation crystal NaI:Tl 1m m x 1mm x 4 mm + H8500 MAPMT Poor energy resolution ~ 14% Pixel Spatial resolution < 1.3 mm Image Spatial resolution > 1.3 mm

  10. Continuous scintillator crystal 1.5 mm step scannig – 0.4 mm Ø Tc99m point source LaBr3:Ce 49 mm x 49 mm x 4 mm + 3 mm glass window H8500 MAPMT • Best Values: • Energy resolution = 9.6 % (@ 1000V) • Overall Spatial Resolution= 1.1 mm • Intrinsic Spatial Resolution= 1.0 mm Very good linearity !!!

  11. Detector assembly: • MAPMT Hamamatsu H8500 • LEGP collimator • (1.5 mm hole, 22 cm lenght) • Multi-anode read-out • Crystal samples: • LaBr3:Ce continuous, 5mm thick • NaI:Tl array, 1.1mm pixel 1.3 pitch • MTF for Continuous Crystal • Spatial Resolution limited to LEGP • Enhancement in Contrast - increased AUC (Area Under Curve) • NO restrictions in image digitization (Nyquist frequency not limited from image pixel) • Continuous position response • Increased detection efficiency Modulation Transfer Function

  12. Scintillation crystal: requirements for SPECT (@140 keV) • Z  40 →Photofraction greater than 70% • High density (> 3 gr/cc) →Reduction of crystal thickness to obtain 80-90% efficiency ( important for light collection) • Refraction index close to 1.5 →To avoid light loosing due to critical angle (continuous crystal) • Decay time  1 ms→To obtain 200 kHz max. • High luminous efficiency (> 20000 at suitable wavelength) → To improve:  Decoding crystal pixel in scintillation array •  Spatial resolution, in continuous crystal •  Energy resolution. • Lowafterglow for high counting rate There are few predictions if energy resolution or light output dominates the intrinsic spatial resolution in light sharing

  13. Scintillator crystals for SPECT

  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 animal imaging. Scintillation decay time  300 ns →To allow good coincidence time resolution. Time resolution better than 0.5 ns can reduce random coincidences (50 % in a 3D PET) and time of flight can be realized. • High luminous efficiency > 8000 ph/MeV → •  To enable block detectors with a greater number of pixel (from • 8  8 BGO to 16 16 LaBr3(Ce) crystal pixel/module). •  Improvementin energy resolution reduces scatter background (25% • Compton scattering / 25% “true” events in a 3D PET). • Lowafterglow for high counting rate

  15. Scintillator crystals for PET

  16. Energy Resolution • Intrinsic Scintillation Contribute • Non homogeneities • Non proportionality of scintillation response Electronic noise Photodetector and preamplifier system [Equivalent noise charge – E. Gatti, NIM Phys Res 1990] Statistical generation of the signal Nph: number of photons in a scintillation flash a : worsening of the Poisson behaviour h : Quantum Efficiency

  17. Intrinsic Scintillator Energy Resolution NaI(Tl)A LaBr3(Ce)B A – Prescott and Narayan, NIM A, 75 (1969) B – G.Bizarri, IEEE TNS, Vol 53,02 (2006) Luminosity (phe @ 662keV - PMT 25% QE) W.Moses, NIM A, 487 (2002)

  18. 1 inch 1 inch Is the QE really useful? 1° PMT HIGH QE: Hamamatsu R7600-200 Crystal Test: LaBr3:Ce Cylinder (½”Ø  ½” thickness) • QE max. = 41.6 % @ 380 nm • Number of dinode = 10 • Gain= 2.0 E+06 @ HV= -700 V

  19. 2° PMT HIGH QE: Hamamatsu R8900-00-C12 • QE max. = 42 % @ 350 nm • Number of dinode = 12 • Gain= 1.0 E+06 @ HV= -800 V Is the QE really useful? Pulse heigh Resolution & Coincidence Resolving Time: Crystal TEST: LSO 4 x 4 x 20 mm3 Source : Na22 (Eg @ 511keV) PMT position *Courtesy of Hamamatsu Photonics K.K. (Iwata City - Japan)

  20. Critical Angle & Q.E. :MC Simulation GEANT 4 • Scintillation crystal : LaBr3:Ce continuous crystal 50 x 50 x 4 mm3 ( white entrance face – black edges) • 8  8 Photodetector array ( 6.0 mm pitch) • 140 keV photon energy No glass window Q.E = 0.22 –Phe n°=1860 3 mm glass window Q.E = 0.22 – Phe n°=1153 No glass window Q.E = 0.60 – Phe n° = 5102 S.R.= 0.75mm E.R. = 2.3% ( 5.1 % including intrinsic energy resolution of LaBr3:Ce) S.R.=0.82 mm E.R. = 5.1 % ( 6.9 % including intrinsic energy resolution of LaBr3:Ce) S.R.= 0.60 mm E.R. = 1.4 % ( 4.8 % including intrinsic energy resolution of LaBr3:Ce)

  21. Conclusion • LaBr3:Ce seems a very promising crystal for SPET ( PET ToF) application • Light sharing on continuous crystal requires position sensitive photodetectors with superior performances • Intrinsic energy resolution of scintillators can seriously limit the energy resolution response of a high Q.E. photodetectors • Removing glass window( critical angle) in scintillator coupling, could strongly enhance imaging performances

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