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INFN and Sapienza-University of Rome Italy

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

6 Position determination in light sharing technique
one γ-ray interaction Scintillation light PSF mm FWHM Image PSF 1mm FWHM Many γ-ray interactions Position linearity Co57 pulse height analisys

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 Pixel Spatial resolution < 1.3 mm Poor energy resolution ~ 14% 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 Modulation Transfer Function
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 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

12 Scintillation crystal: requirements
for SPECT 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 (> at suitable wavelength) → To improve:  Decoding crystal pixel in scintillation array  Spatial resolution, in continuous crystal  Energy resolution. Low afterglow 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
Density (g/cm3) Atten.len. @ 140keV* (mm) Z eff. Photo- fraction (%) Light yield (ph/MeV) Decay time (ns) Refr. index ΔE/E (PMT) Emiss max (nm) NaI:Tl 3.67 3.76 51.0 84 41,000 230 1.85 9% 410 CsI(Tl) 4.51 2.55 52.0 86 66,000 630 1.80 14% 565 YAP 5.50 10.00 36.0 50 21,000 27 1.95 20% 350 LaCl3:Ce 3.86 4.22 49.5 80 40,000 (65%) 1.90 8% LaBr3:Ce 5.07 3.32 47.4 79 63,000 16 (97%) 6% 380 LuI3:Ce 5.60 1.70 - 90 90,000 30 472 535

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).  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. efficiency Coinc. timing res. (ps) ΔE/E @511 keV (PMT) BGO 7.1 480 83 43 9,000 300 100% 3000 10% Lu2SiO5:Ce (LSO) 7.4 420 65 34 26,000 40 90% Lu2(1-x)Y2xSiO5:Ce (LYSO) 54 - 30,000 11% LaCl3:Ce 3.86 350 49.5 15 46,000 20 (65%) 36% 265 4% LaBr3:Ce 5.07 380 47.4 14 63,000 16 (97%) 42% 260 3% LuI3:Ce 5.60 472 535 29 90,000 30 73% 200 <15%

16 Statistical generation of the signal
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 PMT P-I-N APD SSD SiPM 1.25 1 2  1  at pk  30%  80% > 80%  60% M  5 105 < 103 ENC/M  0 370 20 25 where 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 [2]. 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
Crystal Nph/ MeV Nel @ 662 keV ER(%) ERscint. (%) ERst ERnoise Light detector Ref. NaI(Tl) 40000 6000 6.7 5.9 3.2 PMT typical CsI(Tl) 65000 6.6 5.8 XP2254B Philips Allier (1998) 26000 4.3 3.8 1.5 1.2 SDD Fiorini (1997) LaBr3(Ce) 63000 12000 3.6 2.2 2.5 PMT XP20Y0 Photonis Moszynski (2006) 19000 2.7 2.0 1.7 0.5 Fiorini (2006) LSO 2000 5300 8.8 7.8 BGO 9000 880 11.7 8.0 YAP 21000 10300 2.3 2.6 APD – Adv.Phot.Inc. Moszynski (2000) A – Prescott and Narayan, NIM A, 75 (1969) B – G.Bizarri, IEEE TNS, Vol 53,02 (2006) LaBr3(Ce)B NaI(Tl)A Luminosity 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 inch QE max. = nm Number of dinode = 10 Gain= 2.0 HV= -700 V

19 Pulse heigh Resolution & Coincidence Resolving Time:
Is the QE really useful? 2° PMT HIGH QE: Hamamatsu R C12 QE max. = nm Number of dinode = 12 Gain= 1.0 HV= -800 V Pulse heigh Resolution & Coincidence Resolving Time: Crystal TEST: LSO 4 x 4 x 20 mm3 Source : Na22 511keV) PHR (%) CRT (psec) Position Standard Type (QE=22%) HIGH QE A 15.1 14.0 460 400 B 16.0 14.5 500 440 C 16.4 14.8 520 D 15.8 550 480 E 15.4 600 510 F 17.1 590 530 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.= mm 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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