Increase in Photon Collection from a YAP:Ce Matrix Coupled to Wave Lenght Shifting Fibres N. Belcari a, A. Del Guerra a, A. Vaiano a, C. Damiani b, G. Di Domenico b, G. Zavattini b a Department of Physics, University of Pisa and INFN, Pisa b Department of Physics, University of Ferrara and INFN, Ferrara

Two coincidence photons (511 keV) Interaction within the detector Point of positron annihilation PET Principle Determination of the Point of Interaction by means of a matrix of scintillating finger crystals Incident photon Pixel  X&Y coordinates Determination of the lines of flight Image reconstruction Position sensitive readout

WLS Fibres in PETApplications WLS Fibres in PETApplications CsI (Na) CsI(Na) CsI(Na) Light Mixer PMT 1 PMT 2 X-Fibres Z-Fibres Y-Fibres White Reflector W.Worstell et al. IEEE Trans. Nucl. Sci. VOL.45, NO.6, DECEMBER 1998 LSO Y-Fibres X-Fibres Light Pipes M.B.Williams et al. IEEE Trans. Nucl. Sci. VOL.45, No.2, APRIL Pixel HPD LSO crystals CsI H.Herbert et al.IEEE Medical Imaging Conference Record, Seattle 1999

YAP-(S)PET scanner PSPMTs YAP:Ce Scintillator (2x2x30 mm) Dual Modality (PET/SPECT) New Scintillator (YAP:Ce) Position Sensitive PMT Small Animal Imaging

4x4x3cm 3 YAP:Ce Matrix 4x4x3cm 3 YAP:Ce Matrix Name Chemical formula Density Effective atomic number Refractive index Scintillation decay time Peak emission wavelenght Light Yield Yttrium Aluminum Oxide Perovskite YAlO 3 :Ce g/cm ns ph /MeV Material Refractive index Peak absorbance wavelenght Core Cladding Core Cladding Core Cladding 1.05 g/cm g/cm nm440nm Density 1.19 g/cm 3 WLS Fibre Kuraray SCSF-78 WLS Fibre Kuraray SCSF nm Polymethilmethacrylate Polystyrene Peak emission wavelenght Section Size SCSF-38 SCSF-78 1mm x 1mm 2mm x 2mm

Position Measurement SetUp Position Measurement SetUp PSPMT R2486 #2 YAP:Ce 5x5 crystal matrix PMT Philips XP2020 #3 YAP:Ce 19 mm  10 mm thick 22 NaNa source SCSF 78 square section fibres PSPMT R2486 #1 crystal image (PSPMT #1) Fibre image (PSPMT #2) Event: (#1) AND (#2) AND (#3)

Evaluation of the Light Yield of the System (YAP crystal + WLS Fibre + HPD) 14 18% 70% 17.3% 80% 84% 96% 90% 5.7%  /MeV MeV  14 p.e. (both sides)  74 ph (both sides)  44 ph (One side)  255 ph (one side)  319 ph (one side)  379 ph (one side)  421 ph (one side)  468 ph (one side)  8687 ph. (one side) Expected Photoelectrons Detector Quantum Efficiency Reflection At The Fibre Aluminized End Fibre Quantum Efficiency YAP Emission-Fibre Absorption Overlapping Light Transmitted at the Air-Fibre Interface Light Transmitted at the YAP-Air Interface Light Escaping From One Crystal End (geometry + attenuation) YAP Emission Energy Deposit Light escaping from One Fibre End (geometry + attenuation)

Experimental Set-up System Light Yield Measurement Single Pixel HPD 22 Na Black Tape Photopeak event selection HPDspectrum PS-PMTSpectru m YAP:Ce 5x5 crystal matrix (10x10x30 mm 3 ) Hamamatsu R2486 PMT 2x2x1 mm 2 square section fiber

Using WLS Fibres and an HPD to read-out a YAP:Ce matrix we found a signal of about 10 p.e. for 511 keV events. The discrepancy with the expected signal could be due to imperfection in the set-up The readout of a YAP:Ce matrix by WLS Fibres is then feasible but: Low light yield Þ low detection efficiency (expecially for lower energy events) We need to increase the light yield! We need to increase the light yield! Where are we P(0 p.e.)  8.2%  reduction in single detection efficiency of about 8.2% for 122keV events.

Cutting angle New shape of the finger crystal: "V-cut" Uniform scintillation Total light Total reflection Reflective layer The best cutting angle (  ) is 45° “Flat” “V-cut”

Comparison between “Flat” and “V-cut 45°” finger crystal "Uniform*" (+76%) { Light (%) Flat "V-cut" RT: 8.2 Al: 1.3 * Uniform scintillation within the crystal (conservative choice ) Extracted light: Monte Carlo simulation Practical use of “V-cut 45°” crystals Problems Very difficult to be produced High cost (  +70%)

A new solution: "Half Pipe-cut" A new solution: "Half Pipe-cut" "Uniform" (+52%) { RT: 7.7 Al: 1.6 Light (%) Flat "Half Pipe -cut" "Real"* (+63%) RT: 6.9 Al: 1.3 { Advantages: Easier production Not so expensive Easier coupling with more versatile *Considering an exponential attenuation of 511 keV photons within the YAP:Ce ( =2.7 cm) Round WLS fibres

Crystal Back-reflected light (%) Coupling YAP-WLS Fibres: Air or Optical Grease? AIR OPTICAL GREASE Core: Clad : AIR GREASE Trasmitted Lost Core: Clad : 3.1 % % Light trasmission in the fiber AIR GREASE % of light emerging from the crystal Light trasmission in the crystalBack-reflection into the crystals Combined factor AIR GREASE %  18.5% = 1.7% 20.5%  3.1% (5.3%*)= 0.6% (1.1%*) (*Using a double cladding fibre)

Evaluation of the Light Yieldof the System half-pipe shaped YAP crystal (with air interface) + WLS Fibre + HPD 23 (14) 18% 70% 17.3% 80% 84% 96% 90% 9.3 %  /MeV MeV  23 p.e. (both sides)  126 ph. (both sides)  81 ph. (One side)  469 ph. (one side)  586 ph. (one side)  698 ph. (one side)  727 ph. (one side)  809 ph. (one side)  8687 ph. (one side) Expected Photoelectrons Detector Quantum Efficiency Reflection At The Fibre Aluminized End Fibre Quantum Efficiency YAP Emission-Fibre Absorption Overlapping Light Transmitted at the Air-Fibre Interface Light Transmitted at the YAP-Air Interface Light Escaping From One Crystal End (geometry + attenuation) YAP Emission Energy Deposit Light escaping from One Fibre End* (geometry + attenuation) * For sake of semplicity we used the same parameters as the square section fibres

2mm diameter WLS Fibre To PSPMT to 61 pixel HPD Light yield measurement Test of various WLS fibres Imaging capabilities To be done: Future studies: 5  5 YAP:Ce "Half-Pipe" Crystal Matrix Conclusions We simulated various geometry and coupling metods Using “Half-Pipe” shaped crystals and “air” coupling we expect: p.e. (instead of 14 p.e.) for 511keV events. This fact could helps in reducing the efficiency loss due to the low light yield: e.g., for 122 keV we expect to measure 4.1 p.e. (instead of 2.5 p.e.) 1.7% 8.2% corresponding to a reduction in single detection efficiency of 1.7% (instead of 8.2%) Experimental measurements are needed.