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AXIAL PET - HPD --------------------

AXIAL PET - HPD -------------------- AXIAL COORDINATE RECONSTRUCTION WITH WLS STRIPS -------------- 1) Introduction

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AXIAL PET - HPD --------------------

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  1. AXIAL PET - HPD -------------------- AXIAL COORDINATE RECONSTRUCTION WITH WLS STRIPS -------------- 1) Introduction 2) Experimental test set-up 3) Readout of the LYSO crystal bars Performances : luminosity – Energy resolution 4) Readout of the WLS strips Performances : pe statistic – σz resolution 5) Expected performances of a brain PET scanner 6) WLS readout with SiPM J. Seguinot Tests WLS - Readout axial PET - Bari - Janvier 2007

  2. HPD1 HPD1 z y x HPD2 HPD2 N1 N2 Original concept ; New concept : would allow a digital z reconstruction by mean of a transversal readout of each layer of the LYSO crystal bar matrix by mean of WLS strips (33 x 13 strips – L = 6 cm, w=3mm). The principle is well known and largely used in calorimetry Z = 0.5·(λeff.ln(N1/N2) + Lc) σz =(λeff/√2No)sqrt{ ez/λeff + e(L-z)/λeff } For Lc = 10 cm and λeff~ 7 to 8 cm : σz ~4 mm. N1 = (No/2)·e-z/λeffN2 = (No/2)·e-(L-z)/λeff Npe(z) = N1 + N2 No (nber of pe’s detected for λeff= ∞) characterizes the luminosity of the LYSO cristal matrix. Energy resolution : The statistical term σE/E = ENF/sqrt( Npe) is limited by the value of λeff required to minimize σz. (was proposed by D.Schinzel) For a WLS strip width w = 3 mm, σz ≤ w/√12 =.9 mm As λeff ~ 400 mm for polished crystal bars, the energy resolution will also be significantly improved with respect to the original concept. Tests WLS - Readout axial PET - Bari - Janvier 2007

  3. EJ-280 wavelength shifting plastics Emission peak : 490 nm Absorption peak : 425 nm Quantum efficiency : 0.86 Decay time : 8.5 ns Density : 1.02 – ref. index : 1.58 Physical properties : LYSO emission peak Absorption coefficient~ 2.5 mm-1 Using a reflector on the opposite side of the strip more than 80% of the light emitted by the LYSO crystals is absorbed in a WLS of 0.7 mm. Tests WLS - Readout axial PET - Bari - Janvier 2007

  4. Light attenuation length in the ELJEN EJ-280 WLS strips The mean light intensity transmitted over 3 cm (6 cm is the max. length of the strips) is 78 to 88% for a thickness of 0.7 and 1 mm respectively. Tests WLS - Readout axial PET - Bari - Janvier 2007

  5. VME DAQ Readout card Si sensor (300 mm) 208 pads (4×4 mm2) -UPC = 0 -20 kV vacuum pump (turbo) P < 10-5 mbar Pulsed LED (blue) sapphire collimator MgF2 mirror The principle is similar for the tests of the WLS strips : a narrow electron beam (Ф~ 2mm – Δt ~ 10 ns)is generated from the photoemission of a CsI-PC. The light source is a self triggered UV flash lamp. EXPERIMENTAL SET UP FOR HPD - TESTS Advantages : 1) The el. beam energy can be varied by adjusting the nber of pe’s emitted from the PC and the acceleration voltage up to 30 kV. 2) Possibility to scan a surface by moving the mirror. Inconvenient : The set-up must be vacuum pumped to < 10-5 mbar. U.V light beam – Ф~ 2 mm H2 self triggered U.V flash lamp Δt ~ 10 ns f ~ 40 Hz PC collimator Tests WLS - Readout axial PET - Bari - Janvier 2007

  6. Position adjustable U.V light spot beam (~2mm in diameter - Δt~ 10 ns) scanning normal to the WLS strips PMT Photonis XP 3102 (outside the enceinte) Support of the PC (10 nm CsI deposited on a CaF2 crystal) - 25 kV Sapphire window (1mm) Polished LYSO bars – 3x3x100 mm3 - delivered by St Gobain (Fr) 10 mm PMT Hamamatsu R1650 (inside the enceinte) WLS strips – ELJEN – 1 mm thick, 3 x 60 mm2 Set-up is turbo-pumped at ~ 10 exp-6 mbar Tests WLS - Readout axial PET - Bari - Janvier 2007

  7. All the components of the set-up have been produced by the workshop in Bari. U.V light spot beam 2 mm diam.- Δt ~10 ns PMT – Hamamatsu R1650 PMT – Photonis XP3102 - 25 kV Sapphire window Support CaF2 crystal coated with a transmittive CsI photocathode (10 nm thickness) Anode at gnd potential with a mesh (T=.90) above the crystals LYSO – crystal bar 3.2x3.2x10 cm3 The extremities opposite to the PMTs of the LYSO bars and of the WLS strips are aluminized Set-up turbo-pumped at ~10 exp.(-6) mbar Tests WLS - Readout axial PET - Bari - Janvier 2007

  8. Tests WLS - Readout axial PET - Bari - Janvier 2007

  9. Tests WLS - Readout axial PET - Bari - Janvier 2007

  10. Readout of the LYSO bars Gain PMT = 6. 106 Tests WLS - Readout axial PET - Bari - Janvier 2007

  11. Readout of the WLS strips Gain PMT = 1.3.106 Tests WLS - Readout axial PET - Bari - Janvier 2007

  12. Crystal LYSO Single photoelectron response X-HPD DEVELOPMENT Aluminized cylindrical crystal : Ф=12 mm, h= 18 mm ~45 photoelectrons from scintillation are detected without back scattering – Relative Light Output (RLO) = 0.8 Tests WLS - Readout axial PET - Bari - Janvier 2007

  13. Typical pulses from the readout of the LYSO bars Tests WLS - Readout axial PET - Bari - Janvier 2007

  14. WLS test set-up Response of the LYSO bars for Uacc=25 kV and UPMT= - 1 kV The dispersion reflects the statistic on the nber of pe’s emitted from the CsI-PC 850 pe’s Tests WLS - Readout axial PET - Bari - Janvier 2007 ( ADC counts )

  15. ESTIMATIONS OF THE PERFORMANCES OF THE LYSO BARS : Photomultiplier Photonis XP 3102 : Gain G = 6·106 at – 1 kV Attenuation signal = 18 db ( ~ factor 8) ADC : sensitivity = 50 fC/ADC count Electron beam : The number of primary pe’s emitted from the PC is determined by the relation, Npe = (<μ>/σ)2 where <μ> is the mean value of the LYSO charge distribution of ecart type σ. Uacc = acceleration voltage (kV) RLO = Relative Light Output of the scintillation yield with the acceleration voltage (0.8 at 25 kV) Npe(LYSO) = {8 x 50· <μ>· 10-15} / G x 1.6·10-19 = 2.5·106·<μ>/ G = (No/2) · {e-z/λeff + R · e-(L – z)/λeff} R = reflectivity at the end bar Experimental conditions : z = 6 cm, L = 10 cm, λeff = 40 cm, R = 0.8 ? The formula above, if correct, shows that the number of detected pe’s is~ independent of z. This is very good for the discrimination in energy needed for the trigger and, moreover, the resolution in energy is minimized. At 511 keV equivalent energy, No = 1794·<μ> / (Npe·G · Uacc·RLO) with Uacc (kV) and G PMT = 6.10-6 Tests WLS - Readout axial PET - Bari - Janvier 2007

  16. Data :12/9/2006 -U (pc) = 25 kV U(PMT)=1. kV The shape is exactly what one expects for a total crystal width of 6.4 mm and a beam spot of ~ 2mm in diameter LYSO On the plateau, 12 ± .4 primary pe’s from the PC are detected andthe mean charge (1189 ± 24 ADC counts) corresponds to about 500 pe’s from the scintillation light in the LYSO crystal bars, i.e ~40 pe’s/ inc.pe, as expected from the previous tests for the X-HPD project. Assuming an effective light attenuation length λeff of 40 cm for polished crystal bars and a reflectivity of 80% of the Al coating at the end of the bars opposite to the PMT, one can estimate No to ~1400. Tests WLS - Readout axial PET - Bari - Janvier 2007

  17. Data : 1/3/2007 Npe inc. = 17.3 ± 0.7 for 10≤ Uacc ≤ 27.5 120 keV 350 keV LSF : -113.1 + 36.51 x U(kV) Cut-off : 3.1 kV Tests WLS - Readout axial PET - Bari - Janvier 2007

  18. Data : 1/3/2007 LUMINOSITY LYSO : No ~ 1600 In our publication (Il Nuovo Cimento Vol.29 C, N. 4) we have quoted 1525 for LSO assuming a QE of 30% Tests WLS - Readout axial PET - Bari - Janvier 2007

  19. Typical readout pulses from the WLS strips Sum of channels 1 and 2 Tests WLS - Readout axial PET - Bari - Janvier 2007

  20. Uacc = 25 kV Charge distribution : sum of the two WLS strips response. 40 pe’s Tests WLS - Readout axial PET - Bari - Janvier 2007 (ADC counts)

  21. PERFORMANCES OF THE WLS STRIPS : Photomultipliers : Hamamatsu R1650 at 1.2 kV Gain PM = 1.3 106 ADC sensitivity 50 fC/ADC count <QWLS>: mean charge in ADC count (Npe)WLS = 0.24 · <QWLS> At 511 keV equivalent energy : (Npe)WLS = 122.6 · <QWLS> / (Npe·Uacc·RLO) The ratio : R = (Npe)WLS/ (Npe)LYSO= 0.58 · <QWLS>/<μLYSO> (PMT (LYSO) at -1 kV and the PMT(LYSO) at -1.2 kV). is a rough estimation of the ratio of the detected photon yields assuming : QE PMT(LYSO) at 425 nm = .30 QE PMT(WLS) at 500 nm = .15 Tests WLS - Readout axial PET - Bari - Janvier 2007

  22. Data 12/19/2006 3 mm <Npe inc.> = 7.3 ± 1.3 Strip# 1 Strip # 2 WLS At z = 64 mm (centre of strip #1) ) about 60 % of the total charge is detected, i.e ~ 40 pe’s at 511 keV and 40% on strip#2 i.e ~20 pe’s. As both strips are hit when scanning over the total width covered by the strips, a precise digital z reconstruction cannot be obtained unless to apply a high discrimination level proportional to the level of light detected. Indeed, the aperture of the light cone at the input of the WLS is large (~400) and the spot size is still increased because the scintillation light in these tests is generated on the opposite side of the strips since the range of electrons of 25 keV in the crystals is very small. Tests WLS - Readout axial PET - Bari - Janvier 2007

  23. Data : 12/19/2006 Uacc = 25 kV Q = Q2 + Q3 1 mm WLS strips 6 mm WLS strips At z = 66 mm, Q(WLS)/inc. pe = 16 – 4 (bkg subst.) ADC counts For a PM gain = 1.3.106 : Npe(WLS)/20 keV inc. pe = 2.9 i.e, ~ 70 pe’s at 511 keV for a 1 mm WLS strip and a photon detector quantum efficiency of ~15%. The tail is a bkg due to photons which escape from the lateral surface of the LYSO bars. Tests WLS - Readout axial PET - Bari - Janvier 2007

  24. Data : 1/3/2007 Z mir = 67 mm Tests WLS - Readout axial PET - Bari - Janvier 2007

  25. Data : 1/3/2007 Zmir= 67 mm 1 mm WLS strips <Npe> inc.=17.3 ± 0.7 E=120 keV E=350 keV Cut-off : 2.7 kV At 511 keV : Npe (WLS) ~ 60 Tests WLS - Readout axial PET - Bari - Janvier 2007

  26. Data - 12/22/2006 ANALOGUE Z RECONSTRUCTION Q2 = 0 LSF : m = 0.9 Q3 = 0 Q2 Q3 WLS ZREC = 60 + (Q2 – Q3) / (Q2 + Q3) The slope m = 0.9 instead of 1 expected is due to the different detection efficiency of the WLS strips. Tests WLS - Readout axial PET - Bari - Janvier 2007

  27. Uacc = 25 kV ( E = 350 keV ) As the z reconstruction is, “de facto”, analogue, the resolution σZ varies as 1/√Npe(WLS) or 1/√E, and is estimated to ~ 1mm for 511 keV gamma from the extrapolation of the next measurement. Tests WLS - Readout axial PET - Bari - Janvier 2007 Z (mm)

  28. Data : 1/3/2007 10 kV 27.5 kV 120 keV LSF : σz = -0.21 + 9.785/√Npe 350 keV Tests WLS - Readout axial PET - Bari - Janvier 2007

  29. From our publication in Il Nuovo Cimento Vol. 29 C,N. 4 HPD – PET LYSO 1.85 2.35 2.1 ~ 9 mm3 (Digital z reconstruction) FWHM values Expected This is ~ physical limit Tests WLS - Readout axial PET - Bari - Janvier 2007

  30. PET-HPD with LYSO and digital z reconstruction ΔE/E (511 keV) = 9 % ΔV (mm3) = 9 ΔE/E . ΔV (%.mm3) = 81 FoM ~ 3 Tests WLS - Readout axial PET - Bari - Janvier 2007

  31. z y x The concept would allow a better azimuthal coverage with a single HPD. Tests WLS - Readout axial PET - Bari - Janvier 2007

  32. READOUT OF THE WLS STRIPS BY MEAN OF SiPM’s • We need a specific SiPM design : • Active area : 3 x 1 mm2 (cross section of the WLS strip 3 x 0.7 mm2) • Nber of pixels : 600 – i.e 200 / mm2 (dynamic range is sufficient) • Pixel size : 70 x 70 μm2 • Sensitivity to single photons (R quenching ~ 250 k) • Advantages: • By increasing the pixel size : • One increases the active area. Consequently the quantum efficiency could, hopefully, be better than 30% at 500 nm ( ? ) instead of ~15% for a bi-alkali PC, a large gain, especially for the detection of Compton events. • The capacitance per pixel being higher, the gain would be increased. Tests WLS - Readout axial PET - Bari - Janvier 2007

  33. SLIDES FOR COMPLEMENT OF INFORMATION Tests WLS - Readout axial PET - Bari - Janvier 2007

  34. Charge (ADC counts) Z (mm) Tests WLS - Readout axial PET - Bari - Janvier 2007

  35. Gain PMT = 3.10 exp.6 Charge ( V.s.) Tests WLS - Readout axial PET - Bari - Janvier 2007

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