1. Tests WLS - Readout axial PET - Bari - Janvier 2007 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 – σ z resolution 5) Expected performances of a brain PET scanner 6) WLS readout with SiPM J. Seguinot

2. Tests WLS - Readout axial PET - Bari - Janvier 2007 Original concept ; N1N1 N2N2 Z = 0.5·(λ eff.ln(N 1 /N 2 ) + L c ) σ z =(λ eff /√2No)sqrt{ e z/λ eff + e (L-z)/λ eff } For Lc = 10 cm and λ eff ~ 7 to 8 cm : σ z ~4 mm. N 1 = (No/2 )·e -z/λ eff N 2 = (No/2)· e -(L-z)/λ eff N pe (z) = N 1 + N 2 N o (nber of pe’s detected for λ eff = ∞ ) characterizes the luminosity of the LYSO cristal matrix. Energy resolution : The statistical term σ E /E = ENF/sqrt( Npe) is limited by the value of λ eff required to minimize σ z. 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 For a WLS strip width w = 3 mm, σ z ≤ w/√12 =.9 mm As λ eff ~ 400 mm for polished crystal bars, the energy resolution will also be significantly improved with respect to the original concept. (was proposed by D.Schinzel)

3. Tests WLS - Readout axial PET - Bari - Janvier 2007 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 LYSO emission peak Absorption coefficient ~ 2.5 mm-1 Physical properties : 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.

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

5. Tests WLS - Readout axial PET - Bari - Janvier 2007 vacuum pump (turbo) P < 10 -5 mbar DAQ Readout card Pulsed LED (blue) MgF 2 sapphire mirror Si sensor (300  m) 208 pads (4×4 mm 2 ) VME -U PC = 0 -20 kV collimator H 2 self triggered U.V flash lamp Δt ~ 10 ns f ~ 40 Hz collimator U.V light beam – Ф ~ 2 mm 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. PC EXPERIMENTAL SET UP FOR HPD - TESTS 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. Inconvenient : The set-up must be vacuum pumped to < 10 -5 mbar.

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

7. Tests WLS - Readout axial PET - Bari - Janvier 2007 PMT – Hamamatsu R1650 LYSO – crystal bar 3.2x3.2x10 cm 3 PMT – Photonis XP3102 Support CaF2 crystal coated with a transmittive CsI photocathode (10 nm thickness) U.V light spot beam 2 mm diam.- Δt ~10 ns - 25 kV Anode at gnd potential with a mesh (T=.90) above the crystals Set-up turbo-pumped at ~10 exp.(-6) mbar The extremities opposite to the PMTs of the LYSO bars and of the WLS strips are aluminized All the components of the set-up have been produced by the workshop in Bari. Sapphire window

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

10. Gain PMT = 6. 10 6 Readout of the LYSO bars

11. Tests WLS - Readout axial PET - Bari - Janvier 2007 Gain PMT = 1.3.10 6 Readout of the WLS strips

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

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

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

15. Tests WLS - Readout axial PET - Bari - Janvier 2007 ESTIMATIONS OF THE PERFORMANCES OF THE LYSO BARS : Photomultiplier Photonis XP 3102 : Gain G = 6·10 6 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, N pe = ( /σ) 2 where is the mean value of the LYSO charge distribution of ecart type σ. U acc = acceleration voltage (kV) RLO = Relative Light Output of the scintillation yield with the acceleration voltage (0.8 at 25 kV) N pe (LYSO) = {8 x 50· · 10 -15 } / G x 1.6·10 -19 = 2.5·10 6 · / G = (No/2) · {e -z/λ eff + R · e -(L – z)/λeff } R = reflectivity at the end bar Experimental conditions : z = 6 cm, L = 10 cm, λ eff = 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· / (N pe ·G · U acc ·RLO) with U acc (kV) and G PMT = 6.10 -6

16. Tests WLS - Readout axial PET - Bari - Janvier 2007 -U (pc) = 25 kV U(PMT)=1. kV On the plateau, 12 ±.4 primary pe’s from the PC are detected and the 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 λ eff 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 N o to ~1400. LYSO Data :12/9/2006 The shape is exactly what one expects for a total crystal width of 6.4 mm and a beam spot of ~ 2mm in diameter

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

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

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

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

21. Tests WLS - Readout axial PET - Bari - Janvier 2007 PERFORMANCES OF THE WLS STRIPS : Photomultipliers : Hamamatsu R1650 at 1.2 kV Gain PM = 1.3 10 6 ADC sensitivity 50 fC/ADC count : mean charge in ADC count (N pe ) WLS = 0.24 · At 511 keV equivalent energy : (N pe ) WLS = 122.6 · / (N pe ·U acc ·RLO) The ratio : R = (N pe ) WLS / (N pe ) LYSO = 0.58 · / (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

22. Tests WLS - Readout axial PET - Bari - Janvier 2007 Strip # 1 Strip # 2 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. 3 mm Data 12/19/2006 = 7.3 ± 1.3 WLS 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 (~40 0 ) 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.

23. Tests WLS - Readout axial PET - Bari - Janvier 2007 6 mm WLS strips At z = 66 mm, Q(WLS)/inc. pe = 16 – 4 (bkg subst.) ADC counts For a PM gain = 1.3.10 6 : 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%. Data : 12/19/2006 Uacc = 25 kV The tail is a bkg due to photons which escape from the lateral surface of the LYSO bars. Q = Q 2 + Q 3 1 mm WLS strips

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

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

26. Tests WLS - Readout axial PET - Bari - Janvier 2007 Q2Q2 Q3Q3 WLS Z REC = 60 + (Q 2 – Q 3 ) / (Q 2 + Q 3 ) Q 2 = 0 Q 3 = 0 LSF : m = 0.9 The slope m = 0.9 instead of 1 expected is due to the different detection efficiency of the WLS strips. Data - 12/22/2006 ANALOGUE Z RECONSTRUCTION

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

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

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

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

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

32. Tests WLS - Readout axial PET - Bari - Janvier 2007 READOUT OF THE WLS STRIPS BY MEAN OF SiPM’s We need a specific SiPM design : Active area : 3 x 1 mm 2 (cross section of the WLS strip 3 x 0.7 mm 2 ) Nber of pixels : 600 – i.e 200 / mm 2 (dynamic range is sufficient) Pixel size : 70 x 70 μm 2 Sensitivity to single photons (R quenching ~ 250 k) Advantages: By increasing the pixel size : 1)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. 2)The capacitance per pixel being higher, the gain would be increased.

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

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

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