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Time of Flight: the scintillator perspective

Time of Flight: the scintillator perspective. Paul Lecoq CERN, Geneva. Where is the limit ?. Philips and Siemens TOF PET achieve 550 to 650ps timing resolution About 9cm localization along the LOR Can we approach the limit of 100ps (1.5cm)?

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Time of Flight: the scintillator perspective

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  1. Time of Flight: the scintillator perspective Paul Lecoq CERN, Geneva

  2. Whereis the limit? • Philips and Siemens TOF PET achieve • 550 to 650ps timing resolution • About 9cm localizationalong the LOR • Can weapproach the limit of 100ps (1.5cm)? • Can scintillatorssatisfythis goal?

  3. Timing parameters • General assumption , based on Hymantheory • For the scintillator the important parameters are • Time structure of the pulse • Light yield • Light transport • affecting pulse shape, photon statistics and LY decay time of the fast component Photodetector excess noise factor number of photoelectronsgenerated by the fast component

  4. Light output: LYSO example • Statistics on about 1000 LYSO pixels 2x2x20mm3 • produced by CPI • for the ClearPEM-Sonicproject (CERIMED) • Mean value = 18615 ph/MeV • For 511 KeV and 25%QE: 2378 phe • Assuming ENF= 1.1 • Nphe/ENF ≈ 2200 phe

  5. Statistical limit on timing resolution • W(Q,t) is the time interval distribution between photoelectrons • = the probability density that the interval between event Q-1 and event Q is t • = time resolution when the signal is triggered on the Qth photoelectron t = 40 ns t = 40 ns Nphe=2200 LSO Nphe Nphe Nphe Nphe

  6. Light generation 5d Rare Earth 4f

  7. Rise time • Rise time is as important as decay time

  8. Photon countingapproach LYSO, 2200pe detected, td=40ns tr=0.5ns tr=1ns tr=0ns tr=0.2ns

  9. Fasterthan Ce3+?Intrinsiclimitat 17ns • Cross-Luminescentcrystals (veryfast, low LY) • BaF2 (1400ph/MeV) but 600ps decay time produces more photons in the first ns (1100) than LSO (670)! • Direct bandgapsemiconductorsS. Derenzo, SCINT2001 • Sub-nsband-to-bandrecombination in ZnO, CuI,PbI2, HgI2 • Nanocrystals • Bright and sub-nsemission due to quantum confinement • Pr3+ • Pr3+ 5d-4f transition isalways 1.55eV higherthan for Ce3+

  10. UltimatelyfastusingCerenkovemission? • Evenlowenegyg ray produceCerenkovemission in dense, high n materials • This emissionisinstantaneouswith a 1/l2spectrum * Lowwavelengthcut-off set at 250nm for calculations on LSO, LuAG and LuAP Ce absorption bands subtractedfromCerenkovtransparencywindow

  11. LuAGCerenkov/LYSO Scintillation coincidencemeasurement LuAG 2013 (undoped -> shows no scintillation) LSO 1121 22Na FWHM=374ps LuAG=259ps Crystals wrapped on 5 sides with teflon. PMT left (2150V) 8cm 8cm PMT right (1500V) Scope CFD FWHM=650ps LuAG=587ps Coincidence: Th_left=-4mV, th_right=-500mV

  12. Light Transport • -49° < θ < 49° Fastforwarddetection 17.2% • 131° < θ < 229° Delayed back detection 17.2% • 57° < θ < 123° Fast escape on the sides 54.5% • 49° < θ < 57° and 123° < θ < 131° infinitebouncing11.1% For a 2x2x20 mm3 LSO crystal Maximum time spreadrelated to difference in travelpathis 424 pspeak to peak ≈162 ps FWHM

  13. Photoniccrystals to improve light extraction Periodic medium allowing to couple light propagation modes inside and outside the crystal 24% 34% M. Kronberger, E. Auffray, P. Lecoq, Probing the concept of Photonics Crystals on Scintillating Materials TNS on Nucl. Sc. Vol.55, Nb3, June 2008, p. 1102-1106

  14. Expected Light Output Gain for differentcrystals BGO LuAG:Ce Light gain 1.92 Light gain 2.11 LYSO LuAP Light gain 2.08 Light gain 2.1 Litrani + CAMFR simulation

  15. How does the PhCwork? Diffracted modes interfere constructively in the PhC- grating and are therefore able to escape the Crystal Crystal- air interface with PhC grating: Section of the plane crystal- air interface: (EM – fieldplot) θ>θc Total Reflection at the interface θ>θc Extracted Mode

  16. PhC fabrication Nano Lithography • PhC is produced in cooperation with the INL (Institut des Nanotechnologies de Lyon) • Three step approach: • Sputter deposition of an auxiliary layer • Electron beam lithography (EBL) • Reactive ion etching (RIE) RAITH® lithography kit:

  17. PhC fabrication Ion Bombardment z z a Reactive Ion Etching (RIE) • Chemically reactive plasma removes Si3N4 not covered by the resist • Change the composition of the reactive plasma to remove the resist (PMMA) without etching the Si3N4 PMMA Resist Si3N4 Si3N4 Hole depth: 300nm ITO ITO Scintillator Scintillator y y hole diameter: 200nm x x

  18. PhC fabrication Results • Scanning Electron Images: D = 200nm a = 340nm

  19. PhC first results • Use larger LYSO crystal: 10x10mm2 to avoidedgeeffects • 6 different patches (2.6mm x 1.2mm) and 1 (1.2mm x 0.3mm) of differentPhC patterns 0° 45° Preliminary

  20. PhCimproveslight extraction eficiency But also collimation of the extracted light

  21. Conclusions • Timing resolutionimproveswithlowerthreshold • Ultimateresolutionimplies single photon counting • High light yieldismandatory • 100’000ph/MeV achievablewithscintillators • Shortdecay time • 15-20ns is the limit for brightscintillators (LaBr3) • 1ns achievable but withpoor LY • Crossluminescentmaterials • Severelyquenchedself-activatedscintillators • SHORT RISE TIME • Difficult to break the barrier of 100ps

  22. Conclusions New approaches? • Crystalswith a highlypopulateddonor band (ZnO) • Metamaterialsloadedwith quantum dots • Make use of Cerenkov light • Improve light collection withphotoniccrystals

  23. Our Team • CERN • Etiennette Auffray • Stefan Gundacker • HartmutHillemanns • Pierre Jarron • Arno Knapitsch • Paul Lecoq • Tom Meyer • Kristof Pauwels • François Powolny • Nanotechnology Institute, Lyon • Jean-Louis Leclercq • Xavier Letartre • ChristianSeassal

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