P. Lecoq CERN 1 November 2010 Workshop on Timing Detectors - Cracow 2010 Time of Flight: the scintillator perspective Paul Lecoq CERN, Geneva

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 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)? Can scintillators satisfy this goal?

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 For the scintillator the important parameters are –Time structure of the pulse –Light yield –Light transport affecting pulse shape, photon statistics and LY Timing parameters decay time of the fast component Photodetector excess noise factor number of photoelectrons generated by the fast component General assumption, based on Hyman theory

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Light output: LYSO example Statistics on about 1000 LYSO pixels 2x2x20mm 3 –produced by CPI –for the ClearPEM-Sonic project (CERIMED) Mean value = ph/MeV For 511 KeV and 25%QE: 2378 phe Assuming ENF= 1.1 Nphe/ENF ≈ 2200 phe

P. Lecoq CERN 5 November 2010 Workshop on Timing Detectors - Cracow 2010  = 40 ns N phe  = 40 ns N phe 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 Q th photoelectron LSO N phe =2200

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Light generation Rare Earth 4f 5d

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Rise time is as important as decay time Rise time

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Photon counting approach LYSO, 2200pe detected,  d =40ns  r =0ns  r =0.2ns  r =0.5ns  r =1ns

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Cross-Luminescent crystals (very fast, low LY) –BaF 2 (1400ph/MeV) but 600ps decay time produces more photons in the first ns (1100) than LSO (670)! Direct bandgap semiconductors S. Derenzo, SCINT2001 –Sub-ns band-to-band recombination in ZnO, CuI,PbI 2, HgI 2 Nanocrystals –Bright and sub-ns emission due to quantum confinement Faster than Ce 3+ ? Intrinsic limit at 17ns Pr 3+ –Pr 3+ 5d-4f transition is always 1.55eV higher than for Ce 3+

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 MaterialDensity (g/cm 3 ) Radiation length X 0 (cm) Refractio n index n Critica l angle Fondamental absorption (nm) Cerenkov threshold energy for e (KeV) Recoil e range above C threshold (  m) # C photons / 511KeV  ray * PbWO ° LSO:Ce ° LuAG:Ce ° LuAP:Ce ° Ultimately fast using Cerenkov emission? Even low enegy  ray produce Cerenkov emission in dense, high n materials This emission is instantaneous with a 1/ 2 spectrum * Low wavelength cut-off set at 250nm for calculations on LSO, LuAG and LuAP Ce absorption bands subtracted from Cerenkov transparency window

P. Lecoq CERN November Workshop on Timing Detectors - Cracow Na PMT left (2150V)PMT right (1500V) LuAG 2013 (undoped -> shows no scintillation) LSO cm Crystals wrapped on 5 sides with teflon. Scope Coincidence: Th_left=-4mV, th_right=-500mV CFD LuAG Cerenkov/LYSO Scintillation coincidence measurement FWHM=374ps LuAG=259ps FWHM=650ps LuAG=587ps

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Light Transport –-49° <  < 49° Fast forward detection 17.2% –131° <  < 229° Delayed back detection 17.2% –57° <  Fast escape on the sides 54.5% –49° <  < 57° and 123° <  < 131° infinite bouncing 11.1% For a 2x2x20 mm 3 LSO crystal Maximum time spread related to difference in travel path is 424 ps peak to peak ≈162 ps FWHM

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Photonic crystals to improve light extraction Periodic medium allowing to couple light propagation modes inside and outside the crystal 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 %34%

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 LuAP Light gain 2.1 LYSO Light gain 2.08 BGO Light gain 2.11 LuAG:Ce Light gain 1.92 Expected Light Output Gain for different crystals Litrani + CAMFR simulation

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 How does the PhC work? Section of the plane crystal- air interface: (EM – fieldplot) Crystal- air interface with PhC grating: θ>θcθ>θc Total Reflection at the interface Extracted Mode θ>θcθ>θc Diffracted modes interfere constructively in the PhC- grating and are therefore able to escape the Crystal

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 PhC fabrication Nano Lithography PhC is produced in cooperation with the INL (Institut des Nanotechnologies de Lyon) Three step approach: 1.Sputter deposition of an auxiliary layer 2.Electron beam lithography (EBL) 3.Reactive ion etching (RIE) RAITH® lithography kit:

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 PhC fabrication Reactive Ion Etching (RIE) 1. Chemically reactive plasma removes Si 3 N 4 not covered by the resist 2. Change the composition of the reactive plasma to remove the resist (PMMA) without etching the Si 3 N 4 x z y Scintillator ITO Si 3 N 4 a Hole depth: 300nm hole diameter: 200nm x z y Scintillator ITO Si 3 N 4 Ion Bombardment PMMA Resist

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 PhC fabrication Results Scanning Electron Images: a = 340nm D = 200nm

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Use larger LYSO crystal: 10x10mm 2 to avoid edge effects 6 different patches (2.6mm x 1.2mm) and 1 (1.2mm x 0.3mm) of different PhC patterns PhC first results 0°45°

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 PhC improves light extraction eficiency But also collimation of the extracted light

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010Conclusions Timing resolution improves with lower threshold Ultimate resolution implies single photon counting High light yield is mandatory –100’000ph/MeV achievable with scintillators Short decay time –15-20ns is the limit for bright scintillators (LaBr 3 ) –1ns achievable but with poor LY Crossluminescent materials Severely quenched self-activated scintillators SHORT RISE TIME –Difficult to break the barrier of 100ps

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 New approaches?Conclusions Crystals with a highly populated donor band (ZnO) Metamaterials loaded with quantum dots Make use of Cerenkov light Improve light collection with photonic crystals

P. Lecoq CERN November Workshop on Timing Detectors - Cracow 2010 Our Team CERN –Etiennette Auffray –Stefan Gundacker –Hartmut Hillemanns –Pierre Jarron –Arno Knapitsch –Paul Lecoq –Tom Meyer –Kristof Pauwels –François Powolny Nanotechnology Institute, Lyon –Jean-Louis Leclercq –Xavier Letartre –Christian Seassal