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 1November 2010 Workshop on Timing Detectors - Cracow 2010
Time of Flight: the scintillator
perspective
Paul LecoqCERN, Geneva
P. Lecoq CERNNovember 2010 2Workshop 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 CERNNovember 2010 3Workshop 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
€
Δt ∝τ
N phe ENF
Timing parameters
decay time of the fast component
Photodetectorexcess noise factor
number of photoelectrons generated by the fast component
General assumption , based on Hyman theory
P. Lecoq CERNNovember 2010 4Workshop on Timing Detectors - Cracow 2010
Light output: LYSO example
Statistics on about 1000 LYSO pixels 2x2x20mm3 – produced by CPI – for the ClearPEM-Sonic project
(CERIMED)
Mean value = 18615 ph/MeV For 511 KeV and 25%QE:
2378 phe Assuming ENF= 1.1
Nphe/ENF ≈ 2200 phe
P. Lecoq CERN 5November 2010 Workshop on Timing Detectors - Cracow 2010
t = 40 ns
Nphe
t = 40 ns
Nphe
Nphe
Nphe
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
LS
O
Nphe=2200€
WQ t( ) =
N pheQ × 1− e
−t
τ ⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
Q−1
exp −N phe 1− e−
t
τ ⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
⎡
⎣
⎢ ⎢
⎤
⎦
⎥ ⎥e
−t
τ
τ Q −1( ) !
P. Lecoq CERNNovember 2010 6Workshop on Timing Detectors - Cracow 2010
Light generation
€
y(t) = Ae−
t
τ
€
N phe = y(t)dt = Aτ0
∞
∫ Rare Earth4f
5d
P. Lecoq CERNNovember 2010 7Workshop on Timing Detectors - Cracow 2010
Rise time is as important as decay time
Rise time
€
y(t) = Ae− t
τ d 1− e− t
τ r ⎛
⎝ ⎜
⎞
⎠ ⎟
P. Lecoq CERNNovember 2010 8Workshop on Timing Detectors - Cracow 2010
Photon counting approach
LYSO, 2200pe detected, td=40ns
tr=0ns tr=0.2nstr=0.5ns tr=1ns
P. Lecoq CERNNovember 2010 9Workshop on Timing Detectors - Cracow 2010
Cross-Luminescent crystals (very fast, low LY)– BaF2 (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,PbI2, HgI2
Nanocrystals– Bright and sub-ns emission due to quantum confinement
Faster than Ce3+?Intrinsic limit at 17ns
Pr3+
– Pr3+ 5d-4f transition is always 1.55eV higher than for Ce3+
€
τ Pr
τ Ce
≈λ Pr
λ Ce
⎛
⎝ ⎜
⎞
⎠ ⎟
3
≈3.5eV
3.5eV +1.55eV
⎛
⎝ ⎜
⎞
⎠ ⎟3
=1
3
P. Lecoq CERNNovember 2010 10Workshop on Timing Detectors - Cracow 2010
Material Density (g/cm3)
Radiation length X0
(cm)
Refraction index n
Critical angle
Fondamental absorption
(nm)
Cerenkov threshold
energy for e (KeV)
Recoil e range
above C threshold
(mm)
# C photons / 511KeV g ray
*
PbWO4 8.28 0.89 2.2 63° 370 63 51321
LSO:Ce 7.4 1.14 1.82 57° 190 101 52715
LuAG:Ce 6.73 1.41 1.84 57° 177 97 58222
LuAP:Ce 8.34 1.1 1.95 59° 146 84 48728
Ultimately fast using Cerenkov emission?
Even low enegy g ray produce Cerenkov emission in dense, high n materials
This emission is instantaneous with a 1/l2 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 CERNNovember 2010 11Workshop on Timing Detectors - Cracow 2010
22Na
PMT left (2150V) PMT right (1500V)
LuAG 2013 (undoped -> shows no scintillation)LSO 1121
8cm 8cm
Crystals wrapped on5 sides with teflon.
Scope
Coincidence:Th_left=-4mV, th_right=-500mV
CFD
LuAG Cerenkov/LYSO Scintillation coincidence measurement
FWHM=374psLuAG=259ps
FWHM=650psLuAG=587ps
P. Lecoq CERNNovember 2010 12Workshop on Timing Detectors - Cracow 2010
Light Transport
– -49° < θ < 49° Fast forward detection 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°
infinite bouncing 11.1%
For a 2x2x20 mm3 LSO crystalMaximum time spread related to
difference in travel path is424 ps peak to peak
≈162 ps FWHM
P. Lecoq CERNNovember 2010 13Workshop 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. 1102-1106
24% 34%
P. Lecoq CERNNovember 2010 14Workshop on Timing Detectors - Cracow 2010
LuAP Light gain2.1
LYSO Light gain2.08
BGO Light gain2.11
LuAG:Ce Light gain1.92
Expected Light Output Gain for different crystals
Litrani + CAMFR simulation
P. Lecoq CERNNovember 2010 15Workshop 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>θc
Total Reflection at the interface Extracted Modeθ>θc
Diffracted modes interfere constructively in the PhC- grating and are therefore able to escape the Crystal
P. Lecoq CERNNovember 2010 16Workshop 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 CERNNovember 2010 17Workshop on Timing Detectors - Cracow 2010
PhC fabrication
Reactive Ion Etching (RIE)
1. Chemically reactive plasma removes Si3N4 not covered by the resist
2. Change the composition of the reactive plasma to remove the resist (PMMA) without etching the Si3N4 x
z
y
Scintillator
ITOSi3N4
a
Hole depth: 300nm
hole diameter: 200nm
x
z
y
Scintillator
ITOSi3N4
Ion Bombardment
PMMA Resist
P. Lecoq CERNNovember 2010 18Workshop on Timing Detectors - Cracow 2010
PhC fabrication
Results
Scanning Electron Images:a = 340nm
D = 200nm
P. Lecoq CERNNovember 2010 19Workshop on Timing Detectors - Cracow 2010
Use larger LYSO crystal: 10x10mm2 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°
Preliminary
P. Lecoq CERNNovember 2010 20Workshop on Timing Detectors - Cracow 2010
PhC improves light extraction eficiency
But also collimation of the extracted light
P. Lecoq CERNNovember 2010 21Workshop on Timing Detectors - Cracow 2010
Conclusions 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 (LaBr3)
– 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 CERNNovember 2010 22Workshop 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 CERNNovember 2010 23Workshop 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