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Ultimate Time Resolution in
Scintillator-based detectors for
Calorimetry and Time-of-Flight PET
P. Lecoq,
E. Auffray, S. Gundacker
CERN, Geneva, Switzerland
This work is supported under the ERC Grant Agreement N°338953–TICAL
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
1
Why fast timing in HEP?

TOF for
– Particle ID
– Pileup mitigation at high luminosity colliders




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Improve pattern recognition in Cerenkov detectors
Cerenkov/Scintillation differentiation (Dual Readout Cal)
Bring additional information on the shower development
in a segmented calorimeter
Current state of the art for TOF in Alice expt: 75ps
Major advances in detector/enabling technologies
– Fast and high light yield scintillators
– SiPMs, MCPs
– Fast low noise FE electronics (NINO)

A 4D imaging HHCAL is within reach
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
2
Why fast timing in PET?

TOF for rejecting background events (event collimation)
– Requires 200ps TOF resolution for a few cm ROI
(EndoTOFPET-US FP7 project)
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TOF for improving image S/N
– Requires 100ps TOF resolution for x5 S/N improvement, which
brings a potential sensitivity gain (dose reduction)
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TOF for direct 3D information
– Requires 20ps TOF resolution for 3mm resolution along LOR

TOF for restoring image quality for limited angle
tomography
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
3
State of th Art: CTR with NINO
chip (Time over Threshold)
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
4
Influence of crystal length on CTR
S. Gundacker et.al., NIMA, dx.doi.org/10.1016/j.nima.2013.11.025
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
5
State of the art: EndoTOFPET
system performance
• CTR distribution of 168 Modules (4x4 cells each) , 2688 LORs
• The bias voltage applied to each module is fixed to 2.5 Volt over breakdown
Voltage.
• Same threshold and temp for all channels
4x4 cells
3.5x3.5x15mm3
crystals
80mm 3M ESR gap
Discrete Siliconthrough-via
(TPV) MPPC array
Hamamatsu (S12643050CN)
3x3mm2, 0.6mm gap
March 2014
NINO ASIC
239 ps
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
6
The detection chain
Crystal
SiPM
electronics
g
Dt
Dt
+ tk’ ph
Conversion depth
Scintillation
process
tkth pe =
March 2014
q
2
+ ttransit
Transit time
jitter
+ tSPTR
+ tTDC
Single photon
time spread
TDC
conversion time
Random deletion 1
Unwanted pulses 1
Absorption
Self-absorption
DCR, cross talk
Afterpulses
Random deletion 2
Unwanted pulses 2
SiPM PDE
DCR
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
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Modeling the whole chain
SiPM
S. Gundacker
Thesis, CERN,
Feb2014
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
8
Analog vs Digital approach
Cramer-Rao lower bound
S. Gundacker
Thesis, CERN,
Feb2014

Under investigation
– in the frame of the FP7 EndoTOFPET-US project
– with the Philips digital evaluation kit recently ordered
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
9
Parameters of interest to
improve timing resolution
Parameters for LSO: Ce, Ca and Hamamatsu S10931-050P MPPC
Rise time influence limited
by SPTR (66ps)
March 2014
CTR improves like
SQRT (photon time density)
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
10
Factors influencing
scintillator time resolution
P. Lecoq et al, IEEE Trans. Nucl. Sci. 57 (2010) 2411-2416
Besides all factors related to photodetection and readout
electronics the scintillator contributes to the time
resolution through:
1. The scintillation mechanism
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Light yield,
Rise time,
Decay time
2. The light transport in the crystal
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Time spread related to different light propagation modes
3. The light extraction efficiency (LYLO)
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
March 2014
Impact on photostatistics
Weights the distribution of light propagation modes
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
11
Influence of prompt photons
2x2x3mm3 LSO:Ce, Ca with 70ps rise time
and an arbitrary number of prompt photons generated
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
12
Light generation in scintillators
5d
Rare Earth
4f
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
13
Hot intraband luminescence

Wide emission spectrum from UV
to IR
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Ultrafast emission in the ps range
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Independant of temperature
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Independant of defects
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Absolute Quantum Yield
Whn/Wphonon = 10-8/(10-11-10-12)
≈ 10-3 to 10-4 ph/eh pair

Higher yield if structures or dips
in CB? Interesting to look at CeF3
More details in SCINT2013 paper TNS-00194-2013
M. Korzhik, P. Lecoq, A. Vasil’ev
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
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Photon propagation time spread
Photodetector
g
q2
x
L
Dt prop max 
nx
(2L  x) n

c cos(q1) c cos(q 2 )
For L = 20mm LSO (n = 1.82)
ngrease= 1.41  qc = 50.8°
March 2014
with q1
q2
0
qc
Dtmax= 71 ps for x = L
Dtmax= 384 ps for x = 0
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
15
Photonic crystals
Nanostructured interface allowing to couple light
propagation modes inside and outside the crystal
Crystal
θ>θc
air
Crystal- air interface with
PhC grating:
θ>θc
Total Reflection
at the interface θ>θc
March 2014
Extracted Mode
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
16
Photonic crystals increase
the light extraction efficiency
0°
45°
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Use large LYSO crystal: 10x10mm2 to avoid edge
effects
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6 different patches (2.6mm x 1.2mm) and 1
(1.2mm x 0.3mm) of different PhC patterns
A. Knapitsch et al, “Photonic crystals: A novel approach to enhance the light output of scintillation based detectors,
NIM A268, pp.385-388, 2011
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
17
Photonic crystals compress
the light propagation modes
Extract more photons at
first incidence with PhC
= better timing
Regular LYSO
a)
March 2014
b)
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
18
Conclusions

Standard scintillation mechanisms are unlikely to give
access to the 10ps range

A number of transient phenomena could generate ps
measurable signals

Photonic crystals improve scintillator timing
resolution by two means:
– By increasing the light output and therefore decreasing the
photostatistics jitter
– By redistributing the light in the fastest propagation modes
in the crystal
March 2014
Fast timing workshop, Clermont Fd, March 12-14, 2014
P. Lecoq CERN
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