How Photonic Crystals can
improve scintillator
timing resolution
Paul Lecoq, E. Auffray, A. Knapitsch
CERN, Geneva
October 2012
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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


PhC

Light yield,
Rise time,
Decay time
SCINT2013
April 12-19
Shanghai
2. The ligh transport in the crystal
PhC

Time spread related to different light propagation modes
3. The light extraction efficiency (LYLO)


October 2012
Impact on photostatistics
Weights the distribution of light propagation modes
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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
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Extracted Mode
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Photonic crystals
0°
45°

Use large 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
A. Knapitsch et al, “Photonic crystals: A novel approach to enhance the light output of scintillation based detectors,
NIM A268, pp.385-388, 2011
October 2012
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Measurements:
LO angular distribution
LYSO: 1.2x2.6x5mm3
October 2012
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Motivation1: Improve timing
through increased light output
Photonic Crystal Slab
Air
ph α
Crystal
LYSO
1.2 x 2.6 x 5mm3
Wrapping + Glue
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Light Transport
-49° <  < 49° Fast forward detection 17.2%
131° <  < 229° Delayed back detection 17.2%
57° <  < 123° Fast escape on the sides 54.5%
49° <  < 57° and 123° <  < 131°
infinite bouncing
11.1%
Improving light extraction efficiency at first hit on coupling
face to photodetector is the key
–
–
–
–
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Photon propagation time spread
Photodetector
g
2
x
L
t prop max 
nx
(2L  x) n

c cos(1) c cos( 2 )
For L = 20mm LSO (n = 1.82)
ngrease= 1.41  c = 50.8°
October 2012
with 1
2
0
c
tmax= 71 ps for x = L
tmax= 384 ps for x = 0
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Motivation2: Improve Timing
through redistribution of light
propagation modes
Extract more photons at
first incidence with PhC
= better timing
Regular LYSO
a)
October 2012
b)
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Light propagation modes
contribution to timing resolution
Designed PhC pattern
Actual PhC pattern
SEM picture
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Preliminary results
Normalized CTR
 1.05
2


PhC configurations tested (P4 & P6)
Expected
gain for crystal
the measured
LOthe
gain
– compared
to anCTR
unpatterned
cut from
same block
1
Small
crystal size (1.2x2.6x5mm3) limited by electron beam
lithography constrainsts
– small sensitivity to light transport variations
0.95
CTR measurement against a 2x2x10mm3 LYSO ref. crystal
0.9
0.85
0.8
0.75
Normalized LO
0.7
1
Ref
October 2012
1.33
P4
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1.56
P6
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Towards a nanoimprint
mass production technology
Evaluate methods, such as nanoimprint technologies
to implement this approach at an industrial scale

An 8’’ Si stamp has been produced
by UV-lithography and dry etching

Direct application of this hard stamp
does not give optimal results because
of the crystal surface defects

Si stamp was replicated in two
different soft materials to play the
role of soft stamp to imprint nonconformal substrate as BGO
Intermediate polymer stamp
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Polydimethylsiloxane stamp
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First test on BF33 glass substrate

October 2012
Re-do process with newly polished BGO substrates with a
deposited Si3N4 layer from SILSEF
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Conclusions

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

October 2012
Nanoimprint technologies offer attractive
solutions for cost effective mass
production
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