how photonic crystals can improve scintillator timing resolution
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P. Lecoq CERN 1October 2012 NSS-MIC Annaheim, October 30th, 2012
How Photonic Crystals can
improve scintillator
timing resolution
Paul Lecoq, E. Auffray, A. KnapitschCERN, Geneva
P. Lecoq CERNOctober 2012 2NSS-MIC Annaheim, October 30th, 2012
Factors influencing scintillator time resolution
Besides all factors related to photodetection and readout electronics the scintillator contributes to the time
resolution through:1. The scintillation mechanism
Light yield, Rise time, Decay time
2. The ligh transport in the crystal Time spread related to different light propagation modes
3. The light extraction efficiency (LYLO) Impact on photostatistics Weights the distribution of light propagation modes
P. Lecoq et al, IEEE Trans. Nucl. Sci. 57 (2010) 2411-2416
PhC
PhC
SCINT2013April 12-19Shanghai
P. Lecoq CERNOctober 2012 3NSS-MIC Annaheim, October 30th, 2012
Photonic crystals
Crystal
Crystal- air interface with PhC grating:
θ>θc
Total Reflection at the interface Extracted Modeθ>θc
Nanostructured interface allowing to couple light propagation modes inside and outside the crystal
air
θ>θc
P. Lecoq CERNOctober 2012 4NSS-MIC Annaheim, October 30th, 2012
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
0° 45°
Photonic crystals
A. Knapitsch et al, “Photonic crystals: A novel approach to enhance the light output of scintillation based detectors, NIM A268, pp.385-388, 2011
P. Lecoq CERNOctober 2012 5NSS-MIC Annaheim, October 30th, 2012
Measurements: LO angular distribution
LYSO: 1.2x2.6x5mm3
P. Lecoq CERNOctober 2012 6NSS-MIC Annaheim, October 30th, 2012
Motivation1: Improve timing through increased light output
Photonic Crystal Slab
LYSO 1.2 x 2.6 x 5mm3
Wrapping + Glue
αph
Crystal
Air
P. Lecoq CERNOctober 2012 7NSS-MIC Annaheim, October 30th, 2012
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%
Improving light extraction efficiency at first hit on couplingface to photodetector is the key
P. Lecoq CERNOctober 2012 8NSS-MIC Annaheim, October 30th, 2012
Photon propagation time spread
xL
€
Δt prop max = nxc cos(θ1)
− (2L − x) nc cos(θ 2)
with θ1 0 θ2 θc
For L = 20mm LSO (n = 1.82)ngrease= 1.41 θc = 50.8°
θ2
Δtmax= 71 ps for x = LΔtmax= 384 ps for x = 0
Photodetector
g
P. Lecoq CERNOctober 2012 9NSS-MIC Annaheim, October 30th, 2012
Motivation2: Improve Timing through redistribution of light
propagation modes
Regular LYSO
Extract more photons at first incidence with PhC
= better timing
a) b)
P. Lecoq CERNOctober 2012 10NSS-MIC Annaheim, October 30th, 2012
Designed PhC pattern
Actual PhC patternSEM picture
Light propagation modes contribution to timing resolution
P. Lecoq CERNOctober 2012 11NSS-MIC Annaheim, October 30th, 2012
Preliminary results 2 PhC configurations tested (P4 & P6)
– compared to an unpatterned crystal cut from the same block Small crystal size (1.2x2.6x5mm3) limited by electron beam
lithography constrainsts – small sensitivity to light transport variations
CTR measurement against a 2x2x10mm3 LYSO ref. crystal
1 1.33 1.560.7
0.75
0.8
0.85
0.9
0.95
1
1.05N
orm
aliz
ed C
TR
Normalized LO
Ref P4 P6
Expected CTR gain for the measured LO gain
P. Lecoq CERNOctober 2012 12NSS-MIC Annaheim, October 30th, 2012
Towards a nanoimprint mass production technology
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
Evaluate methods, such as nanoimprint technologies to implement this approach at an industrial scale
Intermediate polymer stamp Polydimethylsiloxane stamp
Si stamp was replicated in two different soft materials to play the role of soft stamp to imprint non-conformal substrate as BGO
P. Lecoq CERNOctober 2012 13NSS-MIC Annaheim, October 30th, 2012
First test on BF33 glass substrate
Re-do process with newly polished BGO substrates with a deposited Si3N4 layer from SILSEF
P. Lecoq CERNOctober 2012 14NSS-MIC Annaheim, October 30th, 2012
ConclusionsPhotonic 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
Nanoimprint technologies offer attractive solutions for cost effective mass production
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