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 =nx

c cos(θ1)−

(2L − x) n

c cos(θ 2) with q1 0 q2 qc

For L = 20mm LSO (n = 1.82)ngrease= 1.41 qc = 50.8°

2q

Dtmax= 71 ps for x = LDtmax= 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

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

Nanoimprint technologies offer attractive solutions for cost effective mass production