How Photonic Crystals can improve scintillator timing resolution

1. How Photonic Crystals can improve scintillatortiming resolution Paul Lecoq, E. Auffray, A. Knapitsch CERN, Geneva

2. Factorsinfluencingscintillator time resolution Besides all factorsrelated to photodetection and readoutelectronics the scintillatorcontributes to the time resolutionthrough: • The scintillation mechanism • Light yield, • Rise time, • Decay time • The ligh transport in the crystal • Time spreadrelated to different light propagation modes • The light extraction efficiency (LYLO) • Impact on photostatistics • Weights the distribution of light propagation modes P. Lecoq et al, IEEE Trans. Nucl. Sci. 57 (2010) 2411-2416 SCINT2013 April 12-19 • Shanghai PhC PhC

3. Photoniccrystals Nanostructured interface allowing to couple light propagation modes inside and outside the crystal Crystal- air interface with PhC grating: Crystal air θ>θc θ>θc Total Reflection at the interface θ>θc Extracted Mode

4. Photoniccrystals • Use large LYSO crystal: 10x10mm2 to avoidedgeeffects • 6 different patches (2.6mm x 1.2mm) and 1 (1.2mm x 0.3mm) of differentPhC patterns 0° 45° A. Knapitsch et al, “Photonic crystals: A novel approach to enhance the light output of scintillation based detectors, NIM A268, pp.385-388, 2011

5. Measurements: LO angular distribution LYSO: 1.2x2.6x5mm3

6. Motivation1: Improve timing through increased light output Air Photonic Crystal Slab α ph Crystal LYSO 1.2 x 2.6 x 5mm3 Wrapping + Glue

7. Light Transport • -49° < θ < 49° Fastforwarddetection 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° infinitebouncing11.1% Improving light extraction efficiencyat first hit on coupling face to photodetectoris the key

8. Photonpropagation time spread Photodetector q2 x g L with q1 0 q2qc Dtmax= 71 ps for x = L Dtmax= 384 ps for x = 0 For L = 20mm LSO (n = 1.82) ngrease= 1.41 qc = 50.8°

9. Motivation2: Improve Timing through redistribution of light propagation modes Extract more photons at first incidence with PhC = better timing Regular LYSO a) b)

10. Light propagation modes contribution to timing resolution DesignedPhC pattern ActualPhC pattern SEM picture

11. Preliminaryresults Expected CTR gain for the measured LO gain • 2 PhC configurations tested (P4 & P6) • compared to an unpatternedcrystalcutfrom the same block • Small crystal size (1.2x2.6x5mm3) limited by electronbeamlithographyconstrainsts • smallsensitivity to light transport variations • CTR measurementagainst a 2x2x10mm3 LYSO ref. crystal Normalized CTR Normalized LO Ref P4 P6

12. Towards a nanoimprintmass 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 stampdoes not give optimal resultsbecause of the crystal surface defects • Si stamp was replicated in two different soft materials to play the role of soft stamp to imprint non-conformal substrate as BGO Intermediatepolymerstamp Polydimethylsiloxanestamp

13. First test on BF33 glass substrate • Re-do process with newly polished BGO substrates with a deposited Si3N4 layer from SILSEF

14. Conclusions • Photoniccrystalsimprovescintillator timing resolution by twomeans: • By increasing the light output and thereforedecreasing the photostatisticsjitter • By redistributing the light in the fastest propagation modes in the crystal • Nanoimprint technologies offer attractive solutions for cost effective mass production