How photonic crystals can improve the timing resolution of scintillators

Abstract

Abstract-Photonic crystals (PhCs) and quantum optics phenomena open interesting perspectives to enhance the light extraction from scintillating media with high refractive indices as demonstrated by our previous work. By doing so, they also influence the timing resolution of scintillators by improving the photostatistics. The present contribution will demonstrate that they are actually doing much more. Indeed, photonic crystals, if properly designed, allow the extraction of fast light propagation modes in the crystal with higher efficiency, therefore contributing to increasing the density of photons in the early phase of the light pulse. This is of particular interest to tag events at future high-energy physics colliders, such as CLIC, with a bunch-crossing rate of 2 GHz, as well as for a new generation of time-of-flight positron emission tomographs (TOFPET) aiming at a coincidence timing resolution of 100 ps FWHM. At this level of precision, good control of the light propagation modes is crucial if we consider that in a -mm LSO crystal, the time spread (peak to peak) of extracted photons can be as large as 400 ps considering simple light bouncing only. This paper presents a detailed analysis of the light propagation and extraction modes in a LSO crystal combining the LITRANI light ray tracing and the CAMFR PhC simulation codes. Ongoing measurement results are shown with an attempt to unfold the contribution from the improved photostatistics that result from the total enhanced light output on one side and from the improved contribution of fast propagation mode extraction on the other side. Some results are also shown on a new and more industrial process to produce PhCs by the use of nano-imprint technology.

Analytical calculation of the lower bound on timing resolution for PET scintillation detectors comprising high-aspect-ratio crystal elements

“…Scenario (2) represents the CRLB improvement for the case where light collection efficiency at the exit boundary is increased by 50%. This light collection increase represents a reported simulation value for increased light collection via photonic crystal techniques, for an LSO:Ce/grease exit interface [Lecoq et al 2013]. At this level of light collection, the lower bound approaches 110 ps for SPTR values <200 ps.…”

Time resolution of the plastic scintillator strips with matrix photomultiplier readout for J-PET tomograph

“…. To large extent this research is driven by the endeavor to improve the timing properties of the PET systems ( Conti Eriksson 2009, Moszynski et al 2011, Schaart et al 2009, Kuhn et al 2006, Lecoq et al 2013, and therefore so far the investigations concentrated on the small size crystal scintillators. In the recent work a detailed elaboration of the lower bound for time resolution has been published for most kinds of available crystal scintillators .…”

Combining Photonic Crystal and Optical Monte Carlo Simulations: Implementation, Validation and Application in a Positron Emission Tomography Detector

“…The simulation technique was used to incorporate an optimized PhC coating into a state-of-the-art PET detector module. Our results underline the potential of improving the total light yield and CRT by PhCs, although the reported values are more modest than the results presented in [7], [11]. However, care must be taken for comparing these results with our study, as they were derived for individual scintillators instead of an 8×8 array and used air instead of optical glue to couple the scintillator with the photosensor.…”

“…An efficient extraction of photons at their first incidence on the exit face is expected to improve the CRT because the timing is not exclusively defined by the total amount of detected photons, but also depends on their propagation time distribution [7]. Currently, photon extraction is limited by light trapping inside the scintillator due to total internal reflection (TIR) since common inorganic scintillators such as lutetium oxyorthosilicate (LSO) have a high refractive index (n LSO = 1.82) versus the optical glue (n Glue ≈ 1.5) used to couple the scintillator and the photosensor [8].…”

“…This effect can lead to the extraction of photons with shallow incident angles beyond the TIR threshold. First applications of PhC technology to inorganic scintillators showed improved light output and timing resolution for individual LSO crystals [7], [11].…”