Improving light output and coincidence time resolution of scintillating crystals using nanoimprinted photonic crystal slabs

Scintillating crystals are used in numerous applications of ionizing radiation detectors. In time of flight positron emission tomography (TOF-PET) for example, both energy and coincidence time resolution (CTR) are important characteristics that could significantly benefit if more light from scintillators, otherwise trapped, could be collected by the photodetector. A novel and promising method to extract more efficiently the light produced in crystal scintillators with high index of refraction is to introduce a thin nanopatterned photonic layer on the readout surface. In this paper, we describe the patterning process of a photonic crystal layer made of TiO$_2$ with 390 nm diameter ”pillars” in a square lattice with a periodicity of 580 nm and a structure thickness of 300 nm on one side of a 10x10x10 mm $^3$ LYSO cube. The production process used was nanoimprint lithography. A substantial increase in light yield of ≥ 50% has been measured in good agreement with our simulations. An interesting result from these measurements is that the improvement in light output is independent of whether the crystal is read out from its photonically patterned side or from the one opposite to it. For all cases studied, the energy resolution improved by a factor of 1.1. On the other hand, the CTR, being very threshold dependent, is unlike the light yield not subject to a constant improvement. It turns out that, at low thresholds, the gain (improvement) in CTR is limited to 1.2, and then rapidly increases to a value of more than 2 at higher thresholds. This is mainly explained by an additionally induced light transfer time spread of the photonic pattern. Several configurations with and without Teflon wrapping were investigated.