Photonic Crystals: Enhancing the Light Output of Scintillation Based Detectors

A scintillator is a material which emits light when excited by ionizing radiation. Such materials are used in a diverse range of applications; From high energy particle physics experiments, X-ray security, to nuclear cameras or positron emission tomography. Future high-energy physics (HEP) experiments as well as next generation medical imaging applications are more and more pushing towards better scintillation characteristics. One of the problems in heavy scintillating materials is related to their high index of refraction. As a consequence, most of the scintillation light produced in the bulk material is trapped inside the crystal due to total internal reflection. The same problem also occurs with light emitting diodes (LEDs) and has for a long time been considered as a limiting factor for their overall efficiency. Recent developments in the area of nanophotonics were showing now that those limitations can be overcome by introducing a photonic crystal (PhC) slab at the outcoupling surface of the substrate. Photonic crystals are optical materials which can affect the propagation of light in multiple ways. In this work, we used 2D PhC slabs consisting of hexagonal placed rods of air or square shaped pillars made of silicon nitride. The 300-500nm thick photonic crystal slab changes the reflection properties of the crystal-air interface in different ways. In our case the pattern was optimized to enable the light extraction of photons which are otherwise confined within the scintillator by total internal reflections. In the simulation part we could show light output improvements of a wide range of scintillating materials due to light scattering effects of the photonic crystal grating. For these calculations we used two different programs. At first, a Monte Carlo program called LITRANI was used to calculate the light propagation inside a common scintillation based detector setup. LITRANI calculates the path of a photon starting from the production inside the crystal by an ionizing event all the way through the different materials until it is absorbed or detected. It uses the Fresnel formulas to calculate the light transition and reflection between different absorbing or non-absorbing materials. The second program in our simulations was used to calculate the light extraction efficiency of our PhC structure. This program calculates the 2D photonic crystal interface using a rigorous coupled wave analysis (RCWA) tool. It was used to determine the reflection and transition parameters of the PhC slab for a photon of a certain angle and polarization. With the combination of these two programs the overall gain of the PhC modified scintillators could be calculated and we could show a theoretical light yield improvement of 60-100% by the use of PhCs. In the practical part of this work it is shown how the first samples of PhC slabs on top of different scintillators were produced. The aim of these demonstrator samples was to confirm the simulation results by measurements. Through the deposition of an auxiliary layer of silicon nitride and the adaptation of the standard electron beam lithography (EBL) parameters we could successfully produce several PhC slabs on top of $1.2 imes 2.6 imes 5mm^3$ lutetium oxyorthosilicate (LSO) scintillators. In the characterization process, the PhC samples showed a 30-60% light yield improvement when compared to an unstructured reference scintillator. In our analysis it could be shown that the measured PhC sample properties are in close accordance with the calculations from our simulations.