Optimization of the light extraction from heavy inorganic scintillators

Inorganic scintillators are widely used in modern medical imaging modalities as converter for the X- and gamma-radiation that is used to obtain information about the interior of the body. Likewise, they are applied in high-energy physics to measure the energy of particles that are produced in particle physics experiments. Their use is motivated by the very good detection efficiency of these materials for hard radiation which allows the construction of relatively compact and finely pixelised systems with a high spatial resolution. One key problem in the development of the next generation of particle detectors and medical imaging systems is the optimisation of the energy resolution of the detectors. This parameter is influenced by the statistical fluctuations of the light output of the scintillators, i.e. by the number of photons that are detected when a particle deposits its energy in the scintillator. The light output of the scintillator depends not only on the absolute number of generated photons but also on the geometrical shape of the material, its transmission properties at the wavelength of scintillation, and its refractive index. Especially in tiny detector crystals with small aspect ratio, a significant fraction of photons is lost before conversion into an electronic signal in the photo detector. This effect increases the statistical fluctuations of the light output and therefore, deteriorates the energy resolution. The present work explores possible ways to overcome the problem of total internal reflection at the surface of the scintillator that couples to the photo detector, which is one of the principal reasons for signal losses in heavy inorganic scintillators with high refractive index. The emphasis is placed on the recently developed Ce doped scintillators LuYAP:Ce and LYSO:Ce which are used in the dedicated PET imaging systems ClearPEM and ClearPET developed by the Crystal Clear Collaboration. Starting point of the work are Monte-Carlo simulations of standard LuYAP and LYSO crystals used in ClearPEM and ClearPET with the light ray tracing program LITRANI. These simulations are used to assess the losses induced by total internal reflection and the potential in effective light yield gain. In order to calibrate the model, experiments in several reference setups are performed and the results compared with those from simulations of equivalent systems. This combination of experimental and simulated data is also used for a precise determination of the absolute light yield of the scintillators. The results of the simulations are then used to study the effect of non-planar coupling faces on the light extraction. Two approaches are considered: A macroscopic one, in which trapezoidal grooves with a depth of several hundred micrometer are used to break up total internal reflection at the coupling face; and a microscopic one that uses the unique op tical properties of photonic crystals to assess photons outside the extraction cone of a plain coupling face. The potential of the macroscopic approach is assessed by means of LITRANI simulations and validated by experiments with prototype samples designed on basis of the simulation results. The experimental results show that the gain in light yield is less than predicted from simulations of an idealized system. However, they are found to be well described by a model where the crystal layers immediately beneath the groove surfaces are described by a heavily absorbing and diffusing material. The microscopic approach is probed by means of simulations with a scattering-matrix algorithm. This algorithm is used to evaluate the impact of several structural parameters of the photonic crystal on the transmission properties of the exit surface. From these data, a model is developed which uses the results obtained with LITRANI to calculate the gain in light collection efficiency for a number of possible designs. Finally, possible fabrication techniques for photonic crystal structures on scintillators are briefly discussed. The thesis closes with a short summary of the results and a short outlook on possible future projects based on the presented approaches.