Detailed Studies of Light Transport in Optical Components of Particle Detectors

Scintillator- and fibre-based particle detectors are an indispensable tool in high-energy particle physics, medical physics and other fields of application. The potentially very low light yield, down to a few photons, of the optical detector components in combination with the inevitable light transport to photodetectors necessitate an optimal design and detailed understanding of such detectors. Thus, very detailed simulations are needed, which require a very accurate modelling of the optical physics (optics, scintillation, wavelength-shifting effects,...), of the optical material properties, and of the optical components. To allow for a reliable usage also by less experienced users, the necessary complexity and flexibility of a suitable simulation framework must not lead to an increasing danger of user mistakes. Additionally, the required effort for creating or modifying a detailed simulation has to be minimised in order to allow for the fast creation of flexible simulation setups. In the scope of this thesis, these challenges have been addressed by developing the general simulation framework GODDeSS. It is an extension of the particle-physics simulation tool Geant4 and allows for the easy simulation of optical detector components, especially combinations of scintillators, optical fibres, and photodetectors. To achieve this, the creation of simulated setups is automated as much as possible: The material properties of the optical detector components are specified via easy-to-read text files and new object classes allow for an easy creation of scintillator tiles, optical fibres, reflective wrappings and paints, and photodetectors with basically a single line of code per created object. This results in an increase of flexibility and at the same time in a reduction of complexity. The user can create extensive setups within a few lines of code and typical mistakes are avoided, as the peculiarities of Geant4 regarding the configuration of the optical physics processes are treated automatically by the GODDeSS framework. All this makes GODDeSS an excellent approach to simplify the detailed simulations of optical detector components, which are necessary for designing modern particle detectors and for understanding their response. This thesis introduces the GODDeSS framework, its classes, and its functionality. Furthermore, the extensive efforts to validate it against manufacturer data as well as against test measurements with prototype setups will be presented. Additionally, detailed simulations have been performed in order to investigate the optical properties of optical fibres and of the characteristics of the response of detector modules.