Diplomstudiengang Technische Physik (Physikalische Technik)
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The MAGIC II telescope is, after MAGIC I, the second large diameter Cherenkov telescope build in La Palma. Its goal is to detect extraterrestrial gamma-radiation in the energy region from 30 GeV to 30 TeV. Additionally, MAGIC II has been build with the purpose of stereoscopic observation with MAGIC I. Cosmic particles, such as gamma-rays, protons and heavy particles, generate, when hitting the atmosphere, so-called extended air showers. Ultra-relativistic particles of these showers produce Cherenkov light that can be observed by ground-based telescopes during dark, clear nights. These measurements both provide information about the direction of the primary particle and its energy. From the shower image shape one can distinguish the very rare gamma-rays from the much larger hadronic background. The intensity of the Cherenkov radiation scales in first order with energy. Depending on the shower impact parameter the light flux is typically in the order of 100 photons/1 m2 for a 1 TeV shower and an impact value below about 120 m. For impact parameters above 120 m the intensity of the light flux is rapidly dropping as the light originates from the shower halo particles. Therefore, the optical performance as well as the size of the mirror for Cherenkovtelescopes is of great importance, since it is a key element in the detection chain. The optical parameters like the point spread function, the focused reßectivity and the focal length play a major role for recording high quality images. The mirror performance data have to be used for the Monte Carlo Simulation of the MAGIC telescope because it is impossible to test the telescope with calibrated cosmic ‘test sources’, which do not exist. Note that this is an important difference to particle physics experiments where one normally tests detector elements in test beams. Moreover, it is of great importance to assure the mirror’s resistance against the impact of the environmental, such as against dew, rain water or icing. Therefore, a number of measurements were performed to determine the relevant optical parameters and water tightness for the all-aluminium mirror elements of the MAGIC-II telescope. In this thesis the principles and the results of these measurements will be presented. Additionally, in order to improve the light detection efficiency for future telescope, one considered to use so-called Silicon Photo-Multipliers (SiPM) for Cherenkov light detection. One of the most critical parameters of these novel and still experimental semiconductor light detectors with internal gain is the generation of light in the avalanche gain amplification leading to optical crosstalk and preventing to operate SiPMs with high photon detection efficiency (PDE). As the operation parameters of these devices have also to be used for the Monte Carlo Simulation of future Cherenkov telescopes it is important to evaluate the optical crosstalk rate and its impact on the PDE in great detail. The detailed study of optical crosstalk is also part of this diploma thesis and the analysis and results of this study will be presented here.