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Abstract

In this thesis I describe the motivations behind and design of the lake concept, an alternative detector technology idea for the future Southern Wide-field Gamma-ray Observatory (SWGO), a planned gamma-ray observatory in the Andes. In the lake concept, light-tight bladders that form optically separated water Cherenkov detector (WCD) units would be deployed in a natural or artificial lake. This technology option offers potential cost savings compared to tank-based approaches. In this work I focus on the double-layered WCD design, where a lower layer with reflective lining is used for muon identification in addition to the upper layer that is used for calorimetric detection of gamma-ray initiated extensive air showers. I present simulations of the lake option, showing that the presence of water around the double-layered WCD units provides a shielding advantage for the lower chamber. Furthermore I present a discussion of the water waves that pose a challenge for the lake concept.

In collaboration with colleagues from MPIK, I have designed, evaluated, tested and simulated a full scale prototype detector unit for the lake concept. This detector is designed for the lake idea, however it also provides the first experimental verification of the double-layered design for SWGO and in addition allowed tests of materials. After the evaluation of early prototypes, a final double-layer WCD prototype was built. A flexible inner chamber that forms the lower chamber, named the matryoshka, was inserted into a larger commercially produced single cell bladder, forming the first double-layered WCD prototype for SWGO. A setup utilizing photomultiplier tubes and a full electronics chain, where the WCD prototype was located between two muon tagger detectors, was used to take coincidence data of through-going particles. The data were compared with simulations and a good agreement between the detector simulation and data was seen. The materials used for the prototyping of unit detectors were tested for reflectivity and water degradation. These tests inform the simulations performed to evaluate the double-layer WCD prototype.

Among the sources that would be transiting the field of view of SWGO at low zenith is HESS J1825-137, a middle aged pulsar wind nebula (PWN) that appears to be among the largest sources known in the gamma-ray sky and is one of the most powerful emitters at TeV and ultra-high energies. In this thesis I present an analysis of HESS J1825-137 with the recently revised data from the HAWC observatory, a wide-field gamma-ray observatory in Mexico and find a spectrum consistent with other instruments. Similar to previous observations by H.E.S.S. and Fermi-LAT we observe an energy-dependent morphology of HESS J1825-137 up to hundreds of TeV. The PWN decreases in size with increasing energy, which can be used to constrain particle transport mechanisms, once the morphology at higher energies is resolved better with SWGO. As PWNe are currently the dominant source class in TeV energies, they form one of the important science goals with SWGO.