Primordial nuclides and low-level counting at Felsenkeller

Within cosmology, there are two entirely independent pillars which can jointly drive this field towards precision: Astronomical observations of primordial element abundances and the detailed surveying of the cosmic microwave background. However, the comparatively large uncertainty stemming from the nuclear physics input is currently still hindering this effort, i.e. stemming from the ${}^2$H(p,γ)${}^3$He reaction. An accurate understanding of this reaction is required for precision data on primordial nucleosynthesis and an independent determination of the cosmological baryon density. Elsewhere, our Sun is an exceptional object to study stellar physics in general. While we are now able to measure solar neutrinos live on earth, there is a lack of knowledge regarding theoretical predictions of solar neutrino fluxes due to the limited precision (again) stemming from nuclear reactions, i.e. from the ${}^3$He(α,γ)${}^7$Be reaction. This thesis sheds light on these two nuclear reactions, which both limit our understanding of the universe. While the investigation of the ${}^2$H(p,γ)${}^3$He reaction will focus on the determination of its crosssection in the vicinity of the Gamow window for the Big Bang nucleosynthesis, the main aim for the ${}^3$He(α,γ)${}^7$Be reaction will be a measurement of its γ-ray angular distribution at astrophysically relevant energies. In addition, the installation of an ultra-low background counting setup will be reported which further enables the investigation of the physics of rare events. This is essential for modern nuclear astrophysics, but also relevant for double beta decay physics and the search for dark matter. The presented setup is now the most sensitive in Germany and among the most sensitive ones worldwide.