[en] In the present dissertation, the nuclear reactions ²⁵Mg(α,n)²⁸Si, ²⁶Mg(α,n)²⁹Si, ¹⁸O(α,n)²¹Ne are investigated in the astrophysically interesting energy region from E_α=1000 keV to E_α=2450 keV. The experiments were performed at the Nuclear Structure Laboratory of the University of Notre Dame (USA) with the Van-de-Graaff accelerator KN. Solid state targets with evaporated magnesium or anodized oxygen were bombarded with α-particles and the released neutrons detected. For the detection of the released neutrons, computational simulations were used to construct a neutron detector based on ³He counters. Because of the strong occurrence of background reactions, different methods of data analysis were employed. Finally, the impact of the reactions ²⁵Mg(α,n)²⁸Si, ²⁶Mg(α,n)²⁹Si, ¹⁸O(α,n)²¹Ne on stellar nucleosynthesis is investigated by means of network calculations. (orig.)