Neutron capture reactions of short-lived isotopes

The origin of elements in the universe is attributed to a multitude of processes, that are located in both Big Bang and stellar scenarios. The knowledge of the involved reactions and reaction rates makes it possible to probe the underlying models and thus to make more precise statements about the plausibility of the scenarios. Some of these processes rely on neutron capture reactions in order to reach heavier elements. The activation method is a tool to determine the neutron capture reaction probabilty, provided the target nucleus has detectable radioactivity. The experimental investigation of a reaction with a short-lived product nucleus is a particular challenge, since most of the nuclei created during the activation have already decayed by the time of the measurement. An efficient method to achieve enough statistics for the observed reaction is the cyclic activation analysis, whereby irradiation and counting is repeated several times. Two different applications of cyclic activation were investigated in this thesis. A sample of radioactive ${}^{10}$Be, provided by the Paul Scherrer Institut Villigen was activated at the TRIGA research reactor of the Johannes Gutenberg - Universität Mainz. Using the cadmium difference method, it was possible to separate the thermal and epithermal components of the neutrons, thereby determining both the thermal capture cross section and the resonance integral for the ${}^{10}$Be(n,$\gamma$)${}^{11}$Be reaction. At the Institut für Kernphysik of the Goethe Universität Frankfurt, a quasistellar neutron spectrum corresponding to k${}_B$T ≈ 25 keV was generated via the ${}^7$Li(p,n)${}^7$Be reaction, using a Van - de - Graaff accelerator. The experimental infrastructure for automated cyclic activations was planned and commissioned. Utilizing this setup, the spectrum averaged neutron capture cross sections for multiple reactions were measured. The total capture cross section of the ${}^{19}$F(n,$\gamma$)${}^{20}$F reaction was determined. For the ${}^{45}$Sc(n,$\gamma$)${}^{46}$Sc reaction, the partial cross section into the 142.5 keV isomeric state was measured. The investigation of the ${}^{115}$In(n,$\gamma$)${}^{116}$In reaction provided the partial cross sections into the isomeric states at 289.7 keV and 127.3 keV and into the ground state. Using a hafnium sample, the partial capture cross sections into the 1147.4 keV isomeric state of ${}^{178}$Hf and into the 375 keV isomeric state of ${}^{179}$Hf were measured. A methodology for measurements and analysis using this setup was developed that can be applied to further cyclic activation experiments with different samples.