Neutron Capture at keV Energies - Probing the Stellar Interior

Neutron capture nucleosynthesis, which is producing essentially all elements between Fe and the actinides, takes place in the He burning zones of evolved stars (sprocess) and presumably in supemovae (rprocess). The slow time scale of stellar burning scenarios implies that the associated neutron capture path follows the stability valley and that the resulting abundances are to good approximation inversely proportional to the respective (n,γ) cross sections. The experimental program at the Karlsruhe Van de Graaff accelerator, which is focused on this field, includes time-of-flight measurements with a 4πBaF2array, activations in a quasi-stellar neutron spectrum, and complementing studies of important α-induced reactions. Recent examples will be presented for each of these areas to illustrate the actual status as well as the need for further efforts. The consequences of these laboratory results for deriving constraints on the physical parameters of the He burning zones are discussed at the example of the stellar neutron flux.