Astrophysical quests for neutron capture data of unstable nuclei

The abundances of the chemical elements heavier than iron can be attributed in about equal parts to the r and to the s process, which are taking place in supernova explosions and during the He and C burning phases of stellar evolution, respectively. So far, quantitative studies on the extremely short-lived neutron-rich nuclei constituting the (n, gamma) network of the r process are out of reach. On the contrary, the situation for the s-process is far advanced, as the reaction path of the s process from C-12 to the Pb/ Bi region is located within the valley of stability. Accordingly, a comprehensive database of experimental (n, gamma) cross sections has been established. While for many stable isotopes the necessary accuracy is still to be reached, reliable cross sections for the involved unstable isotopes are almost completely missing. Because of the intrinsic gamma background of radioactive samples, successful time-of-flight measurements are depending on intense pulsed neutron sources. Such data are fundamental for our understanding of branchings in the s-process reaction path, which carry important model-independent information on neutron flux and temperature in the deep stellar interior.