Neutron capture measurements on 62Ni, 63Ni and 197Au and their relevance for stellar nucleosynthesis

Neutron capture reactions in stars are responsible for forming about 99% of the elemental abundances heavier than Fe. Two processes contribute about equally to the overall abundance pattern: the slow neutron capture process (s process) where neutron densities are small and therefore radioactive decay is generally faster than subsequent neutron capture on radionuclides, and the rapid neutron capture process (r process) which takes place in environments of high neutron densities, driving the reaction path towards the neutron rich side. The key nuclear physics input for s process studies are stellar neutron capture cross sections, called MACS (Maxwellian-averaged cross section). In the course of this work, dierent reactions relevant to s process nucleosynthesis have been studied. To improve and check existing information, neutron capture cross sections of most stable Fe and Ni isotopes were measured via the time-of-flight technique at the n TOF facility at CERN. This campaign was triggered by a work of Sneden et al. (Ap. J. 533 (2000) L139-L142), who found that observed abundances in Ultra Metal Poor stars do not agree with calculated values, using existing experimental (n; ) cross sections as input. First results on the cross section of 62Ni(n; ) are presented in this work. Of special interest for studying stellar conditions in s process environments are branching points, i.e. where half lives are long enough so that neutron capture competes with radioactive decay. 63Ni with a half life of 101.1 years represents such a branching and its (n; ) cross section has a crucial influence on the production of Cu. The 63Ni(n; ) reaction, of which no cross section data are available above thermal neutron energies, was measured at n TOF and first results are reported here. For (n; ) reaction measurements, accurate normalization standards play an important role. Two experiments were performed to resolve a discrepancy of the 197Au(n; ) cross section in the lower keV region between the reference cross section used for MACS measurements (Ratynski and K¨appeler, Phys. Rev. C 37 (1988) 595-604) and the ENDF/B-VII.1 evaluation. The 197Au(n; ) cross section, measured at n TOF, has been determined from 5 to 400 keV neutron energy. The results are in agreement with the ENDF/B-VII.1 evaluation. The MACS at kBT = 30 keV is in very good agreement with the MACS calculated from the ENDF evaluation. Within quoted uncertainties it is compatible with the MACS result of Ratynski and K¨appeler which was obtained by the activation technique. We re-measured the neutron spectrum produced by the 7Li(p; n)7Be reaction (that was used for activation measurements) with improved resolution and accuracy at Physikalisch-Technische-Bundesanstalt Braunschweig. The results confirm the spectrum measured by Ratynski and K¨appeler. The 197Au(n; ) cross section averaged over the neutron spectrum obtained in this work agrees with the result using the Ratynski and K¨appeler spectrum, providing increased confidence in the use of this reaction as a standard.