Nuclear astrophysics with neutrons

Neutrons play a crucial role in astrophysics during the heavy element nucleosynthesis. The largest fraction of isotopes heavier than iron is produced by neutron capture processes on short ( r process) and long timescales ( s process). During the ``slow neutron capture process'' ( s process) heavier elements are produced by successive captures of in-situ produced neutrons from the reactions 13 C(α,n) 16 O and 22 Ne(α,n) 25 Mg (with densities of 10 6 −10 10 cm −3 ) in the interior of stars and following β-decays. With this scenario the reaction path runs along the valley of stability up to 209 Bi and produces about 50% of the solar abundances of the heavy elements. Important nuclear physics parameters for s -process nucleosynthesis are neutron capture cross sections (for E n = 0.3–300 keV, corresponding to stellar temperatures between kT = 8 and 90 keV) and β-decay half-lives. Neutron capture measurements can be performed via activation in a quasi-stellar neutron spectrum utilizing several ( p , n ) reactions, or by the time-of-flight technique. The ``rapid neutron capture process'' ( r process) is responsible for the remaining 50% of the solar abundances. Here neutrons with densities of 10 20 −10 30 cm −3 are captured on a very fast timescale (ms) during a Core Collapse Supernova in a region close to the forming neutron star. The r -process nuclei are thus very short-lived, neutron-rich isotopes up to the actinides, which can only be produced and investigated at large-scale radioactive-beam facilities. Here the most important nuclear physics parameters are masses, half-lives, and at later stages also β-delayed neutrons. This paper will summarize the role of neutrons in nuclear astrophysics and give a short overview about the related astrophysics programs at the GSI Helmholtz research center and the FRANZ facility in Germany.