Solving the stellar 62Ni problem with AMS

The nucleosynthesis of elements heavier than iron can be almost completely ascribed to the ’slow’ and the ’rapid’ neutron capture processes. Among the nuclei involved, long-lived radioactive isotopes like 63Ni (t1/2= 100.1 yr) assume key positions, because their beta-decay rate becomes comparable to the neutron capture rate. The resulting competition leads to branchings in the s-process nucleosynthesis path. An accurate knowledge of the stellar neutron capture cross sections of 62,63Ni is required since these two cross sections affect the entire weak s-process flow towards heavier nuclei (A about 90). Until recently the experimental values for the (n,gamma) cross section of 62Ni at stellar temperatures (kT=30 keV) ranged between 12 and 37 mb. Stellar models using the lower value revealed a strong overproduction of 62Ni in postexplosive production factors of supernova type II explosions which intensified the question if this is due to uncertainties in the stellar models or in the nuclear input. This discrepancy could now be solved by two activations with following AMS using the GAMS setup at the Munich tandem accelerator which are also in perfect agreement with a recent time-of-flight measurement. The resulting (preliminary) Maxwellian cross section at kT=30 keV was determined to be <>30keV = 23.4 ± 4.6 mb. Additionally, we have measured for the first time the 64Ni(gamma,n)63Ni cross sections close to threshold which can transformed into the inverse 63Ni(n,gamma) channel by detailed balance. Photoactivations at 13.5 MeV, 11.4 MeV and 10.3 MeV were carried out with the ELBE accelerator at Forschungszentrum Dresden-Rossendorf. A first short AMS measurement of the sample activated at 13.5 MeV revealed a cross section smaller by more than a factor of 2 compared to previous predictions.