Solving the stellar 62Ni problem with AMS

TL;DR

This study accurately measured neutron capture cross sections of 62Ni and 63Ni using AMS, resolving previous discrepancies and providing crucial data for stellar nucleosynthesis models, especially in the weak s process.

Contribution

The paper presents new experimental cross section measurements for 62Ni(n,gamma) and 63Ni(n,gamma), resolving prior uncertainties and discrepancies in stellar reaction rates.

Findings

Measured 62Ni(n,gamma) cross section at 30 keV: 23.4 +/- 4.6 mb.

Found 64Ni(gamma,n) cross section at 13.5 MeV to be significantly lower than predictions.

Resolved previous experimental discrepancies for 62Ni(n,gamma) cross section.

Abstract

An accurate knowledge of the neutron capture cross sections of 62,63Ni is crucial since both isotopes take key positions which affect the whole reaction flow in the weak s process up to A=90. No experimental value for the 63Ni(n,gamma) cross section exists so far, and until recently the experimental values for 62Ni(n,gamma) at stellar temperatures (kT=30 keV) ranged between 12 and 37 mb. This latter 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 <sigma>30keV = 23.4 +/- 4.6 mb. Additionally, we have measured the 64Ni(gamma,n)63Ni cross section close to threshold. 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 AMS measurement of the sample activated at 13.5 MeV revealed a cross section smaller by more than a factor of 2 compared to NON-SMOKER predictions.