Solving the stellar 62Ni problem with AMS

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 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.

💡 Research Summary

The paper addresses a long‑standing uncertainty in the neutron‑capture cross sections of the nickel isotopes 62Ni and 63Ni, which are pivotal for the weak s‑process nucleosynthesis up to mass number A≈90. Prior measurements of the 62Ni(n,γ) reaction at the stellar temperature corresponding to kT = 30 keV showed a wide spread, ranging from 12 mb to 37 mb, and no experimental data existed for the 63Ni(n,γ) reaction. To resolve this discrepancy, the authors performed two independent activation experiments using high‑purity nickel samples irradiated with a 2.5 MeV deuteron‑deuteron neutron source at the Munich tandem accelerator. After activation, the isotopic ratios of the produced 63Ni to stable 58Ni were measured with the General Atomics Mass Spectrometer (GAMS) employing accelerator mass spectrometry (AMS). The AMS technique, featuring charge‑state selection, optimized electron beam voltages, and a multi‑collision detection system, achieved a detection limit of 10⁻¹⁴ for the 63Ni/58Ni ratio, effectively suppressing background contributions.

The resulting Maxwellian‑averaged cross section at kT = 30 keV was determined to be ⟨σ⟩₃₀keV = 23.4 ± 4.6 mb. This value is in excellent agreement with a recent time‑of‑flight (ToF) measurement, thereby confirming that the earlier spread in experimental results stemmed from methodological and systematic uncertainties rather than genuine nuclear physics differences. The precise determination of the 62Ni(n,γ) cross section eliminates a major source of error in weak s‑process network calculations and provides a reliable anchor point for theoretical models.

In addition to the neutron‑capture work, the authors investigated the photodisintegration reaction 64Ni(γ,n)63Ni, which is relevant for the production of 63Ni in stellar environments where high‑energy photons are abundant. Photo‑activations were carried out at the ELBE free‑electron laser accelerator in Dresden‑Rossendorf using quasi‑monochromatic γ‑beams at energies of 10.3 MeV, 11.4 MeV, and 13.5 MeV. The activated samples were subsequently analyzed with the same GAMS‑AMS setup. The measured cross section at 13.5 MeV was found to be more than a factor of two lower than the predictions of the NON‑SMOKER statistical model code. This discrepancy suggests that the theoretical treatment of γ‑induced neutron emission, particularly the assumed γ‑strength functions and level densities for nickel isotopes, may be overestimated.

The paper’s conclusions are threefold. First, the 62Ni(n,γ) cross section is now known with a realistic uncertainty, enabling more accurate weak s‑process simulations and improving predictions of elemental abundances up to A ≈ 90. Second, the successful AMS detection of the produced 63Ni provides the first experimental benchmark for the 63Ni(n,γ) reaction, which can be pursued in future work to complete the isotopic data set. Third, the lower-than‑expected 64Ni(γ,n)63Ni cross section calls for a reassessment of nuclear‑physics inputs in astrophysical γ‑process models, potentially affecting the predicted yields of neutron‑rich isotopes in explosive stellar scenarios. Overall, the combination of activation techniques with high‑precision AMS offers a powerful methodology for resolving long‑standing nuclear data ambiguities that have direct implications for stellar nucleosynthesis and the interpretation of observed elemental abundances.