PNNL

Abstract

The 9Be neutron capture cross section has significant implications for Be materials in the nuclear industry as well as the a-process in stellar nucleosynthesis. While the cross section is well-constrained at thermal neutron energies, there is a lack of experimental data at higher neutron energies and the evaluated nuclear data libraries can differ by up to two orders of magnitude. Purpose: Calculate the 9Be(n,?)10Be integral cross section at fission neutron energies in an effort to resolve disagreements amongst the nuclear data libraries. Methods: Foil irradiation experiments were performed using the Flattop critical assembly at the National Criticality Experiments Research Center with either the highly enriched U or Pu cores, with target foil stacks placed at multiple locations to exploit different neutron energy profiles. Accelerator mass spectrometry was used to measure the 10Be/9Be ratio in irradiated Be foils, while all other activation products were quantified through gamma spectrometry. The experiments were simulated using the Monte Carlo N-Particle radiation transport code and combined with experimental results to determine the total neutron fluence, while the STAYSL-PNNL suite and FISPACT-II code were used to validate the model and assess the systematic uncertainty. Results: The new 9Be(n,?)10Be integral cross sections calculated in this work are: 26.5 ± 2.2 µb at 0.59 ± 0.07 MeV, 24 ± 3 µb at 0.98 ± 0.14 MeV, 21.7 ± 1.3 µb at 1.26 ± 0.11 MeV, 21.8 ± 1.3 µb at 1.32 ± 0.11 MeV, and 18.6 ± 1.1 µb at 1.46 ± 0.13 MeV. These results do not agree with integral cross sections from any of the nuclear data library evaluations. Conclusions: Discrepancies between the new integral cross sections reported here and the nuclear data libraries suggest a more complex cross-section structure in the MeV range which allows for more resonance contributions, and more work is needed to further constrain the evaluated cross sections.