Determining the 9Be (n, γ) 10Be integral cross section at fission neutron energies

摘要

Background: The 9Be neutron capture cross section has significant implications for Be materials in the nuclear industry as well as the alpha 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: We calculate the 9Be (n, gamma )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, gamma )10Be integral cross sections calculated in this work are 26.5 +/- 2.2 mu b at 0.59 +/- 0.07 MeV, 24 +/- 3 mu b at 0.98 +/- 0.14 MeV, 21.7 +/- 1.3 mu b at 1.26 +/- 0.11 MeV, 21.8 +/- 1.4 mu b at 1.32 +/- 0.11 MeV, and 18.6 +/- 1.1 mu b 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.