Determining the ${}^9\mathrm{Be}(n,\gamma ){}^{10}\mathrm{Be}$ integral cross section at fission neutron energies

Background: The ${}^9\mathrm{Be}$ neutron capture cross section has significant implications for Be materials in the nuclear industry as well as the α 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 ${}^9\mathrm{Be}(n,\gamma ){}^{10}\mathrm{Be}$ 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 ${}^{10}\mathrm{Be}/{}^9\mathrm{Be}$ 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 ${}^9\mathrm{Be}(n,\gamma ){}^{10}\mathrm{Be}$ integral cross sections calculated in this work are $26.5\pm 2.2\mu \mathrm{b}$ at $0.59\pm 0.07$ MeV, $24\pm 3\mu \mathrm{b}$ at $0.98\pm 0.14$ MeV, $21.7\pm 1.3\mu \mathrm{b}$ at $1.26\pm 0.11$ MeV, $21.8\pm 1.4\mu \mathrm{b}$ at $1.32\pm 0.11$ MeV, and $18.6\pm 1.1\mu \mathrm{b}$ at $1.46\pm 0.13$ 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.

Figure 1

Plot of ${}^9\mathrm{Be}(n,\gamma ){}^{10}\mathrm{Be}$ cross sections from the major libraries and existing experimental data [5-16]. The BROND-3.1 library, equivalent to JEFF-3.3, and the TENDL-2019 library, equivalent to ENDF/B-VIII.0, have been omitted for clarity.

Figure 2

Block diagram of the methodology coupling experiment and simulation to calculate the integral cross section for the ${}^9\mathrm{Be}(n,\gamma ){}^{10}\mathrm{Be}$ reaction. Blue rectangle–simulation; Tan oval–experiment; Orange trapezoid–validation; Green hexagon–result.

Figure 3

Target foil pack layouts for the a) FT-HEU and b) FT-Pu experiments containing Au, Be, stainless steel (SS) and its elemental components (Cr, Mn, Fe, Ni), as well as highly enriched uranium (HEU, 93.2% ${}^{235}\mathrm{U}$) and depleted uranium (DU, 99.9% ${}^{238}\mathrm{U}$) as spectral indicators. Additional SS and Co foils were included in the FT-Pu experiment. The Ir-LANL foil was included as part of a separate ride-along experiment. Foil packs were surrounded by either HEU, natural U, or Pu plugs, depending on location, to fill the sample tube.

Figure 4

Simulated neutron spectra at the Boundary position in the FT-HEU simulation for the four Au foils covering the transition from HEU to natural U in the Flattop assembly, with Capsule-6 Left (dash-dot, blue) being the most inward on the HEU side and Capsule-5 Right (short dot, red) being the most outward on the natural U side. Individual bin uncertainties omitted for clarity.

Figure 5

Total neutron fluence (a) and average neutron energy (b) in Au foils for the FT-HEU (square, black) and FT-Pu (triangle, red) experiments as a function of distance from the center of the assembly. Error bars are 1σ combined uncertainty (see Sec. 5, Table 4 for systematic component).

Figure 6

Simulated position-averaged neutron energy distributions for the foil pack locations in the FT-HEU and FT-Pu experiments. Bin uncertainties omitted for clarity.

Figure 7

The relative difference between the STAYSL-PNNL adjusted neutron spectra and the MCNP input spectra is shown for each location within the assembly.

Figure 8

Plot of the integral cross sections from this work (FT-HEU: square, black; FT-Pu: triangle, green) along with the current nuclear data libraries and only other existing data (red, circle) in this energy range. The X-axis error bars for this work represent 95% of the neutron energy distributions.