New Precision Measurements of the ${}^{235}\mathbf{U}(n,\gamma )$ Cross Section

The neutron capture cross section of ${}^{235}\mathrm{U}$ was measured for the neutron incident energy region between 4 eV and 1 MeV at the DANCE facility at the Los Alamos Neutron Science Center with an unprecedented accuracy of 2–3% at 1 keV. The new methodology combined three independent measurements. In the main experiment, a thick actinide sample was used to determine neutron capture and neutron-induced fission rates simultaneously. In the second measurement, a fission tagging detector was used with a thin actinide sample and detailed characteristics of the prompt-fission gamma rays were obtained. In the third measurement, the neutron scattering background was characterized using a sample of ${}^{208}\mathrm{Pb}$. The relative capture cross section was obtained from the experiment with the thick ${}^{235}\mathrm{U}$ sample using a ratio method after the subtraction of the fission and neutron scattering backgrounds. Our result indicates errors that are as large as 30% in the 0.5–2.5 keV region, in the current knowledge of neutron capture as embodied in major nuclear data evaluations. Future modifications of these databases using the improved precision data given herein will have significant impacts in neutronics calculations for a variety of nuclear technologies.

Figure 1

(a) Comparison of ${}^{235}\mathrm{U}(n,f)$ reaction counts from experiments with PPAC (squares) and the thick target (solid red line) as a function of incident neutron energy between 100 eV and 1 MeV. (b) Difference of the two spectra $\Delta$ divided by the statistical uncertainty $\sigma$ of the measurements.

Figure 2

Total $\gamma$-ray energy spectra obtained for neutron incident energies $E_n=0.2–10\mathrm{keV}$ and $\gamma$-ray multiplicities $M_{\gamma }=3–5$: black solid line, ${}^{235}\mathrm{U}$ thick target; green dashed line, the fission background component; blue dotted line, spectrum after the fission background removal; red dot-dashed line. the scattering background normalized in the region $E_{\mathrm{sum}}=7.5–10\mathrm{MeV}$. (See text.)

Figure 3

${}^{235}\mathrm{U}(n,\gamma )$ cross section measured at the DANCE facility (black squares) compared to ENDF/B-VII.1 (red line) and JENDL-4.0 (blue dashed line) for incident neutron energies between 4 eV and 20 keV.

Figure 4

(Top) ${}^{235}\mathrm{U}(n,\gamma )$ cross section measured at the DANCE facility (black squares) compared to ENDF/B-VII.1 (red line) and JENDL-4.0 (blue dashed line) and available experimental data for incident neutron energies between 1 keV and 1 MeV. (Bottom) Difference in percent between available data and results of this work.

Figure 5

(Top) A variance ${\sigma }^2$ of variables in Eq. (4) contributing to the total variance of the ratio $R=N_{n\gamma }/N_{nf}$ (see text for details). (Bottom) Relative uncertainties of ${}^{235}\mathrm{U}(n,\gamma )$ cross section obtained at the DANCE facility.