Validation of Nuclear Data for Fusion Applications Using the Quasi-Differential Neutron Scattering Experimental Method

Neutronics calculations of fusion systems require accurate neutron transport simulations to predict quantities such as shielding effectiveness, material damage and transmutation, and reaction rates. Evaluated nuclear data from the U.S. Evaluated Nuclear Data Library (ENDF), the Joint Evaluated Fission and Fusion File (JEFF), or the Fusion Evaluated Nuclear Data Library (FENDL) are frequently used for these calculations. Therefore, the results from these calculations are highly dependent on the accuracy of the cross sections and angular distributions provided in these evaluated nuclear data libraries. The quasi-differential neutron scattering method was developed to validate the accuracy of evaluated cross sections and angular distributions. These measurements are particularly well suited to validate nuclear data pertaining to nuclear fusion applications. The incident neutron energy region of the measurement, 0.5 to 20 MeV, corresponds with the neutron energy range most relevant to fusion systems where neutron scattering reactions dominate.

Modeling neutron scattering reactions accurately is important since neutrons will only have a few interactions before being absorbed or leaking from a system. In quasi-differential neutron scattering experiments, a white pulsed neutron beam is scattered from a thick sample and neutrons are detected by an array of detectors surrounding the sample. The experimental data are then compared with detailed radiation transport simulations using multiple evaluated nuclear data sets. This comparison provides information about the accuracy of these evaluated data for the sample of interest. The advantage of such a method is the high sensitivity to the nuclear data of the measured sample material with minimal interference from other materials. This method has been successfully utilized to obtain nuclear reaction data of several materials that are important to nuclear fission reactors and criticality safety applications, such as Be, Pb, Fe, Ta, Mo, ²³⁸U, Zr, and F. Moreover, data from these measurements informed certain evaluations in the ENDF/B-VIII.0 and ENDF/B-VIII.1 nuclear data libraries. We propose that quasi-differential neutron scattering will be of great value for the validation of fusion-related materials, such as Si, W, Li, Ti, Nb, and O, and that additional measurements are needed for more materials of interest.