Modeling the Detection of Delayed Neutron Signatures in MCNP6 and Comparisons with Measurements of ²³³U, ²³⁵U, and ²³⁹Pu

Study of the magnitude and temporal behavior of delayed neutrons (DNs) enables the identification of fissile isotopes and a determination of their relative quantities. Thus, the ability to model accurately these neutrons and the methods of their detection is of relevance to nuclear forensics and counterterrorism. The capability of MCNP6 to model these emissions was examined and compared to measurements of the DNs produced by ²³³U, ²³⁵U, and ²³⁹Pu after neutron-induced fission. Fissile samples were irradiated in a SLOWPOKE-2 research reactor for 60 s and were then conveyed via pneumatic tubing to an array of six ³He detectors embedded in a paraffin moderator. Several MCNP6 input files were created to reproduce irradiation conditions, temporal DN emission, and the detection arrangement. Nuclear reactions and other effects within the ³He detectors were reproduced by MCNP6, and detection efficiencies of this modeled arrangement determined by MCNP6 were in agreement with experimental measurements. Finally, the library and model DN emission options in the MCNP6v1 release were evaluated and compared to the measured magnitudes and temporal behavior of ²³³U, ²³⁵U, and ²³⁹Pu. Significant discrepancies observed between the DN model option and measurements for count times >100 s are discussed.