Measurement of Neutron-Induced Fission Product Yields in ²³⁸U and ²³⁵U at Energies Below 4 MeV

When a heavy nucleus undergoes fission, it splits into two lighter fission products.  Fission product yields (FPY) refer to the quantity of a particular fission product produced per fission.  As a quantifiable feature of the fission process, FPY measurements provide insight into the mechanics underlying fission and hold significant implications for the fields of nuclear forensics, stockpile maintenance, and nuclear energy.  While it is known that FPY vary with excitation energy, there is a lack of accurate data on the energy dependence of FPY, particularly at low energies.  This study continues a LANL-LLNL-TUNL collaboration which was formed in order to collect accurate data on the energy dependence of neutron-induced FPY at energies below 14 MeV.  Uranium targets were activated with a mono-energetic neutron beam; the target was held between two reference foils of the same isotopic composition in a dual fission chamber (DFC) in order to accurately measure total fissions.  Following irradiation, the activated targets were γ ray counted with HPGe detectors.  The resulting spectra were used to identify fission products by their characteristic γ rays and decay curves; FPY were quantified by comparing γ ray counts in the detector to total fission counts in the DFC.  FPY in ²³⁸U and ²³⁵U were measured at 2.0, 2.4, and 3.6 MeV.