FIER: Software for analytical modeling of delayed gamma-ray spectra

A new software package, the Fission Induced Electromagnetic Response (FIER) code, has been developed to analytically predict delayed $\gamma$-ray spectra following fission. FIER uses evaluated nuclear data and solutions to the Bateman equations to calculate the time-dependent populations of fission products and their decay daughters resulting from irradiation of a fissionable isotope. These populations are then used in the calculation of $\gamma$-ray emission rates to obtain the corresponding delayed $\gamma$-ray spectra. FIER output was compared to experimental data obtained by irradiation of a $^{235}$U sample in the Godiva critical assembly. This investigation illuminated discrepancies in the input nuclear data libraries, showcasing the usefulness of FIER as a tool to address nuclear data deficiencies through comparison with experimental data. FIER provides traceability between $\gamma$-ray emissions and their contributing nuclear species, decay chains, and parent fission fragments, yielding a new capability for the nuclear science community.