Prompt fission neutron spectra (PFNS) have a significant role in nuclear science and technology. In this study, the PFNS for \(^{239}\text {Pu}\) are evaluated using both differential and integral experimental data. A method that leverages integral criticality benchmark experiments to constrain the PFNS data is introduced. The measured central values of the PFNS are perturbed by constructing a covariance matrix. The PFNS are sampled using two types of covariance matrices, either generated with an assumed correlation matrix and incorporating experimental uncertainties or derived directly from experimental reports. The joint Monte Carlo transport code is employed to perform transport simulations on five criticality benchmark assemblies by utilizing perturbed PFNS data. Extensive simulations result in an optimized PFNS that shows improved agreement with the integral criticality benchmark experiments. This study introduces a novel approach for optimizing differential experimental data through integral experiments, particularly when a covariance matrix is not provided.
