The evaluated nuclear data widely used in computational tools suffer from inaccuracies and large relative uncertainties, particularly with respect to short-lived and low-yield fission products. The existing data are often based on nuclear models, and limited experimental measurements of these fission product yields have been performed. This work seeks to validate and improve, where applicable, the current fission product yield data in the ENDF library using experimental measurements, particularly for the photofission of ²³⁸U and ²³²Th.
Much of the measurable fission products would have decayed away in experiments that require sample transfer from the linear accelerator to an auxiliary detection setup, resulting in low count rates and large uncertainty in the calculated yield of fission products for those that are short-lived (half-life of less than 50 s). A high-purity germanium detector and a pneumatic transfer system were employed for measurements of short-lived fission products, collecting data between accelerator irradiation and counting cycles. These experiments resulted in measured cumulative fission product yields for ²³⁸U and ²³²Th at bremsstrahlung X-ray endpoint energies of 8, 14, and 20 MeV. These are reported with their excitation energies to allow for reliable comparison among other experimental data sets.