Fission product yield measurements using monoenergetic photon beams

Background: High-accuracy and self-consistent fission product yield (FPY) data are needed to advance microscopic/macroscopic descriptions of the nuclear fission process, to improve the predictive power of phenomenological models, and for applications in nuclear energy, nuclear forensics, and homeland security. Purpose: In a collaboration between the Triangle Universities Nuclear Laboratory (TUNL), Los Alamos National Laboratory (LANL), and Lawrence Livermore National Laboratory (LLNL), the dependence of a number of cumulative FPYs on the incoming neutron energy has been measured and unexpected energy dependencies of certain fission products have been reported [M. E. Gooden, Nucl. Data Sheets 131, 319 (2016)]. To investigate whether this observation is unique to neutron-induced fission, a program has been initiated to measure FPYs in photon-induced fission. Method: The photon-induced FPYs were measured by a combination of fission counting using a specially designed dual-fission chamber and gamma-ray counting. The measurements were carried out with a monoenergetic photon beam at the HI gamma S facility. Gamma-ray counting of the activated targets was performed with well-shielded high-purity germanium (HPGe) detectors over a period of two months after irradiation to properly identify the decay history of fission products. Results: We report on our photofission product yield measurements on U-235, U-238, and Pu-239 using a monoenergetic photon beam of E-gamma = 13 MeV. More than 40 fission products were uniquely identified, and their yield values were computed. The use of the fission chamber with post-activation measurements has provided absolute fission product yield data with minimal uncertainties. Conclusion: The photon-induced cumulative fission product yields of U-235, U-238, and Pu-239 are compared with previous photon- and neutron-induced fission measurements. In the near future data will be obtained at lower and higher photon energies.