Measurement of cumulative photofission yields of ²³⁵U and ²³⁸U with a 16 MeV Bremsstrahlung photon beam

In the frame of a long-term research program on the characterization of large radioactive waste packages by photofission, the Nuclear Measurement Laboratory of CEA IRESNE, France, has measured cumulative photofission yields of U-235 and U-238 short-lived and long-lived fission products by using a Bremsstrahlung photon beam produced by a 16 MeV electron linear accelerator (LINAC). To this aim, a characterization of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium and zirconium. The residual neutron flux exiting the LINAC head (lead collimator, borated polyethylene and cadmium shield) has also been characterized by neutron activation analysis with indium samples to estimate the contribution of photoneutron fissions in the uranium samples used to assess the photofission yields. Finally, 49 fission product yields are reported for U-238 and 26 for U-235, with half-lives ranging from 64 s to more than 3 days, some of them not recorded so far in the literature Some photofission products cumulative yields show significant differences between U-235 and U-238, which confirms the possibility of an isotopic discrimination method based on delayed gamma-ray ratios analysis for radioactive waste characterization.