Theoretical Beta Spectrum from Uranium-235 Fission Fragments in Secular Equilibrium

The theoretical beta spectrum of the thermal fission fragments of ²³⁵U in secular equilibrium was calculated using recent fission yields, nuclear decay scheme data, and very recent semi-empirical mass formulas to predict beta Q values of nuclides with unknown energy level structure. Better agreement with experiment is achieved when these isotopes are assumed to contain all of the excited states of isotopes with known decay schemes with the same atomic number Z and with neutron numbers N differing by even integers. The beta branching ratios for the unknown isotopes were assumed to be the renormalized collection of branching ratios found in all known isotopes of the families described above. The results obtained with these narrower restrictions are in better general agreement with experiment than those that replace the excited states and branching ratios of the unknown nuclides with those obtained by taking broad averages over known isotopes. There still appears to be some disagreement between theory and experiment, particularly at the high-energy end of the spectrum.