Distribution of Delayed Neutron Yields Versus Proton, Neutron, and Mass Numbers: Application to Proton Pairing in Fission Yields

Evaluated delayed neutron emission probabilities for 74 precursors and estimated probabilities for another 174 precursors have been combined with ENDF/B-V fission yields to map out delayed neutron yields as a function of mass number, proton number, and neutron number. For all fissioning systems, more than 90% of the delayed neutron yield comes from precursors with measured P_n values. Most of the remaining yield comes from the precursors ⁸⁸As, ^100–103Y, and ¹³⁷Sb. High priority should be given to P_n measurements for these precursors. Periodic peaks about five mass numbers apart are seen when delayed neutron yields are plotted versus mass number. This behavior is explained by the very strong enhancement of yields from odd Z precursors. Even Z precursors contribute <10% of the total delayed neutron yield. Plots of delayed neutron yields versus neutron number show dramatic decreases at the closed shell numbers 50 and 82 and smaller effects at subshell numbers 40 and 56. A procedure based on calculated and total delayed neutron yields is proposed for determining proton pairing factors in the fission yield model.