The influence of pairing and nuclear structure on the thermal-neutron-induced fission of235U

The mass separated fission product beam provided by the mass separator “Lohengrin” has been used to determine the nuclear charge distribution for the thermal-neutron-induced fission of²³⁵U for all light fission products in the region 80≦A≦107. The measurements were performed at the most probable kinetic energy of the fission products. By using the known fission product mass yields, the independent yields for a total number of 100 nuclides were obtained under the condition of the most probable kinetic energy. The proton pairing effect modulates the average nuclear charge of the fission fragments and the isobaric charge distribution widths in a regular fashion. The probabilities of breaking a pair and of forming fragments with an energetically unfavourable neutron-to-proton ratio are found to compete with each other. Both probabilities depend on the mass split and reach their maximum values in the region of the most probable masses. The odd-even-proton effect is found to vary smoothly between 16% for the most abundant mass splits and 40% for the rare mass splits. The odd-even-neutron effect exhibits maxima nearN=50 andN=60, where it reaches 16%. These maxima and the extremely low Tcyield (0.13±0.05%) are discussed with regard to fragment shell effects.