A study of the de-excitation of primary fission fragments from the neutron-induced fission of 235U

Two separate three-dimensional experiments have been performed in which the energies of coincident fragment pairs and γ-rays or internal conversion electrons, emitted within ≈ 1.6 nsec of the thermal-neutron-induced fission of ²³⁵U, were recorded event by event. The fragment kinetic energies were used for mass identification. The self-consistency of the values of electron energy, γ-ray energy and fragment charge, and its agreement with X-ray selection data, were used to identify the atomic numbers of the fragments. The analysis of the γ-ray and conversion electron spectra resulted in the assignment of many transitions to new isotopes as well as improvement in or confirmation of many assignments from the ²⁵²Cf spontaneous fission data. Limited information on the multipolarities of the transitions in even nuclei is presented. The relative yield of electrons per fragment indicates softness to deformation in the mass region 100–110. Data are presented supporting the assignment of a 193 keV transition as the 2⁺ → 0⁺ transition in ⁹⁸Sr. An examination of the 2⁺ → 0⁺ level systematics of neighbouring even nuclei suggests a transition from vibrational to rotational behaviour in the light fragments between neutron numbers 58 and 60.