Shell effects in the symmetric-modal fission of pre-actinide nuclei

Abstract Mass distributions of fragments in the low-energy fission of nuclei from 187 Ir to 213 At have been analysed. This analysis has shown that shell effects in symmetric-mode fragment mass yields from the fission of pre-actinide nuclei could be described if one assumes the existence of two strongly deformed neutron shells in the arising fragments with neutron numbers N 1 ≈ 52 and N 2 ≈ 68. A new method has been proposed for quantitatively describing the mass distributions of the symmetric fission mode for pre-actinides with A ≈ 180–220.