Neutron odd-even effect of low energy fission

Contrarily to the isotopic distributions of fission fragments that are fixed right at the exit point (the physical scission point), the isotonic distributions are deduced from the mass distributions which are altered by prompt neutron evaporation. They are very staggered; the yields of the even neutron number N are systematically enhanced in comparison with the neighbouring ones. This effect is the so called neutron odd-even effect. Taking into account the fact that the binding energy of the last neutron of fragment with even N is greater than the one of the fragment with N±1, one can claim that an initial neutron odd-even effect is completely washed out by the neutron evaporation process and that the observed odd-even effect is simply due to the neutron evaporation cascade: the cascades stop preferentially at fragment with even N. The recoil mass spectrometer Lohengrin of the Laue Langevin institute at Grenoble-France was used for a multi parametric study of the 249Cf(nth, f) reaction. A detailed investigation of the isotonic distributions at different fragment kinetic energies has been made. In this paper we discuss the obtained results and compare them with those from the literature. It seems that the neutron evaporation process cannot explain coherently all the observations