We develop a model for pair-breaking in order to explain the behavior of the even-odd effects displayed both by the fragment yields and the fragments' kinetic energies in low energy nuclear fission. Neutron and proton pair breaking are taken into acount. Two pair-breaking mechanisms are considered. In the first, pairs are broken at the saddle point and the individual members of the broken pairs are assumed to localise independently in the fragments. In the second, pairs are broken at scission and individual members of the broken pairs are assumed to end in different fission fragments. With this simple model all existing experimental data can be explained, including results of cold-fragmentation.It is now well established that charge distributions of fission fragments show, in certain cases, a strong hindrance [1, 2] of odd-charge fragments as compared to even-charge fragments. Such a behavior requires, of course, that the fissionning nucleus be itself an even-charge one. These so-called even odd effects depend strongly on the fissioning system. They tend to decrease with its fissility or excitation energy. For example, they are very strong for thermal neutron induced fission of the Thorium isotopes, intermediate for that of the Uranium isotopes and practically absent for that of the Plutonium isotopes [2]. The dependance upon the excitation energy has been shown to be strong. While the average amplitude of the even-odd effect amounts to 22 % in slow neutron induced fission of U235 it falls to less than 6 % when fission is induced by 3 MeV neutrons [2]. Furthermore even-odd effects have also been found on the fragments' kinetic energy [-2, 4, 5]. It may be shown 1-2] that the even-odd effects and their variations are difficult to reconcile with the statistical model of fission [63 and are not easily explained in the frame of the thermodynamical model of Wilkins et al. [7]. We have suggested [2,4,8,9-1 an alternative explanation where the magnitude of the even-odd effects reflected the number of broken proton pairs just before scission. In a first approach, we treated neutrons and protons on an independent footing [9] and showed that it was possible do deduce, from the magnitude of the even-odd effects, an estimate of the average number of broken pairs at scission. We found that this number, albeit small, is not much smaller than the maximum number of broken pairs allowed by the amount of free energy [43 at scission. It is clear therefore, that one needs to treat neutron and proton pair breaking coherently. In the following, we develop a model where the competition between neutron and proton pair breaking is explicitly taken into account. A complete treatment of the model will only be made in a schematic case. After the schematic treatment we shall examine in which way "real life" may induce more complexity. These modifications will be shown to be especially important in the case of cold fission. A Schematic Model for Pair-BreakingWe first consider one family of particles in order to allow the reader to obtain fam...
It is shown that the smallness of even-odd effects on the fission fragments mass yields at high kinetic energies implies a late pair breaking mechanism and contradicts the assumption of strong viscosity of the fission process
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