Nucleon pair breaking in the thermal neutron induced fission of 233U and 235U

2014 The absence of a systematic even-odd effect in the primary fragment mass distributions, which has been recently observed, is discussed in terms of nucleon pair breaking during the saddle to scission transition. It is concluded that the existing experimental data are compatible with the assumption of a process where at least one nucleon pair is broken. J. Physique 44 (1983) 785-790 JUILLET 1983, ] Classification Physics Abstracts 25.85 It is well known that thermal induced fission of 233U and 235 U proceeds through a very small number of exit channels, which, according to A. Bohr [1], are associated with transition states of the fissioning nucleus at the saddle point. The available energy of the fissioning nucleus at saddle point lies below the expected value of the pairing gag [2]. Therefore the contribution of channels lying above this gap is hindered. The dominant channels are expected to be collective states built on the fundamental state of the even-even nucleus 234U or 236U. In other words, when the fissioning nucleus starts its descent towards scission, it would be in a superfluid state where both neutrons and protons always come in pairs. Consequently, the problem of viscosity of nuclear matter (quasi particle excitation) during the saddle to scission transition could be approached through the evaluation of the number of nucleon pairs broken at scission, reflected in the even-odd effects on the primary fragment mass and charge distributions. The problem of odd-even effects in fission has been very much studied until today. Odd-even effects on total charge distribution [3-6] and on average fragments kinetic energy [4, 5, 7] were found. These effects were related to the number of broken proton pairs [4, 7-13]. The notation utilized in this paper is inspired from the above indicated references. In recent publications [14, 15], it has been shown that, for events with high kinetic fragment energy, fragmentation into two odd primary masses are just as probable as fragmentation into two even primary masses. Even more, the maximal kinetic energy, as a function of the mass ratio, does not exhibit any evenodd effect. These experimental results were interpreted as a proof that, between the saddle and the scission points, at least one pair of nucleons must be broken whatever the particular mass fragmentation. However, Nifenecker et al. [12] having reanalysed the same experimental results, came to the conclusion that very late nucleon pair breaking is actually the predominant mechanism. In this paper, we shall therefore argue and show that first, Nifenecker et al. have misinterpreted the experimental data taken from references [13] and [14], and second, that the hypothesis of early breaking of at least one pair is compatible with all verifiable experimental facts gathered so far. The relative importance of an « even-odd-effect » on the mass distributions of the fission fragments can best be assessed by means of a parameter, 6A, defined by : where AYe is a pure number representing the sum of all the even-mass fragment-yields, and A Yo the sum Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01983004407078500