Evidence for the predominant influence of the asymmetry degree of freedom on the even–odd structure in fission-fragment yields

Abstract

Abstract. Based on a comprehensive set of fission-fragment distributions measured in low-energy fission, the even-odd staggering in the fission-fragment element yields is investigated. The well-established evolution of the global even-odd effect with the fissioning system is found to be only a partial aspect of the even-odd structure. Indeed, it is shown that the global even-odd effect varies systematically with the mean asymmetry of the fission-fragment distribution, and that the general increase of the even-odd staggering with asymmetry depends on the fissioning system. Thus, the dependency of the even-odd effect with the fissioning system is accredited in part to the asymmetry evolution of the charge distribution, and not solely to the dissipated energy as it has been done earlier. This interpretation is strongly supported by data measured in inverse kinematics, which cover the complete fission-fragment charge distribution and include precise yields at symmetry. The relevance of the ordering parameter for the description the even-odd effect in fission-fragment yields as a general property is explored.PACS numbers: 24.75.+i, 25.85.Ec, 25.85.Jg

The SOFIA experiment: Measurement of236U fission fragment yields in inverse kinematics

“…Figure 5 represents the local proton even-odd staggering, expressed following the prescription of [8], as a function of Z for the four reactions 234,235,236,238 U (γ,f). A similar trend of an increase of the even-odd effect with the asymmetry of the fission is observed, in accordance with a general phenomenon already observed [9] and explained as the manifestation of the energy sorting mechanism [10] : a common relatively low even-odd effect is observed in the four cases around Z = 46 and is then regularly increasing with the asymmetry. Finally, elemental yields for the fission of 236 U are compared with evaluated data from two libraries, B-VII.1 and JEFF-3.1, for Neutron-induced fission of 235 U at different energies on figure 6.…”

Odd-even effect dependence on the excitation energy in low energy fission

“…Surprisingly, in the case of cold fission, where the excitation energy of the fragments is so small that no neutrons can be emitted, an inversion of the oddeven effect is produced. As evidenced experimentally [7,8] for U isotopes, in the cold region regime the oddodd fragmentations dominate at excitations energies lower than [4][5][6] MeV. This behavior is typical, being observed also for the fission of Cf [9,10].…”

“…In the low energy fission, the mass and charge distributions of fragments show a preference for even numbers of neutrons and protons [1][2][3][4][5]. When the excitation energy increases, the magnitude of this odd-even effect is lowered [6].…”

Excitation-energy influence at the scission configuration

“…Figure 5 presents the global even-odd staggering in nuclear charge as a function of the fissility parameter (Z 2 /A), which relates the mass and the nuclear charge of the fissioning system. Present data (black) are compared with (n th , f ) data (green) [20]. Even-Z systems follow a decreasing behavior with Z 2 /A with a shift between present data and (n th , f ) data produced by the difference in E x : a system with an excitation energy E x at the ground state, reaches the saddle point with intrinsic excitation energy (E x − B f ), being B f the fission barrier.…”

“…6 for 238 U and 240 Pu, shows a decreasing exponential behavior produced by the saturation of the pair-rupture at high E x . The shift between both systems reflects the different asymmetry of the systems [20].…”