Calculated fission-fragment yield systematics in the region74≤Z≤94and90≤N≤150

Abstract

Background: In the seminal experiment by Schmidt et al. [Nucl. Phys. A 665, 221 (2000)] in which fissionfragment charge distributions were obtained for 70 nuclides, asymmetric distributions were seen above nucleon number A ≈ 226 and symmetric below. Because asymmetric fission had often loosely been explained as a preference for the nucleus to always exploit the extra binding of fragments near 132 Sn it was assumed that all systems below A ≈ 226 would fission symmetrically because available isotopes do not have a proton-to-neutron Z/N ratio that allows division into fragments near 132 Sn. But the finding by Andreyev et al. [Phys. Rev. Lett. 105, 252502 (2010)] did not conform to this expectation because the compound system 180 Hg was shown to fission asymmetrically. It was suggested that this was a new type of asymmetric fission, because no strong shell effects occur for any possible fragment division.Purpose: We calculate a reference data base for fission-fragment mass yields for a large region of the nuclear chart comprising 987 nuclides. A particular aim is to establish whether 180 Hg is part of a contiguous region of asymmetric fission and if so its extent, or if, in contrast to the actinides there are scattered smaller groups of nuclei that fission asymmetrically in this area of the nuclear chart. Methods:We use the by now well benchmarked Brownian shape-motion method and perform random walks on the previously calculated five-dimensional potential-energy surfaces. The calculated shell corrections are damped out with energy according to a prescription developed earlier. Results:We have obtained a theoretical reference data base of fission-fragment mass yields for 987 nuclides. These results show an extended region of asymmetric fission with approximate extension 74 ≤ Z ≤ 85 and 100 ≤ N ≤ 120. The calculated yields are highly variable. We show 20 representative plots of these variable features and summarize the main aspects of our results in terms of "nuclear-chart" plots showing calculated degrees of asymmetry versus N and Z.Conclusions: Experimental data in this region are rare: only ten or so yield distributions have been measured, some with very limited statistics. We agree with several measurements with higher statistics. Regions where there might be differences between our calculated results and measurements lie near the calculated transition line between symmetric and asymmetric fission. To draw more definite conclusions about the accuracy of the present implementation of the Brownian shape-motion approach in this region experimental data, with reliable statistics, for a fair number of suitably located additional nuclides are clearly needed. Because the nuclear potential-energy structure is so different in this region compared to the actinide region additional experimental data together with fission theory studies that incorporate additional, dynamical aspects should provide much new insight.

Dynamical effects in fission investigated at high excitation energy

“…The timedependent formulation of this model using a Fokker-Planck equation with a dissipative term describing the coupling between the collective and singleparticle degrees of freedom was introduced by Grangé and collaborators [6]. An analogue formulation is the one based on the Langevin equation [2] or as a random walk, in the limit of strong dissipative coupling [7], on a multidimensional macroscopic-microscopic potential energy surface [8].…”

Mass distribution of fission fragments within the Born-Oppenheimer approximation

“…They can be divided into two groups: timedependent and quasi-static approaches. Among the timedependent approaches one could mention the time-dependent density functional theory [1], the time-dependent generator coordinate method [2], the Brownian shapemotion approach for nuclear fission [3], the models based on the solution of Langevin equations for the shape degrees of freedom [4][5][6][7]. The quantum mechanical timedependent approaches, like time-dependent density functional theory (TDDFT) [8], are unfortunately very time consuming.…”

Recent Achievements In Fission Dynamics Investigated At High Energies

“…The time-dependent formulation of this model, using a Fokker-Planck equation with a dissipative term describing the coupling between the collective and single-particle degrees of freedom, was introduced by Grangé and collaborators [4]. Analogue formulations make use of the Langevin equation [5] or the random walk algorithm, in the limit of strong dissipative coupling [6], on a multidimensional macroscopic-microscopic potential energy surface [7].…”