CORRELATED TRANSITIONS IN TKE AND MASS DISTRIBUTIONS OF FISSION FRAGMENTS DESCRIBED BY 4-D LANGEVIN EQUATION

 

F.A. Ivanyuk¹, M.D. Usang², C. Ishizuka², and S. Chiba²

 

¹ Institute for Nuclear Research of NAS of Ukraine, Kyiv, Ukraine

²Tokyo Institute of Technology, Tokyo, Japan

 

We have decomposed the mass-TKE fission fragment distributions calculated by 4-dimensional Langevin approach to symmetric and asymmetric modes and observed how the dominant fission mode and symmetric mode change as functions of Z²/A^1/3 of the fissioning system in the actinides and trans-actinide region [1]. As a result, we found that the symmetric mode makes a sudden transition from super-long to super short fission mode around ²⁵⁴Es. The dominant fission modes on the other hand, are persistently asymmetric except for ²⁵⁸Fm, ²⁵⁹Fm and ²⁶⁰Md when the dominant fission mode suddenly becomes symmetric although it returns to the asymmetric mode around ²⁵⁶No. These correlated transitions has been known empirically by Darleane Hoffman and her group back in 1989, but for the first time we have given a clear explanation in terms of a dynamical model of nuclear fission.

By describing the fission process in terms of 4-D Langevin equations we have shown that the anomalously high TKE seen in ²⁵⁸Fm, ²⁵⁹Fm and ²⁶⁰Md is the inevitable results of splitting of nucleus into two almost double magic fragments that are symmetric. However, unlike ²³⁶U that demonstrate super-long fission modes for the symmetric splitting, these three nuclei of ²⁵⁸Fm, ²⁵⁹Fm, ²⁶⁰Md and other fissioning systems around them have super-short fission modes.

The differences between super-long fission modes and the super-short fission modes shows itself in the Coulomb repulsion energy between the fission fragments that is stronger in the latter fission mode due to its shorter shape. Then a further investigation revealed that the super-short fission modes are present for all nuclei heavier than Einsteinium. Hence, the mystery on why the super-short fission modes are preferred in ²⁵⁸Fm, ²⁵⁹Fm, ²⁶⁰Md are solved.

We even see the slow disappearance of super-long fission modes, prominent in ²³⁶U and hardly identifiable for ²⁵⁰Cf.

 

[1] M.D. Usang, F.A. Ivanyuk, C. Ishizuka, and S. Chiba, Scientific Reports 9 1525 (2019)