Contrasting fission potential-energy structure of actinides and mercury isotopes

Background: Fission-fragment mass distributions are asymmetric in the fission of typical actinide nuclei for nucleon number A in the range 228 not less than or equal to A not less than or equal to 258 and proton number Z in the range 90 not less than or equal to Z not less than or equal to 100. For somewhat lighter systems it has been observed that fission mass distributions are usually symmetric. However, a recent experiment showed that fission of Hg-180 following electron capture on Tl-180 is asymmetric. Purpose: We calculate potential-energy surfaces for a typical actinide nucleus and for 12 even isotopes in the range Hg-178-Hg-200, to investigate the similarities and differences of actinide compared to mercury potential surfaces and to what extent fission-fragment properties, in particular shell structure, relate to the structure of the static potential-energy surfaces. Methods: Potential-energy surfaces are calculated in the macroscopic-microscopic approach as functions of five shape coordinates for more than five million shapes. The structure of the surfaces is investigated by the use of an immersion technique. Results: We determine properties of minima, saddle points, valleys, and ridges between valleys in the five-dimensional shape-coordinate space. Along the mercury isotope chain the barrier heights and the ridge heights and their persistence with elongation vary significantly and show no obvious connection to possible fragment shell structure, in contrast to the actinide region, where there is a deep asymmetric valley extending from the saddle point to scission. Conclusions: The mechanism of asymmetric fission must be very different in the lighter proton-rich mercury isotopes compared to the actinide region and is apparently unrelated to fragment shell structure. Isotopes lighter than Hg-192 have the saddle point shielded from a deep symmetric valley by a significant ridge. The ridge vanishes for the heavier Hg isotopes, for which we would expect a qualitatively different asymmetry of the fragments.