Abstract
It is well known that most actinides fission into fragments of unequal size. This contradicts liquid-drop-model theory from which symmetric fission is expected. The first attempt to understand this difference suggested that division leading to one of the fragments being near doubly magic 132Sn is favored by gain in binding energy. After the Strutinsky shell-correction method was developed an alternative idea that gained popularity was that the fission saddle might be lower for mass-asymmetric shapes and that this asymmetry was preserved until scission. Recently it was observed [Phys. Rev. Lett. 105 (2010) 252502] that 180Hg preferentially fissions asymmetrically in contradiction to the fragment-magic-shell expectation which suggested symmetric division peaked around 90Zr, with its magic neutron number N=50, so it was presented as a “new type of asymmetric fission”. However, in a paper [Phys. Lett. B 34 (1971) 349] a “simple” microscopic mechanism behind the asymmetry of the actinide fission saddle points was proposed to be related to the coupling between levels of type [40ΛΩ] and [51ΛΩ]. The paper then generalizes this idea and made the remarkable prediction that analogous features could exist in other regions. In particular it was proposed that in the rare-earth region couplings between levels of type [30ΛΩ] and [41ΛΩ] would favor mass-asymmetric outer saddle shapes. In this picture the asymmetry of 180Hg is not a “new type of asymmetric fission” but of analogous origin as the asymmetry of actinide fission. This prediction has never been cited in the discussion of the recently observed fission asymmetries in the “new region of asymmetry”, in nuclear physics also referred to as the rare-earth region. We show by detailed analysis that the mechanism of the saddle asymmetry in the sub-Pb region is indeed the one predicted half a century ago.
Highlights
An initial qualitative theoretical interpretation for the experimental observations of asymmetric fission was that fissioning systems favor division into a heavy fragment near the doubly magic 132Sn because the magic proton number Z = 50 and neutron number N = 82 and associated microscopic effects result in an extra binding of about 12 MeV in132Sn relative to liquid-drop theory
Since we showed that the microscopic mechanism between the fission saddle asymmetry is the same in the actinide region and in the rare-earth region: is the asymmetry of 18800Hg fission “a new type of asymmetry”
As discussed above many of the theories about the origin of the asymmetry of fission fragment mass distributions were intuitive, and not always anchored in a well specified approach that could routinely be applied to any fissioning system
Summary
An initial qualitative theoretical interpretation for the experimental observations of asymmetric fission was that fissioning systems favor division into a heavy fragment near the doubly magic 132Sn because the magic proton number Z = 50 and neutron number N = 82 and associated microscopic effects result in an extra binding of about 12 MeV in132Sn relative to liquid-drop theory In this interpretation asymmetric fission would be roughly limited to the actinide region because no nuclei outside this region can divide into a doubly magic fragment while preserving the Z/N of the fissioning nucleus, a necessity due to the rapid increase in the symmetry energy for deviations from this ratio. Schmidt performed his seminal study of fission-fragment charge distributions of 70 different fissioning species [13] and tried to find corresponding calculations that showed the transition region between symmetric and asymmetric fission that he observed in the regions he studied (85 ≤ Z ≤ 94) he found only a 30-year old, very simple study that covered the region of interest [10]
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