The left and right figures correspond to the fission from excitation energy of 15-MeV and 18-MeV in ²⁵⁸Md, respectively. Theoretical calculations are shown in the bottom two panels, and they agree well with the experimental data, including enhancement of mass-asymmetric fission yield with excitation energy.

The heaviest element that can exist, and hence, to what extent the periodic table of elements extends, is largely determined by the stability of the nuclei against fission. This stability depends on how the nucleus breaks apart through various forms. However, except for uranium, the fission process of heavier elements larger than atomic number 100 remain largely unknown.

This study is the first to observe nuclear fission in the mendelevium nucleus ²⁵⁸Md (atomic number 101). To produce this nucleus, we collided a helium nucleus (⁴He) accelerated by the JAEA tandem accelerator with an einsteinium target (²⁵⁴Es), which is specially prepared. As a result, three distinct fragmentation patterns could be identified in ²⁵⁸Md, as shown in the Fig. 1 top. Furthermore, we discovered that an asymmetric fission component, which produces larger and smaller fission fragments, increases along with the excitation energy in ²⁵⁸Md. These phenomena are not observed in uranium. In addition, we could reproduce the experimental data using newly developed, precise theoretical calculations (Fig. 1 bottom).

This achievement is one step toward understanding the limits of elemental existence. We believe the developed theoretical fission model can be widely used in atomic energy applications.

The einsteinium isotope was obtained from Oak Ridge National Laboratory, U.S. Calculations were performed using JAEA supercomputer HPE SGI8600.

Paper URL: https://doi.org/10.1103/PhysRevC.111.044609

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