Microscopic theory of nuclear fission

6 problems

• How fission fragments acquire their angular momentum

  Contested

  Fission fragments spin with several units of angular momentum even when the parent nucleus barely rotates. A 2021 experiment found that the spins of the two fragments are unrelated in size, which challenges the idea that they are set by shared motions before the split.

• Origin of asymmetric fission in neutron-deficient mercury isotopes

  Partially resolved

  Mercury-180 was expected to split into two equal halves, zirconium-90 nuclei, but a 2010 experiment found it splits unevenly. Which nuclear structure makes the split uneven in this region, far from the uranium-like nuclei where uneven fission is common, is only partly understood.

• Predictive time-dependent density-functional dynamics of fission yields

  Open

  Time-dependent density-functional theory, a method that follows the motion of all nucleons through an average field they create together, can simulate one fission event but gives only average outcomes. Turning it into a method that predicts full distributions of fragment masses and energies without tuning to fission data is still unsolved.

• Energy sharing and dissipation between fragments at scission

  Open

  The energy released in fission ends up partly as the motion of the fragments and partly as heat inside them, and the light and heavy fragments get different amounts of heat. How much energy is turned into heat on the way to scission, and why extra input energy goes mostly to the heavy fragment, are not understood from first principles.

• Distribution of the angle between the two fragment spins

  Open

  Even if the sizes of the two fragment spins are unrelated, their directions could still be linked, and different theories predict different answers. Measuring the angle between them would tell which spinning motions are at work.

• Fission barrier heights of superheavy nuclei to 1 MeV

  Open

  Superheavy elements exist only because shell effects (extra stability from filled nucleon orbits) create an energy barrier against fission. Models disagree by several MeV on how high this barrier is, which changes predicted production rates by orders of magnitude.