A uranium-235 nucleus that captures a slow neutron doesn't immediately fly apart into fragments. Instead, it seems to get stuck in two 'pits'—states with different shapes. The first, the ground state, is a slightly elongated drop. The second, the isomeric state, is a drop stretched further but not yet torn. The transition from one pit to the other is like rolling over a hill, and then over an outer barrier, after which an irreversible rupture begins.
For the first time, scientists have tracked the motion of the nucleus in each of these pits separately. They used a computational method previously applied only to the final stage — from the 'saddle' (the point where the nucleus is ready to break) to the actual rupture. It turned out that in both pits, the motion is highly 'viscous': the nucleus's shape changes not freely but as if in a thick syrup. But there's a difference. In the ground-state pit, any initial deviation in shape quickly decays — the nucleus almost immediately adopts a smooth, symmetric shape. In the isomeric pit, it's different: the nucleus's shape begins to oscillate with large amplitude, almost like a pendulum, taking a long time to settle down.
Another finding: in both pits, the nucleus has a small probability of emitting a neutron. This means that even before rupture, uranium can lose part of its mass. Such neutrons can influence the chain reaction in a reactor or a bomb.
Previously, these early stages of fission were described only by rough models. Now there's a microscopic calculation that can be connected to the already known picture of the final stage. The result is a complete story: from neutron capture to fragment separation. This will help more accurately predict how much energy will be released and which isotopes will be born in reactors.
as written by the authors arXiv:2608.08875v1
In fission induced by low energy neutrons, the mother nucleus spends a significant fraction of the time in the ground state and isomer wells, eventually passing beyond the outer barrier, where the primary fission fragments properties are defined. Despite this, the dynamics of these two early stages have not been investigated using microscopic models. This study examines the evolution of the mother nucleus in both wells separately, using time-dependent density functional theory, which has been previously used to treat the saddle-to-scission stage of fission for $$^{235}$$U(n,f) reactions. These two early stages of fission are essential blocks in the final theory of the formation and evolution of a compound nucleus. The present study shows that the dynamics in both wells is strongly dissipative, similar to the dynamics from saddle to scission. It also reveals that while the initial mass asymmetry of the system quickly settles to very small fluctuations in the ground state well, in the isomeric well, the mass asymmetry oscillates with a rather large amplitude, in almost harmonic motion. Furthermore, with low probability, neutrons are emitted in both wells.
The isomeric pit of uranium-235 was discovered back in the 1960s, but its role in fission remained a mystery. Now it turns out it acts as a 'swing' for the nucleus's shape.