When a heavy atomic nucleus splits, it doesn't break into even halves. First, two fragments form that look like pears pressed together at their narrow ends. Then the junction begins to smooth out, like two droplets merging and the surface quickly leveling.

In that brief moment, the surface moves inward, and particles inside the nucleus can bounce off it, like a ball off an oncoming wall. Such a bounce adds to their speed. In the new study, scientists traced the fate of these bounced particles and found: a few percent of neutrons can fly out with very high energy.

This mechanism helps explain where fast neutrons from fission come from. They are important both for nuclear energy and for understanding processes inside stars.