A nickel crystal irradiated with ions resembles a hall where extra chairs are left after a concert. Those chairs are atoms squeezed between lattice sites. They first gather into clusters, then arrange into rings: some rings lie flat on the floor, immobile, while others roll across the hall like hoops. The question of why some roll and others do not has long troubled materials scientists.

To understand this, the scientists built perfect nickel crystals in a computer and manually inserted extra atomic planes. Then they heated these crystals to 400–1200 K and watched how the rings changed shape and position. It turned out that a prismatic loop does not move like a rigid hoop but like a relay race: each step involves only part of the atomic rows, and the number of participants grows with loop size. Therefore, larger loops, although they have the same migration barrier, move more slowly: more runners need to pass the baton.

But in a real crystal, loops get stuck. In a thin nickel foil irradiated with gallium ions, loops jumped between traps about 5 nm wide. Their effective diffusion was $10^5$–$10^9$ times lower than the calculation predicts for a free loop. This means that impurities and local stresses play the main role, not the properties of the loop itself.

Thus, the dance of loops in irradiated metal turned out to be not free sliding but an obstacle course. Previously this was only assumed; now it has been measured quantitatively. The mobility and stability parameters of loops can be directly incorporated into models of radiation swelling and hardening of nickel alloys used in nuclear reactors.