SokolovTernov

The Sokolov–Ternov effect is a quantum electrodynamical phenomenon in which electrons (and positrons) circulating in a storage ring become spontaneously polarized due to synchrotron radiation. The effect was theoretically described in 1964 by Arkady A. Sokolov and Igor M. Ternov, who showed that radiative spin-flip transitions in curved magnetic fields lead to a net build-up of polarization over time.

The mechanism relies on the asymmetry of spin-flip transitions accompanying photon emission in bending magnets. As electrons emit radiation while circulating, transitions that flip the spin in one direction occur with slightly different probabilities than those flipping in the opposite direction. Over many emissions, this imbalance drives the beam toward a steady-state polarization aligned with the direction dictated by the ring’s magnetic field configuration. In an ideal planar ring, the maximum asymptotic polarization is about 92.4 percent, with the actual level depending on ring geometry, energy, and depolarizing effects such as betatron coupling and misalignments.

Polarization builds up on timescales that depend on beam energy, ring size, and magnetic lattice, ranging from tens of seconds to several hours in typical accelerator conditions. The Sokolov–Ternov effect has practical significance for accelerator physics, providing a method to polarize electron beams and serving as a diagnostic for beam energy and lattice properties. It remains a foundational concept in the study of radiative spin dynamics and quantum electrodynamics in strong-field environments.