Electron-positron pair production by photons in nonuniform strong fields

A probability of electron-positron pair production by photons in strong nonuniform fields is derived by applying a model trajectory method in the frame of a semiclassical approach. In addition to the well known invariant field parameter chi, a new invariant parameter nu is introduced to characterize the nonuniformity of the field. For nu>>1, the obtained expression is reduced to the uniform-field approximation while it approaches the Bethe-Heitler formula for nu<<1. The pair production is predicted for relatively weak external fields where the uniform-field approximation gives no effect. The theory agrees well with the experimental results of crystal-assisted pair production.

Standard radiation spectrum of relativistic electrons: beyond the synchrotron approximation

Radiation emitted by an electron in arbitrary, extreme relativistic motion, has been described for the first time in terms of a standard spectrum of nonsynchrotron type. Ultimately, such a nonsynchrotron spectrum is dependent not only on instantaneous trajectory curvature, but also upon its first two time derivatives and helicity, to provide a basic correction to the synchrotron approximation (SA). A strong deviation from SA has been predicted for above GeV electrons in oriented crystals.