Interplay and specific features of radiation mechanisms of electrons and positrons in crystalline undulators

We predict peculiar changes in spectral distributions of radiation emitted by ultra-relativistic positrons and electrons in periodically bent crystals (PBCs) with variation of the bending amplitude. It is shown that the changes, being sensitive to the projectile energy, manifest themselves differently for negatively and positively charged projectiles. We demonstrate that the features observed are due to the interplay of different radiation mechanisms occurring in PBCs. The observations are important for design, practical realization and channeling experiments with PBCs as the key element of the novel light sources. The analysis presented is based on the grounds of accurate numerical simulations of the channeling process. (letter)