Channeling 2012

Symmetry Properties in Angular Distribution of Radiation in Thin Crystals

24 Sept 2012, 09:30

15m

Hotel Calabona (Hotel Calabona)

S1.1 Coherent Bremsstrahlung

Speaker

Dr Micola Bondarenco (Kharkov Institute of Physics and Technology)

Description

Radiation from an electron passing through a sufficiently thin oriented crystal is of dipole type, and so its spectral-angular distribution must be isomorphic for all species of crystals, depending only on the direction and anisotropy of the electron deflection in the crystal. The dipole radiation angular distribution, even at photon energies commensurable with the electron energy, possesses a high degree of symmetry elucidated by the notion of stereographic projection [1]. In particular, it proves that radiation polarization in the plane of small radiation angles at absolutely anisotropic scattering is distributed along a family of circles intersecting in two knots. One of those circles, with the opening radius exactly $\theta =1/\gamma$, has the centre at zero angle, thus being invariant under rotations relative to the beam axis. That observation may be used to explain the fact that at this emission angle the radiation polarization is always 100\%, irrespective of the scattering anisotropy degree. When the photon to electron energy ratio becomes sizeable, the radiation becomes more isotropic. The notion of stereographic projection proves valid for non-dipole radiation as well, provided the electron deflection anisotropy degree is close to unity, as realized in undulators. Possibilities of experimental verification of the polarized dipole radiation symmetry properties are discussed, for various types of crystals capable of providing a high anisotropy degree of electron scattering. References [1] M.V. Bondarenco, Phys. Rev. A 82 (2010) 042723.