Polarization of bremsstrahlung at electron scattering in an anisotropic medium

Abstract

Bremsstrahlung from relativistic electrons is considered under conditions when some transverse direction of momentum transfer is statistically preferred. It is shown that in the dipole approximation all the medium anisotropy effects can be accumulated into a special modulus-bound transverse vector, N . To exemplify a target with N 2 ∼ 1, we calculate radiation from electron incident at a small angle on an atomic row in oriented crystal. Radiation intensity and polarization dependence on the emission angle and frequency for constant N is investigated. Net polarization for the angleintegral cross-section is evaluated, which appears to be proportional to N 2 /2, and decreases with the increase of the photon energy fraction. A prominent feature of the radiation angular distribution is the existence of an angle at which the radiation may be completely polarized, in spite of the target complete or partial isotropy -that owes to existence of an origin-centered tangential circle for polarization in the fully differential radiation probability kernel. Possibilities for utilizing various properties of the polarized bremsstrahlung flux for preparation of polarized photon beams and for probing intrinsic anisotropy of the medium are analyzed.

Interference in spectrum of radiation from doubly scattered charged particle

“…Volume reflection develops over a length ∆z VR ∼ Rθ c , and for the case of positively charged particles (for which it works somewhat better) leads to deflection to an angle χ ≈ π 2 θ c [28], where θ c = 2V 0 /E ∼ 50 eV/E is the critical channeling angle. The intrinsic relative spread of the deflected beam is ∆χ/χ ∼ 2R c /R, where R c = E/|F max | = 0.2 m E GeV is the critical radius, and R the crystal bending radius, which must be in excess of 4R c .…”

Mean volume reflection angle

“…In fact, for silicon crystal in orientation (110) the parabolic approximation is known to be rather satisfactory. It is this approximation that was used in [15], but only for small R/R c . More generally, if the effective potential is strongly tilted, only relatively small portions of the wells contribute to the mean VR angle (cf.…”

“…The first one is to recall that the experimental procedure for determination of χ consists in fitting the measured angular distribution of transmitted particles by a superposition of Gaussian-related functions, each of which is associated with a certain particle fraction (channeled, dechanneled or VR). But for negatively charged VR particles, the angular distribution has a rather long 'tail' towards the crystal bending due to "orbiting" of particles at the tops of broad interplanar potential maxima [15]. Such particles can be misidentified as not belonging to the VR fraction (instead being included to the quickly dechanneled and multiply scattered fraction), and their subtraction increases the mean VR angle.…”

“…To get a grasp of various parameter dependencies in the VR problem, in [15] it was solved for the simplest example -parabolic model for the interplanar potential. It was demonstrated that the complicated dependencies of the VR angle on R and E greatly simplify if an ex- * Electronic address: bon@kipt.kharkov.ua pansion in the small parameter R c /R [with R c (E) being the critical radius] is carried out.…”

Multiphoton effects in coherent radiation spectra

“…It tells that at multi-GeV positron energies, and L L d /10, the photon multiplicity must achieve values w 1c 1. Multiphoton effects in channeling radia- 24 Instead of actually increasing the radiator length, it may suffice to self-convolve the spectrum from the same radiator two or more times (the procedure adopted in [27,40]).…”