144 research outputs found
Emission of polarized photons from unpolarized electrons moving in crystals
Radiation emitted by unpolarized high-energy electrons penetrating crystals
may be linearly polarized. This occurs when the particle velocity makes an
angle, with respect to some major crystal axis, being sufficiently larger than
the axial-channelling angle. For such orientation, a complete description of
spectral and polarization characteristics of the radiation is derived. At
planar channelling, a non-perturbative contribution to the probability of the
process appears caused by the plane field, and we must solve exactly a
one~-~dimensional mechanical problem. For that, the approximate form of the
actual plane potential is suggested which provides a precise fit for any
crystal plane and an analytical solution to the motion problem. In a practical
case, we must consider electron-photon showers developing in sufficiently thick
crystals. For the first time, this development is described taking into account
the polarization of photons. We discuss qualitative features of the phenomenon,
present results of numerical calculations for thin and thick crystals, and
evaluate the possibility of the use of differently oriented crystals in a
polarized hard photon source.Comment: 16 pages, 7 PostScript figure
Generation of circularly polarized photons for a linear collider polarized positron source
Various methods of obtaining longitudinally polarized positrons for future
linear colliders are reviewed. Special attention is paid to the schemes using
circularly polarized high-energy photons for positron production. Most
effectively such photons are obtained from electrons passing through a helical
undulator or colliding with a circularly polarized laser wave. Spectrum and
polarization of radiation emitted during helical motion of electrons are
considered in detail. A new simple presentation of known formulas is used to
account for the influence of the wave intensity, of the electron-beam angular
divergence, of the collimation of radiation, and of the lateral and temporal
profiles of the laser bunch on the radiation properties.Comment: 16 pages, 6 figure
Hard Polarized Photon Emission in Single Crystals by High Energy Electrons for Planar Crystal Orientations
The radiation emission spectra of polarized photons emitted from charge
particle in single crystal are obtained in semiclassical theory in
Baer-Katkov-Strakhovenko approximation for planar crystal orientation. The
range of applicability of this approximation is estimated by comparing the
results with calculations in exact semiclassical theory. Optimal crystal
orientations for producing unpolarized and polarized photon beams are also
founded.Comment: 10 pages, 5 figures, (wpapfig,epsfig
Polarization effects in non-relativistic scattering
The cross section which addresses the spin-flip transitions of a proton
(antiproton) interacting with a polarized non-relativistic electron or positron
is calculated analytically. In the case of attraction, this cross section is
greatly enhanced for sufficiently small relative velocities as compared to the
result obtained in the Born approximation. However, it is still very small, so
that the beam polarization time turns out to be enormously large for the
parameters of beams available now. This practically rules out a use
of such beams to polarize stored antiprotons or protons.Comment: 9 pages; version accepted for publication in Nucl.Instr.Methods B
where cross sections without averaging over directions of relative velocities
are adde
Photon splitting in atomic fields
Photon splitting due to vacuum polarization in the electric field of an atom
is considered. We survey different theoretical approaches to the description of
this nonlinear QED process and several attempts of its experimental
observation. We present the results of the lowest-order perturbation theory as
well as those obtained within the quasiclassical approximation being exact in
the external field strength. The experiment where photon splitting was really
observed for the first time is discussed in details. The results of this
experiment are compared with recent theoretical estimations.Comment: 45 pages, 24 figure
Coulomb effects in the spin-dependent contribution to the intra-beam scattering rate
Coulomb effects in the intra-beam scattering are taken into account in a way
providing correct description of the spin-dependent contribution to the beam
loss rate. It allows one to calculate this rate for polarized beams
at arbitrarily small values of the ratio ,
characterizing relative change of the electron energy in the laboratory system
during scattering event.Comment: 8 pages, 2 figure
Spin effects in interaction and their possible use to polarize antiproton beams
Low energy interaction is considered taking into account the
polarization of both particles. The corresponding cross sections are calculated
using the Paris nucleon-antinucleon optical potential. Then they are applied to
the analysis of the polarization buildup which is due to the interaction of
stored antiprotons with polarized protons of a hydrogen target. It is shown
that, at realistic parameters of a storage ring and a target, the filtering
mechanism provides a noticeable polarization in a time comparable with the beam
lifetime.Comment: 10 pages, 4 figure
On measurements of the energy and polarization distributions of high-energy gamma-beams
A possibility to measure the energy and polarization distributions of high
intensity gamma-beams is considered. This possibility is based on measurements
of the number of electron-pozitron pairs in such media as laser waves and
single crystals. The method may be useful for future gamma-gamma and e-gamma
colliders.Comment: 6 pages, 3 eps figures, Latex, epsfig.st
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