V.M. Strakhovenko

Theory of pair creation in aligned single crystals

Abstract The theory of pair creation by a photon in aligned single crystals, which is valid at any energies and incident angles, is developed. For small angles of incidence, the theory describes pair creation in the field of the axis (plane) of a single crystal. For this case, the corrections to the constant field approximation are found. For large enough angle of incidence, the general expressions are transformed into the standard theory of coherent pair creation. A modification of the coherent pair creation theory, which is valid in a wider interval of the incidence angles, is obtained.

Cascade processes in the fields of the axes of aligned single crystals

Abstract A theory is developed of electron-photon showers occurring in the fields of the axes of aligned single crystals. At very high energies and high energy thresholds of the particles involved in a cascade the processes in the field of the axes should be taken into account alone (hard cascade). The analytical solution has been derived for the cascade-theory equations for this case. When lowering the energy threshold the Beth-Heitler processes on separate nuclei should be taken into account as well (mixed and soft cascades). These cascades are analysed using computer simulation. Results are presented concerning the recent experiments on radiation and pair creation in aligned single crystals.

Axial channeling of relativistic electrons in crystals as a source for positron production

Abstract In the framework of studies of positron sources dedicated to linear colliders, crystals are considered as photon radiators in association with thin amorphous converters or as compact positron sources where photon production and pair creation occur in the same medium. The main features of such sources in comparison with conventional amorphous sources are described. Taking linear collider requirements for such sources, a crystal converter suited for the Japanese Linear Collider (JLC) is considered. Limitations caused by radiation damage are mentioned and indications on critical fluences are given. A proof-of-principle experiment has been undertaken in Orsay, observing radiation enhancement in a tungsten crystal oriented along the 〈111〉 axis and submitted to a 2 GeV electron beam. The main results are reported.

Radiation intensity from ultrarelativistic particles during axial channeling and its orientation dependence

Abstract The radiation intensity I of ultrarelativistic electrons and positrons during axial channeling in thin crystals is calculated using a realistic model for the axial potential. The approach used avoids a cumbersome analysis of radiation from concrete trajectories. A general expression for the intensity I (ϑ o ) as a function of the incident angle ϑ o of the particles, which is valid for any ax potential U (ρ), is derived. Explicit formulas for the potential [eq. (29)] are given. Intensities I (ϑ o ) for axial and planar chann compared.

Electromagnetic showers in crystals at GeV energies

Abstract Specific features of electromagnetic showers in crystals initiated by electrons with energies of a few GeV are considered, basing on a model form of radiation spectrum at axial alignment. Formulae describing the influence of ordered crystalline structure on incoherent processes are derived, using Yukawa potential for a separate atom. Positron yield from crystal and amorphous targets for the kinematics inherent in positron sources is calculated, showing definite advantages of crystals.

Polarization effects in non-relativistic ep scattering

Abstract 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 e ± beams available now. This practically rules out a use of such beams to polarize stored antiprotons or protons.

Hard photon emission from high energy electrons and positrons in single crystals

Abstract A theory is developed, which describes hard photon emission from charged particles in single crystals taking into account both the change of the effective electron mass in the plane wave field and the influence of the field caused by the continuous potential of crystal planes. A significant enhancement of radiation processes is found in single crystals in comparison with amorphous materials. The difference is revealed in the hard part of the radiation spectra from channeled electrons and positrons.

Set-up optimization for an experimental test of a positron source using channeling

Abstract A new experiment is suggested in order to investigate the positron yield at the exit of crystal tungsten targets of different thicknesses submitted to 10 GeV electron beam at one of the SPS CERN transfer lines. The Set-up consists of the beam control counters, targets mounted on the goniometer, positron detector and photon detector. The positron detector is based mainly on a drift chamber inserted between the poles of the magnet. It allows to reconstruct the track of the charged particles, to define the vertex coordinates and initial angle of the particles at the exit of the target, to measure the total and transverse momenta. A Monte Carlo simulation corresponding to the theoretical description of chanelling radiation and also numerical calculation were provided for the estimations of the expected distributions and detector response. The optimization of the Set-up is considered, as well as the status of the project in general.

Electromagnetic cascades developing along crystal axes

Abstract The development of an electromagnetic cascade at axial alignment of a single crystal is discussed. For the initial electron energies of a few GeV special attention is paid to the production of positrons in given phase-space, providing the possibility of further acceleration of them. Numerical calculations are performed for energies of incident electrons and photons up to 300 GeV. Shower characteristics obtained are in a good agreement with corresponding experimental data.

Nucleon polarization in the process e+e−→NN¯ near threshold

Abstract The process ee → NN̄ is studied nearby a threshold with account for polarizations of all initial and final particles. The nucleon polarization ζN reveals a strong energy dependence due to that of the nucleon electromagnetic form factors GE(Q ) and GM (Q ) caused by the final-state interaction of nucleons. It is shown that the modulus of the ratio of these form factors and their relative phase can be determined by measuring ζN along with the differential cross section. The polarization degree is analyzed using Paris NN̄ optical potential for calculation of the form factors. It turns out that |ζN | is high enough in a rather wide energy range above the threshold. Being especially high for longitudinally polarized beams, |ζN | is noticeable even if both ee beams are unpolarized.