V.M. Katkov

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.

Mechanism of electron-positron pair production by high-energy photons in a single crystal

Abstract A mechanism of pair production by photons in an electric field of the axes (planes) of a single crystal is discussed. Simple expressions for the process probability and spectral distribution at κ ≲1 are presented. An analysis of conditions to observe the effect is made.

Quantum effects in radiation emitted by ultrahigh energy electrons in aligned crystals

Abstract The energy losses and the radiation spectra of electrons moving in the field of the single crystals axes have been calculated in the energy range where the radiation in an external field becomes essentially quantum. Comparison with the experimental data has been made.

Pair creation by a photon in an electric field

Abstract The process of pair creation by a photon in a constant and homogeneous electric field is investigated basing on the polarization operator in the field. The total probability of the process is found in a relatively simple form. At high energy the quasiclassical approximation is valid. The corrections to the standard quasiclassical approximation (SQA) are calculated. In the region of relatively low photon energies, where SQA is unapplicable, the new approximation is used. It is shown that in this energy interval the probability of pair creation by a photon in electric field exceeds essentially the corresponding probability in a magnetic field. This approach is valid at the photon energy much larger than the “vacuum” energy in electric field: ω ≫ e E / m . For smaller photon energies the low energy approximation is developed. At ω ≪ e E / m the found probability describes the absorption of soft photon by the particles created by an electric field.

Radiation in oriented single crystals: Status of theory and experiment

Abstract It is shown that the peak in the energy loss distribution, observed by Belkacem et al., can be described in the frame of an approach developed by the authors. This means that the whole set of experimental data agrees quite satisfactorily with the theory.

On the radiation theory of high energy electrons in aligned single crystals

Abstract A general approach is developed for a description of the radiation emitted by high energy particles in aligned single crystals. At small angles of incidence, this theory describes the radiation in the field of separate axes and, at large angles of incidence, goes over to the coherent bremsstrahlung theory.

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.

Electron-positron pair creation by photons in an aligned single crystal: comparison of theory with experiment

Abstract The creation of electron-positron pairs by 50–110 GeV photons directed along the 〈110〉 axis of a cooled crystal is considered. The results obtained are compared with a very recent experiment performed in CERN. Radiation from electrons is also discussed.