v 1 1 D ec 1 99 7 Propagation γ - quanta in single crystals

Propagation of γ-quanta in the single crystals, oriented in a region of the coherent pair production is considered. The qualitative description of the process is also discussed. The theory of γ-quanta propagation in the anisotropic medium is illustrated by the help of the particular calculations of such a process in silicon single crystals. It is shown that the single crystals are sensitive to the initial circular polarization of γ-beam despite the fact that the cross section of absorption is independent of it. The reason is that the normal electromagnetic waves (an eigenfunctions of the problem) are elliptically polarized. The speeds of absorption and motion of both the normal waves are different and as a result the process of γ-quanta propagation depend on the initial polarization state. The calculated value of asymmetry is about 20% for 25 GeV γ-quanta propagating in 100 cm silicon single crystal. The obtained results are useful in creating of polarimeters for high energy electrons and γ-quanta. 1 Introduction It is well known that the optical properties of a medium may be described with the use of a permittivity tensor [1, 2]. The anisotropic medium referred as such a medium in which the permittivity tensor is a symmetric tensor [1, 2]. In the general case the components of this tensor are complex values and, because of this, the complex permittivity tensor does not reduce to principal axes, except in specific cases. The simplest example of such an anisotropic medium of the general type is a combination of the two linearly polarized monochromatic laser waves with different frequencies (dichromatic wave) moving in the same direction. In general the directions of linear polarization of these monochromatic waves are different. Another more complicated example is a single crystal oriented in the region of coherent e − e +-pair production [3, 4, 5]. The permittivity tensor was found for both these examples in papers[6, 7]. Besides, in [6] the theory of γ-quanta propagation in the anisotropic medium of a general type is also considered. One of the main results of these papers is the prediction of sensitivity of the anisotropic medium to the initial circular polarization of propagating γ-quanta. It is important for creating devices measuring the circular polarization of γ-quanta or longitudinal polarization of electrons. In the last case the electron beam is transformed into bremsstrahlung γ-beam by the use of amorphous target [8]. The circular polarization of the end …