Angular distributions of gamma-radiation of 1.2 GeV electrons in silicon monocrystals of great thickness

In the motion of relativistic electrons at a small angle to one of the crystallographic axes, coherence and interference effects manifest themselves in the radiation, owing to which the gamma-radiation intensity of particles in the crystal may far exceed the intensity of radiation in amorphous medium. These effects can be used as the basis for creation of intense radiation sources with a high spectral-angular density of radiation. The intensity of electron radiation in the crystal is known to be proportional to the target thickness. Therefore, to create gamma-sources, it is advantageous to use thick crystals. However, with an increasing target thickness the average square of the angle of multiple electron scattering by atoms also increases, and this results in broadening of the angular distribution of gamma-quanta emitted, and also in the attenuation of the coherence effect of electron radiation in the crystal. Besides, in thick crystals the radiation yield can be appreciably influenced by electron energy losses and by the absorption of emitted gamma-quanta. The present paper is concerned with investigating spectral-angular distributions of 1 GeV electron radiation in thick single crystals. The main attention is here focused on the analysis of influence of the above-mentioned factors on the radiation, and to the determination of the optimum crystalline target thickness from the viewpoint of elucidating the conditions, at which the maximum spectral-angular gamma-radiation density magnitude is attained. Let us consider the radiation with the relativistic electron beam incident on the crystal along one of its crystallographic axes. In thick crystals the greater part of beam particles executes an infinite above-barrier motion with respect to crystal atom strings lying parallel to the crystallographic axis. Therefore, to the first approximation one can assume that this group of particles makes the decisive contribution to the radiation. In its above-barrier motion the electron sequentially collides with different strings of atoms. If the motion occurs at angles to the crystallographic axis, ψ, of about several critical angle values of axial channeling, ψc, the scattering and radiation of the particle from different atom strings can be considered independent [1]. In this case, the spectral-angular radiation distribution will be determined, first of all, by the special features of electron radiation in the field of a single atom row string. As a result of incoherent multiple scattering of the particle by crystal atoms, the particles are redistributed in the angles ψ. If the average scattering angle values exceed the characteristic value of the angle of relativistic electron radiation υk ~ γ, then the formation of spectral-angular distribution of radiation is significantly influenced by multiple scattering of the particle in the crystal. With due regard for the multiple scattering, the average spectral-angular radiation density can be written as