Radiation yield of high‐energy electrons in thick crystals

Abstract

The radiation of high-energy electrons moving near the crystalline axes in thick crystals is analysed. The radiation brightness and yield are discussed. The results obtained are compared with experiment.IIpOBeJeH aHaJIYI3 M3JIYZIeHMFI 3JIeKTpOHOB 60~1buoCi 3HeprllM IlpM ,I(BMXeHMll B6nH3H 1ipMCTaJIZMYeCKMX OCen B TOJICTbIX MOHOKPMCTaJIJIaX. 06cylrcfiam~cfi HPKOCTb PI BMXOa Ii3JIyYeHHFI. nOJIy'4eHHbIe pe3yJIbTaTbI CpaBHMBaIOTCH C 3KCIlepHMeHTOM. Description of Radiation in Thick CrystalsA theoretical description of the specific radiation which occurs when high-energy particles are moving near the crystalline planes, or axes, in single crystals, depends essentially on whether a crystal is thin or thick. I n thin crystals, by definition, the distribution function (DF) in a transverse (to the direction of the axes, or planes) phase space does not vary when the particles pass through a crystal and is defined by the initial conditions. I n thick crystals, a dependence of the D F on the depth of penetration of the particles into a crystal should be taken into account. Multiple scattering and radiation energy losses are referred to the basic processes which govern the kinetics of this function.To apply the effect under discussion as a source of hard and directed radiation concentrated within a comparatively narrow frequency range, it is thick crystals2) that are of greatest interest, in the range of not too high energies. One of the most important characteristics is the total radiation intensity into a given solid angle. I n this case, the interesting question is to find an optimal thickness of the crystal, L = Lo, for which the energy of radiation yield, a t a given collimation of the photon beam, is maximum. This range of problems was recently treated by the authors for the case of motion along the crystalline axes According t o the performed analysis [l, 21 dealing with the kinetics of the distribution, which is due t o multiple scattering, the D F for large depths ( I ) is of the form Prospekt Lavrenteva 11, 630090 Novosibirsk, USSR. z, In thin crystals [l], the particles lost is noticeable fraction of their energy only a t superhigh electron energies 6 2 eCT, beginning with tens of GeV.

Comparison of theory with experiment for positron production from high-energy electrons moving along crystal axes

“…This alignment alone will be considered below. According to [1], the radiation intensity in a crystal exceeds that of the conventional bremsstrahlung starting with electron energies " 1 GeV. Simple estimations of the width of the power spectrum indicated a soft character of this spectrum.…”

“….'' was proposed in [1]. Based on mentioned properties of the photon emission process, the use of this phenomenon in the positron source for future accelerators was proposed [2,3].…”

A positron source using an axially oriented crystal associated to a granular amorphous converter

“…The energy spectrum and the yield of the photons produced in the crystal depend, obviously, on the electron incident energy. For that, it is interesting to consider a figure of merit [1] given by the ratio:…”

“…R= I ch /I br (1) whereI ch is the intensity of channeling radiation and I br , that of bremsstrahlung. This ratio depends on the electron energy and on the crystal.…”

Summary of experimental studies, at CERN, on a positron source using crystal effects

“…The most pronounced effects take place at the orientation where the electrons are incident along the major axes of the crystal. At such orientation, the incident electrons are exposed to the highest mean electric field provided by the atomic rows [1]. The enhancement depends on the particle energy and the crystal orientation (see [2] for further details concerning electromagnetic processes in crystals).…”