Yu.V. Kononets

Cascade processes in radiation by high-energy electrons in crystals: The radiation peak

Abstract The problem of radiation by high-energy electrons in crystals is analysed under conditions where the radiation has a substantially synchrotronic character and the number of γ-quanta emitted by a single electron may be large. A generalized kinetic equation involving both the radiative cooling and the multiple scattering of the radiating electrons is formulated. The processes of emitting high-energy γ-quanta are shown to play a particular role in the deceleration kinetics of superrelativistic channeled electrons and to give rise to a peak in the hard region of the radiated-energy spectrum. An effective numerical method of solving the generalized cascade equation is developed. It provides a theoretical description of the main details of the electron and γ-quantum distributions, which were recently observed by Belkacem et al. in 〈110〉-axial channeling of electrons with initial energy E 0 = 150 GeV in a Ge single crystal.

GENERATION AND DETECTION OF THE POLARIZATION OF MULTI-GEV PHOTONS BY USE OF TWO DIAMOND CRYSTALS

Abstract Presented are experimental results for the difference in pair production probability (the asymmetry) for 5–150 GeV photons polarized parallel and perpendicular to a (110) plane in a 1.5 mm thick diamond 〈100〉 crystal. The photons are produced by interaction of 150 GeV electrons with an aligned diamond 〈100〉 crystal of 0.5 mm thickness. A significant asymmetry is found over the whole energy range, which corresponds to a high degree of linear polarization of the photons as well as a difference in the refractive index. This proof-of-principle result gives the possibility of producing high energy photons with circular polarization by use of a crystal. This might open for several opportunities in high energy physics like for instance the investigation of the contribution of the gluons to the spin of the nucleon.

Enhanced electromagnetic showers initiated by 20–180 GeV gamma rays on aligned thick germanium crystals

Abstract The distribution of the energy released in a silicon detector placed on the downstream side of thick germanium single crystals bombarded by 20–180 GeV gamma rays along directions close to the 〈1 1 0〉 axis or along a random direction has been investigated. A large enhancement of the shower for axial incidence of the gamma rays has been found. The response of the system composed of a germanium crystal and a silicon detector to single gamma rays as a function of their energy has been deduced and compared with existing Monte Carlo simulations.

Radiative-cooling and multiple-scattering effects in the deceleration kinetics of superrelativistic channeled electrons

Abstract The generalized cascade kinetic equation developed in a previous publication is used to study the evolution of energy distributions of superrelativistic electrons in crystals under the influence of intense synchrotron-like γ-radiation. The contributions of radiative cooling and multiple scattering to deceleration kinetics are analysed. The fluctuations of the transverse energy during the radiation and scattering processes, and, particularly, the radiative transitions between above-the-barrier and below-the-barrier states are shown to shape in an essential way the electron distributions in a hard region of the emitted energy spectrum. The structure of the peak found by Belkacem et al. under 〈110〉-axial channeling in Ge is investigated theoretically in a wide range of crystal thicknesses at an incident beam energy E 0 = 150 GeV. The peculiarities of the peak formation are discussed at higher incident energies where the effects of hard-γ-quantum emission manifest themselves more strongly, whereas the influence of the multiple scattering is sharply weakened.

Monte Carlo calculation of characteristics of γ-beams produced by coherent bremsstrahlung at Fermilab

Abstract The energy spectra of the γ-beams which can be produced at Fermilab are calculated by modeling the production and absorption in single crystals, taking into account the energy and angular divergence of the electron beams. The results confirm that, replacing the amorphous targets by crystals, one can increase the γ-beam intensity in the high-energy region.

Multiple scattering and wake effects in interactions of fast diatomic molecules with thin foils

Abstract At small angles between the diatomic-cluster axis and the direction of motion the multiple scattering on medium atoms is shown to be of a coherent character and intensifies considerably the wake effects. As a consequence, even at small dwell times on the energy-angle plane singular curves arise, on which the break-up particle distribution function has integrable singularities. At confluence points of the symmetric branches of the curves the singularities strengthen (“wake focalizing”). The theory of the phenomenon is based on the use of an effective Hamiltonian including the coherent multiple scattering and the wake-potential interactions. The motion of the centre-of-mass (CM) is described by a Fokker-Planck-type equation. Our analysis allowed us to find the positions and the structure of the distribution function singularities depending on the experimental conditions. The explicit dependence of the number of fast ions of the H2+ or 4HeH+ type, transmitted through a foil in the R regime, upon parameters of the incident ions and the foil is established theoretically.

Yield oscillations of inelastic processes under channeling conditions at angles greater than the critical angle

Abstract A quantum-mechanical analysis of the angular dependence of the yield of inelastic processes in thin crystals under planar-channeling conditions reveals strong intensity oscillations at a given depth for incidence angles ψ > ψc. The dependence of the period of these oscillations on the distance z from the surface of incidence is investigated, and pronounced oscillations of the yield versus z are shown to occur simultaneously at fixed ψ.

Coherent bremsstrahlung, coherent pair production, birefringence, and polarimetry in the 20-170 GeV energy range using aligned crystals

The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20-170 GeV. The experimental arrangement allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active debate and development. With the approach used in this paper, both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high-energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high-energy SOS photons.

The peculiarities of multiple scattering of fast dinuclear clusters in amorphous matter

Abstract It is shown that the properties of anisotropic distributions of particles arising from the break-up of fast diatomic ions in thin solid films cannot be explained without exceeding the limits of the wake interaction between the two bare atomic nuclei in matter. At small angles between the dinuclear-cluster axis and the direction of motion the multiple scattering by the atoms of the material is found to lead to the appearance of some effective forces playing an important role in the problem. These forces are calculated in explicit form and a quantitative comparison with the polarization wake forces in the Echenique-Ritchie-Brandt model is made. With increasing velocity the forces of the coherent multiple scattering prevail, which explains the observed tendency towards equalizing structures of “forward” and “backward” peaks of the break-up particles while retaining a sharp angular anisotropy of the distribution.

The quantum recoil effects in radiative losses of superrelativistic electrons in crystals

Abstract By means of the Baier-Katkov operator method a general solution of the radiative-loss problem for fast electrons in crystals is obtained, which takes into account the quantum recoil during the photon emission and needs no analysis of the radiation frequency spectrum. At ultrahigh energies E the classical dependence E2 for radiative losses under channeling conditions is shown to be replaced by the quantum synchrotron law E 2 3 . The transitional energy region is investigated and the possibility of observing pronounced quantum effects in modern experiments with crystals of heavy elements in demonstrated.