Andrei V. Korol

PHOTON EMISSION BY AN ULTRA-RELATIVISTIC PARTICLE CHANNELING IN A PERIODICALLY BENT CRYSTAL

This paper is devoted to a detailed analysis of the new type of undulator radiation generated by an ultra-relativistic charged particle channeling along a crystal plane, which is periodically bent by a transverse acoustic wave, as well as to the conditions limiting the observation of this phenomenon. This mechanism makes feasible the generation of electromagnetic radiation, both spontaneous and stimulated, emitted in a wide range of the photon energies, from X- up to γ-rays.

Coherent radiation of an ultrarelativistic charged particle channelled in a periodically bent crystal

We suggest a new type of the undulator radiation which is generated by an ultrarelativistic particle channelled along a periodically bent crystallographic plane or axis. The electromagnetic radiation arises mainly due to the bending of the particles trajectory, which follows the shape of the channel. The parameters of this undulator, which totally define the spectrum and the angular distribution of the radiation (both spontaneous and stimulated), depend on the type of the crystal and the crystallographic plane (axis), the type of projectile and its energy, and on the shape of the bent channel and, thus, can be varied significantly by varying these characteristics. As an example, we consider the acoustically induced radiation (AIR) which is generated by ultrarelativistic particles channelled in a crystal which is bent by a transverse acoustic wave. The AIR mechanism allows us to make undulators with the main parameters varying over wide ranges, which are inaccessible in the undulators based on the motion of particles in the periodic magnetic fields and also in the field of the laser radiation. The intensity of AIR can easily be made larger than the intensity of the radiation in a linear crystal and can be varied over a wide range by varying the frequency and the amplitude of the acoustic wave in the crystal. A possibility to generate stimulated emission of high-energy photons (in keV - MeV region) is also discussed.

CHANNELING OF POSITRONS THROUGH PERIODICALLY BENT CRYSTALS: ON THE FEASIBILITY OF CRYSTALLINE UNDULATOR AND GAMMA-LASER

The electromagnetic radiation generated by ultra-relativistic positrons channeling in a crystalline undulator is discussed. The crystalline undulator is a crystal whose planes are bent periodically with the amplitude much larger than the interplanar spacing. Various conditions and criteria to be fulfilled for the crystalline undulator operation are established. Different methods of crystal bending are described. We present the results of numeric calculations of spectral distributions of the spontaneous radiation emitted in the crystalline undulator and discuss the possibility to create the stimulated emission in such a system in analogy with the free electron laser. A careful literature survey covering the formulation of all essential ideas in this field is given. Our investigation shows that the proposed mechanism provides an efficient source for high energy photons, which is worth studying experimentally.

Simulation of ultra-relativistic electrons and positrons channeling in crystals with MBN Explorer

A newly developed code, implemented as a part of the MBN Explorer package (Solov?yov et al., 2012; http://www.mbnexplorer.com/, 2012) 1,2] to simulate trajectories of an ultra-relativistic projectile in a crystalline medium, is presented. The motion of a projectile is treated classically by integrating the relativistic equations of motion with account for the interaction between the projectile and crystal atoms. The probabilistic element is introduced by a random choice of transverse coordinates and velocities of the projectile at the crystal entrance as well as by accounting for the random positions of the atoms due to thermal vibrations. The simulated trajectories are used for numerical analysis of the emitted radiation. Initial approbation and verification of the code have been carried out by simulating the trajectories and calculating the radiation emitted by e = 6.7 GeV and e = 855 MeV electrons and positrons in oriented Si(110) crystal and in amorphous silicon.The calculated spectra are compared with the experimental data and with predictions of the Bethe-Heitler theory for the amorphous environment.

Revealing the mechanism of the low-energy electron yield enhancement from sensitizing nanoparticles.

We provide a physical explanation for the enhancement of the low-energy electron production by sensitizing nanoparticles due to irradiation by fast ions. It is demonstrated that a significant increase in the number of emitted electrons arises from the collective electron excitations in the nanoparticle. We predict a new mechanism of the yield enhancement due to the plasmon excitations and quantitatively estimate its contribution to the electron production. Revealing the nanoscale mechanism of the electron yield enhancement, we provide an efficient tool for evaluating the yield of the emitted electron from various sensitizers. It is shown that the number of low-energy electrons generated by the gold and platinum nanoparticles of a given size exceeds that produced by the equivalent volume of water and by other metallic (e.g., gadolinium) nanoparticles by an order of magnitude. This observation emphasizes the sensitization effect of the noble-metal nanoparticles and endorses their application in novel technologies of cancer therapy with ionizing radiation.

Photon emission by ultra-relativistic positrons in crystalline undulators: the high-energy regime

This paper discusses the undulator radiation emitted by high-energy positrons during planar channeling in periodically bent crystals. We demonstrate that the construction of the undulator for positrons with energies of 10GeV and above is only possible if one takes into account the radiative energy losses. The frequency of the undulator radiation depends on the energy of the particle. Thus, the decrease of the particles energy during the passage of the crystal should result in the destruction of the undulator radiation regime. However, we demonstrate that it is possible to avoid the destructive influence of the radiative losses on the frequency of the undulator radiation by the appropriate variation of the shape of the crystal channels. We also discuss a method by which, to our mind, it would be possible to prepare the crystal with the desired properties of its channels.

The influence of the dechannelling process on the photon emission by an ultra-relativistic positron channelling in a periodically bent crystal

We investigate, both analytically and numerically, the influence of the dechannelling process on the parameters of undulator radiation generated by ultra-relativistic positron channelling along a crystal plane, which is periodically bent. The bending might be due either to the propagation of a transverse acoustic wave through the crystal, or due to the static strain as it occurs in superlattices. In either case the periodically bent crystal serves as an undulator which allows the generation of x-ray and γ-radiation. We propose a scheme for the accurate quantitative treatment of the radiation in the presence of dechannelling. The scheme includes: (a) the analytic expression for the spectral-angular distribution which contains, as a parameter, the dechannelling length; (b) the simulation procedure of the dechannelling process of a positron in periodically bent crystals. Using these we calculate the dechannelling lengths of 5 GeV positrons channelling in Si, Ge and W crystals, and the spectral-angular and spectral distributions of the undulator over broad ranges of the photons. The calculations are performed for various parameters of channel bending.

Radiation from multi-GeV electrons and positrons in periodically bent silicon crystal

A periodically bent Si crystal is shown to efficiently serve for producing highly monochromatic radiation in a gamma-ray energy spectral range. A short-period small-amplitude bending yields narrow undulator-type spectral peaks in radiation from multi-GeV electrons and positrons channeling through the crystal. Benchmark theoretical results on the undulator are obtained by simulations of the channeling with a full atomistic approach to the projectile-crystal interactions over the macroscopic propagation distances. The simulations are facilitated by employing the MBN Explorer package for molecular dynamics calculations on the meso-, bio- and nano-scales. The radiation from the ultra-relativistic channeling projectiles is computed within the quasi-classical formalism. The effects due to the quantum recoil are shown to be significantly prominent in the gamma-ray undulator radiation.

Electron Production by Sensitizing Gold Nanoparticles Irradiated by Fast Ions

The yield of electrons generated by gold nanoparticles due to irradiation by fast charged projectiles is estimated. The results of calculations are compared to those obtained for pure water medium. It is demonstrated that a significant increase in the number of emitted electrons arises from collective electron excitations in the nanoparticle. The dominating enhancement mechanisms are related to the formation of (i) plasmons excited in a whole nanoparticle and (ii) atomic giant resonances due to excitation of d electrons in individual atoms. Decay of the collective electron excitations in a nanoparticle embedded in a biological medium thus represents an important mechanism of the low-energy electron production. Parameters of the utilized model approach are justified through the calculation of the photoabsorption spectra of several gold nanoparticles, performed by means of time-dependent density-functional theory.

Quantum and classical features of the photoionization spectrum of C60

By considering photoionization of the C