A. V. Solov’yov

Ion-induced electron production in tissue-like media and DNA damage mechanisms

This work is the first stage in the development of an inclusive approach to calculation of the DNA damage caused by irradiation of biological tissue by ion/proton beams. The project starts with an analysis of ionization caused by the projectiles and the characteristics of secondary electrons produced in tissue-like media. We consider interactions with the medium on a microscopic level and this allows us to obtain the energy spectrum and abundance of secondary electrons as functions of the projectile’s kinetic energy. The physical information obtained in this analysis is related to biological processes responsible for the DNA damage induced by the projectile. In particular, we consider double strand breaks of DNA caused by secondary electrons and free radicals, and local heating in the ion’s track. The heating may enhance the biological effectiveness of electron/free radical nteractions with the DNA and may even be considered as an independent mechanism of DNA damage. Numerical estimates are performed for the case of carbon-ion beams. The obtained dose-depth curves are compared with results of the MCHIT model based on the GEANT4 toolkit.

Formalism of collective electron excitations in fullerenes

A formalism for the description of collective electron excitations in fullerenes by inelastic scattering of fast electrons within the plasmon resonance approximation is presented. Considering the system as a spherical shell of a finite width, we show that the differential cross section is defined by three plasmon excitations, namely two coupled modes of the surface plasmon and the volume plasmon. The interplay of the three plasmons appears due to the electron diffraction of the fullerene shell. Plasmon modes of different angular momenta provide dominating contributions to the differential cross section depending on the transferred momentum.

On the fragmentation of biomolecules : Fragmentation of alanine dipeptide along the polypeptide chain

The interaction potential between amino acids in alanine dipeptide has been studied for the first time taking into account exact molecular geometry. Ab initio calculation has been performed in the framework of density functional theory taking into account all electrons in the system. The fragmentation of dipeptide along the polypeptide chain, as well as the interaction between alanines, has been considered. The energy of the system has been analyzed as a function of the distance between fragments for all possible dipeptide fragmentation channels. Analysis of the energy barriers makes it possible to estimate the characteristic fragmentation times and to determine the degree of applicability of classical electrodynamics for describing the system energy.

Double strand breaks in DNA resulting from double ionization events

Abstract.A mechanism of double strand breaking in DNA due to the action of two electrons is considered. These are the electrons produced in the vicinity of DNA molecules due to ionization of water molecules with a consecutive emission of two electrons, making such a mechanism possible. The transport of secondary electrons, including the additional electrons, is studied in relation to the assessment of radiation damage due to incident ions. This work is a stage in the inclusion of double ionization events into the multiscale approach to ion-beam cancer therapy.

Droplet model of an atomic cluster at a solid surface

A method for calculating the characteristics of the stability, energy, and geometry of an atomic cluster at a solid surface is proposed, which is based on a droplet model that takes into account the cluster-solid interaction. As an example, the interaction of a neutral argon cluster with a (001) surface of graphite is considered. The results of calculations performed within the framework of the droplet model show good agreement with the results of numerical simulation based on a dynamic search for the most stable isomers in the course of cluster growth. It is shown that the droplet model can be used for simple evaluation of the geometry, stability, and energy characteristics of clusters at solid surfaces.

Potential energy surface of alanine polypeptide chains

The multidimensional potential energy surfaces of the peptide chains consisting of three and six alanine (Ala) residues have been studied with respect to the degrees of freedom related to the twist of these molecules relative to the peptide backbone (these degrees of freedom are responsible for the folding of such peptide molecules and proteins). The potential energy surfaces have been calculated ab initio within the framework of the density functional theory taking into account all electrons in the system. The probabilities of transitions between various stable conformations of polypeptide molecules are evaluated. The results are compared to the data obtained by molecular dynamics simulations and to the available experimental data. The influence of the secondary structure of the polypeptide chain on its conformational properties with respect to rotations has been studied. It is shown that, in a chain of six amino acid (Ala) residues, the secondary structure type (helix or sheet conformation) influences the stable isomer states of the polypeptide.

The influence of the structure imperfectness of a crystalline undulator on the emission spectrum

We study the influence of an imperfect structure of a crystalline undulator on the spectrum of the undulator radiation. The main attention is paid to the undulators in which the periodic bending in the bulk appears as a result of a regular (periodic) surface deformations. We demonstrate that this method of preparation of a crystalline undulator inevitably leads to a variation of the bending amplitude over the crystal thickness and to the presence of the subharmonics with smaller bending period. Both of these features noticeably influence the monochromatic pattern of the undulator radiation.

Method for Calculating Photoionization Cross Sections of Fullerenes in the Local Density and Random Phase Approximations

A method for calculating the photoionization cross sections of fullerenes taking into account many-electron correlations on the basis of the local density and random phase approximations is proposed and implemented. Calculations are made specifically for fullerenes C60 and C20. It is shown that the photoionization spectrum of C60 acquires a plasmon resonance whose position and magnitude are in good agreement with experimental results [I.V. Hertel, H. Steger, J. de Vries et al., Phys. Rev. Lett. 68, 784 (1992)] and with the results of other calculations [M.J. Puska and R.N. Nieminen, Phys. Rev. A47, 1181 (1993)]. The emergence of a giant resonance is predicted in the photoionization spectrum of fullerene C20 with the center at a photon energy on the order of 27 eV, which corresponds to the frequency of resonant surface plasmon oscillations in a conducting sphere.

Dynamical Screening of an Endohedral Atom

The dynamical screening factor for Xe@C60 is presented. These results are based on our original model and based on the modified framework developed in [S. Lo, A. V. Korol and A. V. Solov’yov Phys. Rev. A 79 063201 (2009)]. In this modified framework, the σ and the π plasmons of the fullerene are taken into account. Due to the finite width of the fullerene, each plasmon splits into two eigenmodes, resulting in there being four peaks in the structure of the modified dynamical screening factor. The photoabsorption cross section of the two model fullerenes used are shown. These give a qualitative description of the fullerene’s photoabsorption cross section. The manifestation of the two modes of the πplasmon in the cross section are also presented. The model photoionization cross section is compared with quantum mechanical calculations [V. Ivanov et al. J. Exp. Theor. Phys 96, 658 (2003)] and to experimental data [A. K. Belyaev et al. Phys. Scr. 80 048121 (2009)].

A multi‐scale approach to the physics of ion beam cancer therapy

We are developing a multi‐scale approach to understanding the physics related to ion/proton‐beam cancer therapy and the calculation of the probability of DNA damage as a result of irradiation of tumours with energetic ions (up to 430 MeV/u). This approach is inclusive with respect to different scales, starting from the long scale, defined by the ion stopping, followed by a smaller scale, defined by secondary electrons and radicals, and ending with the shortest scale, defined by interactions of secondaries with the DNA. We present calculations of the probabilities of single and double strand breaks of DNA and suggest a way to further elaborate on such calculations.