Spontaneous and stimulated undulator radiation by an ultra-relativistic positron channeling in a periodically bent crystal

We discuss the radiation generated by positrons channeling in a crystalline undulator. The undulator is produced by periodically bending a single crystal with an amplitude much larger than the interplanar spacing. Different approaches for bending the crystal are described and the restrictions on the parameters of the bending are discussed. We also present numeric calculations of the spontaneous emitted radiation and estimate the conditions for stimulated emission. Our investigations show that the proposed mechanism could be an interesting source for high energy photons and is worth to be studied experimentally.Comment: long version of our contribution to the 22nd International Free Electron Laser Conference, Durham, NC, USA, 13-18 August 2000, Reprinted from Nuclear Instruments and Methods A, Volume 474, 1--3, in press, with permission from Elsevier Science. http://www.elsevier.com/locate/nim

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 10 GeV 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 particle's 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.Comment: submitted for the proceedings of the International Workshop on ``Electron-Photon Interaction in Dense Media'' in Nor-Hamberd, Armenia, 2001; 10 pages, 9 figures, LaTe

Semi-spheroidal Quantum Harmonic Oscillator

A new single-particle shell model is derived by solving the Schr\"odinger equation for a semi-spheroidal potential well. Only the negative parity states of the $Z(z)$ component of the wave function are allowed, so that new magic numbers are obtained for oblate semi-spheroids, semi-sphere and prolate semi-spheroids. The semi-spherical magic numbers are identical with those obtained at the oblate spheroidal superdeformed shape: 2, 6, 14, 26, 44, 68, 100, 140, ... The superdeformed prolate magic numbers of the semi-spheroidal shape are identical with those obtained at the spherical shape of the spheroidal harmonic oscillator: 2, 8, 20, 40, 70, 112, 168 ...Comment: 4 pages, 3 figures, 1 tabl

Channeling of ultra-relativistic positrons in bent diamond crystals

Results of numerical simulations of channeling of ultra-relativistic positrons are reported for straight and uniformly bent diamond crystals. The projectile trajectories in a crystal are computed using a newly developed module of the MBN Explorer package which simulates classical trajectories in a crystalline medium by integrating the relativistic equations of motion with account for the interaction between the projectile and the crystal atoms. The Monte Carlo method is employed to sample the incoming positrons and to account for thermal vibrations of the crystal atoms. The channeling parameters and emission spectra of incident positrons with a projecti le energy of 855 MeV along C(110) crystallographic planes are calculated for different bending radii of the crystal. Two features of the emission spectrum associated with positron oscillations in a channel and synchrotron radiation are studied as a function of crystal curvature

Photoionization of multishell fullerenes studied by ab initio and model approaches*

11 págs.; 7 figs.; 1 tab. Part of the collections Topical Issue: Atomic Cluster Collisions (7th International Symposium)Photoionization of two buckyonions, C@C and C@C, is investigated by means of time-dependentdensity-functional theory (TDDFT). The TDDFT-based photoabsorption spectrum ofC@C, calculated in a broad photon energy range, resemblesthe sum of spectra of the two isolated fullerenes, thus illustrating the absence of strongplasmonic coupling between the fullerenes which was proposed earlier. The calculatedspectrum of the smaller buckyonion, C@C, differs significantly from the sum of the crosssections of the individual fullerenes because of strong geometrical distortion of thesystem. The contribution of collective electron excitations arising in individualfullerenes is evaluated by means of plasmon resonance approximation (PRA). An extension ofthe PRA formalism is presented, which allows for the study of collective electronexcitations in multishell fullerenes under photon impact. An advanced analysis ofphotoionization of buckyonions, performed using modern computational and analyticalapproaches, provides valuable information on the response of complex molecular systems tothe external electromagnetic field. c EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2016A.V. acknowledges the support by the FP7 Multi-ITN Project “ARGENT” (Grant agreement No. 608163). A.V.K. acknowledges the support from the Alexander von Humboldt FoundationPeer Reviewe

Toward the exploration of the NiTi phase diagram with a classical force field

10 pags., 7 figs., 3 tabs.Classical force fields, used for atomistic modeling of metal materials, are typically constructed to match low-temperature properties obtained in experiments or from quantum-level calculations. However, force fields can systematically fail to reproduce further fundamental parameters, such as the melting point. In this work, we present a modified force field for modeling metallic compounds, which has been implemented in the MBN Explorer software package. It is employed to simulate different regions of the composition-temperature-size phase diagram of nickel-titanium nanoalloys with particular focus on the evaluation of the melting point of NiTi (x = 0.45-0.55) systems. A near-equiatomic NiTi alloy is of paramount interest for biomedical and nanotechnology applications due to its shape memory behavior, but experiments and theory are inconsistent regarding its structural ground-state properties. The presented force field is used to predict the ground-state structure of an equiatomic NiTi nanoalloy. We observe that this compound does not possess the shape memory capacity because it stabilizes in the austenite instead of the required martensite crystalline phase. All results of our atomistic approach utilizing molecular dynamics and Monte Carlo techniques are in agreement with respective ab initio calculations and the available experimental findings. (Figure Presented). Copyright © 2016 American Chemical SocietyA.V.V. acknowledges the support by the European Commission through the FP7 Initial Training Network “ARGENT” (grant agreement no. 608163), and A.V.K. acknowledges the support from Alexander von Humboldt-Foundation. We thank the Center for Scientific Computing Frankfurt for providing the opportunity to perform calculations on the clusters FUCHS and LOEWE-CSC.Peer Reviewe

Radiation damage of biomolecular systems: Nano-scale insights into Ion-beam cancer therapy. 2nd Nano-IBCT conference

The second Nano-IBCT conference of the COST Action MP1002: Nanoscale Insights into Ion Beam Cancer Therapy was held in Sopot, Poland, from May 20th to May 24th, 2013. The Nano-IBCT action had been launched in December 2010 and brings together experts from different disciplines (physics, chemistry, biology, hadron-therapy centres, medical institutions), with specialisms in the radiation damage of biological matter. This meeting follows up the first one that was held in October, 2011 in Caen, France and we were pleased to see again so many of the participants of the previous meeting as well as to welcome some new colleagues joining and sharing their knowledge and expertise in this field

Development of collisional data base for elementary processes of electron scattering by atoms and molecules

We present a progress report on the development of the Belgrade electron/molecule data base which is hosted by The Institute of Physics, University of Belgrade and The Astronomical Observatory Belgrade. The data base has been developed under the standards of Virtual Atomic Molecular Data Centre (VAMDC) project which provides a common portal for several European data bases that maintain atomic and molecular data. The Belgrade data base (BEAMDB) covers collisional data of electron interactions with atoms and molecules in the form of differential (DCS) and integrated cross sections as well as energy loss spectra. The final goal of BEAMDB becoming both a node within the VAMDC consortium and within the radiation damage RADAM data base has been achieved