Coherent X-radiation of relativistic electrons in a single crystal under asymmetric reflection conditions

Coherent X-radiation of a relativistic electron crossing a single crystal plate with constant speed is considered in the two-wave approximation of the dynamic diffraction theory [Z. Pinsker, Dynamical Scattering of X-rays in Crystals, Springer, Berlin, 1984] in a Laue geometry. Analytical expressions describing the spectral-angular distribution of parametric X-radiation (PXR) and diffracted transition radiation (DTR) formed on a system of parallel atomic planes situated at an arbitrary angle δ to the surface of the crystal plate (asymmetric reflection) are derived. The dependences of the PXR and DTR spectral-angular density and their interference with angle δ are studied.     

Transition radiation in the pre-wave zone for an oblique incidence of a particle on the flat target

Backward transition radiation of an electron with an arbitrary energy is studied when an ideally-conducting target is flat and its normal is not collinear with the particle velocity (oblique incidence). A model for radiation registered with a small flat detector placed at a finite distance (including the radiation in the pre-wave zone) in the vicinity of specular reflection direction is developed. Characteristics of the radiation in the far-field zone are in complete agreement with well-known results. The calculations for pre-wave zone show that the angular distribution of the radiation intensity is distorted compared to the far-field case, and the radiation asymmetry (having a place in the far-field for moderately relativistic energies) is also preserved in the pre-wave zone. Some numerical estimations of the radiation asymmetry at a finite distance are also given. The technique developed may be used for estimations of coherent transition radiation intensity in the mm-wavelength range.     

The Borrmann effect in parametric X-radiation under asymmetric reflection conditions

Parametric X-radiation (PXR) of a relativistic electron traversing a single crystal plate is considered in Laue geometry. The expressions describing spectral-angular distributions of PXR formed on the atomic planes situated under arbitrary angle δ to surface of the plate (asymmetric reflection) obtained on basis of two-wave approximation of dynamic diffraction theory are used for definition of the conditions of the most pronounced manifestation of the Borrmann effect (optimal value of angle δ) are clarified. This effect leads to considerable increase of the intensity of the quasi-monochromatic tuning source of coherent X-radiation built on basis of PXR.     

Resonant diffraction radiation from inclined gratings and bunch length measurements

A simple scheme for the measurement of sub-mm bunch lengths using coherent resonant diffraction radiation (CRDR) from a tilted grating is proposed. The CRDR spectral-angular characteristics have been calculated using an adapted Kirchhoff model, taking into account the pre-wave zone effect. It is shown that the latter leads to a distortion of the CRDR monochromaticity. Choosing the appropriate distance between grating and detector such that the pre-wave zone effect becomes negligible, it is possible to measure the CRDR yield in the sub-THz range by a broadband detector. While changing the grating inclination angle with respect to the beam axis, the CRDR line is shifted and it is possible to obtain information about the bunch length, measuring the signal ratio from two detectors located at fixed observation angles instead of complicated spectral measurements which rely on absolute values of the intensity.     

Focusing of transition radiation from a paraboloidal target

For the first time the focusing effect of coherent transition radiation generated in a paraboloidal target by electrons with energy of 6.1 MeV has been observed experimentally. A comparison of the angular distribution of detected radiation was made for flat and paraboloidal targets under focusing and defocusing conditions. Using paraboloidal targets one can considerably increase the spectral-angular density of the radiation in the paraboloidal focus without any additional optical devices.     

Imaging of optical diffraction radiation in pre-wave zone and spatial resolution of non-invasive beam size diagnostics

A simple model to evaluate the imaging shape of an optical diffraction radiation (ODR) source focused by a lens on a detector, taking into account the pre-wave zone effect has been developed. The characteristic size of an ODR image does not depend on the Lorentz-factor and is defined by the impact-parameter (minimal distance between a particle trajectory and ODR target edge) only. Using the ODR intensity component polarized parallel to the target edge it is possible to significantly improve the spatial resolution of an ODR beam profile monitor.     

First observation of coherent Smith-Purcell radiation in the highly relativistic regime

Coherent Smith-Purcell (SP) radiation has already been applied as a technique to measure the longitudinal bunch profile of charged particle beams in the low to intermediate energy range. However, with the advent of the International Linear Collider and the need to develop a non-invasive method of measuring the bunch profile, it has become necessary to carry out experiments at the highest possible energies. The paper summarizes some recent work at intermediate (45 MeV) energy and presents the first observations of SP radiation from a 28.5 GeV beam at SLAC. The experimental challenges and future possibilities of coherent Smith-Purcell radiation as a longitudinal bunch profile diagnostic tool are also discussed.     

Formation of spectral characteristics of channeling radiation from 800 to 2000 MeV electrons and positrons in a thin silicon crystal

The influence of the tails of particle trajectories on planar channeling radiation (CR) spectra from relativistic (800-2000 MeV) electrons and positrons in a thin silicon crystal is investigated. It is shown that the trajectory tails significantly change the CR spectra from electrons and positrons in specific parts of the spectra compared to calculations which do not take into account this effect.     

Angular distributions of diffracted X-ray radiation from channeled electrons in Si and LiF Crystals: Influence of energy levels band structure

It is shown that the band structure of the energy levels of planar channeled electrons qualitatively changes the angular distributions of X-rays emitted at Bragg angles.     

Applications of parametric X-rays for X-ray diffraction analysis

Until now parametric X-rays (PXR) have not had practical applications because of the lack of a modern compact accelerator providing the required beam current and consequently high X-ray photon flux. PXR sources even with the intensities achievable at present may be applied to a number of X-ray reflectometry and diffractometry measurements which are important for the characterization of crystals and multi-layer nanostructures. In the paper we present some proposals for possible PXR applications for a number of X-ray measurements based on the smooth energy tuning, high monochromaticity and directed emission of this radiation. The theoretical background and numerical evaluations for PXR applications for determining ingredient concentration in a solid solution in the range of anomalous dispersion of the defect atoms, determination of the phase structure of a crystal, and selective PXR action in organic compounds, important for medical and biological research, are considered.     

Local field effect in diffraction radiation from a periodical system of dielectric spheres

We present a theory of diffraction radiation from a two-dimensional system that consists of small spherical particles on a metal substrate. The interaction between a moving charge and single particles is described in the frames of local field theory. The local field effects are proved to lead to a sharp increase of the radiation intensity at certain frequencies, similar to the effect of giant Raman scattering. The case of nanoparticles is explored and it is shown that the possible enhancement of radiation can reach some thousands of times in the THz range. The Smith-Purcell effect is investigated for the case when the system of particles is periodically arranged.     

Cherenkov radiation from relativistic heavy ions taking account of their slowing down in radiator

Combining the SRIM’06 computer code and the formula for spectral-angular distribution of radiation from a particle moving in a medium in arbitrary trajectory, we studied numerically the structure of angular distributions of Cherenkov radiation from moderately relativistic heavy ions (RHI) taking into account the decrease of the ion velocity due to stopping in the radiator. The results obtained clearly show that the width and the fine structure of the Cherenkov radiation in the vicinity of the Cherenkov cone depend strongly on several factors, e.g. the mean and total energy losses, the radiator thickness, the square of the ion charge, the emission wavelength and the refractive index of the radiator material. This might be useful for experiments using RICH detectors for relativistic heavy ions.     

Quantum interference in grazing scattering of swift He atoms from LiF(0 0 1) surfaces: Surface eikonal approximation

This work deals with the interference effects recently observed in grazing collisions of few-keV atoms with insulator surfaces. The process is studied within a distorted-wave method, the surface eikonal approximation, based on the use of the eikonal wave function and involving axial channeled trajectories with different initial conditions. The theory is applied to helium atoms impinging on a LiF(0 0 1) surface along the 〈1 1 0〉 direction. The role played by the projectile polarization and the surface rumpling is investigated. We found that when both effects are included, the proposed eikonal approach provides angular projectile spectra in good agreement with the experimental findings.     

RBS/EBS/PIXE measurement of single-walled carbon nanotube modification by nitric acid purification treatment

Single-walled carbon nanotubes (SWCNTs) have many potential applications, including a number of promising biological applications. Nitric acid treatment solubilises CNTs by introducing functional groups, as well as removing amorphous carbon contaminants. Here, we report simultaneous RBS/EBS/PIXE measurements of nitric acid treated SWCNTs, focussing on the metal, nitrogen and oxygen content. We found that nitrogen remains constant in the samples despite washing and dialysis indicating it has either bound irreversibly via intercalation with the SWCNT and/or has been included in functional groups. We also found that the ratio between oxygen and platinum (catalyst) remains constant with treatment time (sampled at 2, 4, 6 h), indicating no more functional groups are made after 2 h exposure.     

Interference and shadow effects in the production of light by charged particles in optical fibers

A charged particle passing through or near a narrow optical fiber induces, by polarisation, coherent light guided by the fiber. In the limit of zero crossing angle, the radiation tends towards a Cherenkov radiation with a discrete spectrum, studied by different authors. If the particle crosses a bent fiber at regularly spaced points, interference gives quasi-monochromatic lines. If the particle passes near an end of the fiber, light is produced by the capture of virtual photons through the end face. An alternative way consists in sticking a metallic ball to the fiber: the passing particle induces plasmons which are then evacuated as light in the fiber. Interferences can occur between lights from several ends or balls. Applications of these various light signals to beam diagnostics are discussed. The shadow effect, which reduces the photon yield when the particle runs parallel to a row of balls, is pointed out and an upper bound -dE/dz≤C(Ze/b)² for the particle energy loss is conjectured (Ze is the particle charge, b the impact parameter and C a numerical constant). This bound should also apply to other kinds of light sources, in particular to Smith-Purcell radiation.     

Coherent pair production by energetic gamma rays incident on quasicrystals

We develop a Born-approximation theory of coherent pair production (CPP) of electrons by energetic gamma rays incident on an icosahedral quasicrystal, described by a schematic model (K model) that includes phonon and phason disorder. Our main result is a formula for the cross-section dσ_cpp/dε₊ for CPP, differential with respect to the positron energy ε₊ and of order α² in the fine-structure constant α ≈ 1/137, but which is otherwise exact. We discuss results of numerical calculations of dσ_cpp/dε₊ versus y = ε₊/k for gamma rays of energies k = 20 MeV, 200 MeV, and 3 GeV, incident on icosahedral Al-Mn-Si, described as a special case of the K model (vertex model). This consists in placing an Mn atom at each vertex of the relevant Ammann tiles. Our calculations include CPP of types A and B. Both types exhibit vertical intensity drops at irregularly distributed y-values, many of these drops being so large that they should be observable experimentally. They are analogous to the large intensity drops exhibited by coherent bremsstrahlung in quasicrystals. We predict that CPP drops also occur for realistic models of i-Al-Mn-Si at the same y-values as for the vertex model, but whose magnitudes may differ from those predicted by this model.     

Cell irradiation setup and dosimetry for radiobiological studies at ELBE

The radiation source ELBE delivers different types of secondary radiation, which is used for cell irradiation studies in radiobiological research. Thereby an important issue is the determination of the biological effectiveness of photon radiation as a function of photon energy by using low-energetic, monochromatic channeling radiation (10-100 keV) and high-energetic bremsstrahlung (up to 40 MV). Radiobiological studies at the research facility ELBE demand special technical and dosimetric prerequisites. Therefore, a cell irradiation system (CIS) has been designed, constructed and installed at the beam line. The CIS allows automatic irradiation of a larger cell sample number and the compensation of spatial inhomogeneity of the dose distribution within the beam spot. The recently introduced GafChromic^® EBT radiochromic film model has been used to verify the cell irradiation dose deposition achieving a dose uncertainty of <5%. Both, the installed cell irradiation system and the developed dosimetric procedure based on the use of the EBT film have been experimentally tested at ELBE. The biological effectiveness of 34 MV bremsstrahlung with respect to 200 kV X-rays from a conventional X-ray tube has been determined. An RBE value of 0.75 has been measured in good agreement with literature.     

The inverse problem for the dipole field

The inverse problem for an electromagnetic field produced by a dipole is solved. It is assumed that the field of an arbitrary changing dipole is known. Obtained formulae allow calculation of the position and dynamics of the dipole which produces the measured field. The derived results can be used in investigations on radiative process in solids caused by changing of the charge distribution. For example, generation of the electromagnetic field caused by oscillations of atoms or electron gas at the trace of a particle channeling in a crystal, or fields arising at solids cracking or dislocation formation - in any case when one is interested in the details of the dipole field source.