Showing papers on "Synchrotron radiation published in 1999"

Holotomography: Quantitative phase tomography with micrometer resolution using hard synchrotron radiation x rays.

Abstract: Because the refractive index for hard x rays is slightly different from unity, the optical phase of a beam is affected by transmission through an object. Phase images can be obtained with extreme instrumental simplicity by simple propagation provided the beam is coherent. But, unlike absorption, the phase is not simply related to image brightness. A holographic reconstruction procedure combining images taken at different distances from the specimen was developed. It results in quantitative phase mapping and, through association with three-dimensional reconstruction, in holotomography, the complete three-dimensional mapping of the density in a sample. This tool in the characterization of materials at the micrometer scale is uniquely suited to samples with low absorption contrast and radiation-sensitive systems.

Generation of Magnetic Fields in the Relativistic Shock of Gamma-Ray Burst Sources

Abstract: We show that the relativistic two-stream instability can naturally generate strong magnetic fields with 10-5-10-1 of the equipartition energy density, in the collisionless shocks of gamma-ray burst (GRB) sources. The generated fields are parallel to the shock front and fluctuate on the very short scale of the plasma skin depth. The synchrotron radiation emitted from the limb-brightened source image is linearly polarized in the radial direction relative to the source center. Although the net polarization vanishes under circular symmetry, GRB sources should exhibit polarization scintillations as their radio afterglow radiation gets scattered by the Galactic interstellar medium. Detection of polarization scintillations could therefore test the above mechanism for magnetic field generation.

Soft-x-ray polarimeter with multilayer optics: complete analysis of the polarization state of light.

Abstract: The design of a versatile high-precision eight-axis ultrahigh-vacuum-compatible polarimeter is presented. This multipurpose instrument can be used as a self-calibrating polarization detector for linearly and circularly polarized UV and soft-x-ray light. It can also be used for the characterization of reflection or transmission properties (reflectometer) or polarizing and phase-retarding properties (ellipsometer) of any optical element. The polarization properties of Mo/Si, Cr/C, Cr/Sc, and Ni/Ti multilayers used in this polarimeter as polarizers in transmission and as analyzers in reflection have been investigated theoretically and experimentally. In the soft-x-ray range, close to the 2p edges of Sc, Ti, and Cr, resonantly enhanced phase retardation of the transmission polarizers of as much as 18° has been measured. With these newly developed optical elements the complete polarization analysis of soft-x-ray synchrotron radiation can be extended to the water-window range from 300 to 600 eV.

Attenuation length of electrons in self-assembled monolayers of n-alkanethiols on gold

Abstract: The interaction of both photoelectrons and X-rays with self-assembled monolayers of n-alkanethiols on gold has been measured using synchrotron radiation as the photon source in the energy range 140−1100 eV. The attenuation length of photoelectrons (λ) was found to vary from a minimum of ∼5 A at an electron kinetic energy (E) of 100 eV up to ∼23 A at a kinetic energy of 1000 eV and can be described by the expression λ = 0.3E0.64 in the range 300−1000 eV. Exposure of the self-assembled monolayer to X-rays leads to fission of the C−S bond with a cross section of the order of 10-17 cm2 which diplays no apparent dependence on the incident photon energy.

Optical and Radio Polarimetry of the M87 Jet at 0.2" Resolution

Abstract: We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry observations of the M87 jet taken during 1994-1995. The angular resolution of both of these observations is ∼ 0.2 ′′ , which at the distance of M87 corresponds to 15 pc. Many knot regions are very highly polarized (∼ 40 −50%, approaching the theoretical maximum for optically thin synchrotron radiation), suggesting highly ordered magnetic fields. High degrees of polarization are also observed in interknot regions. The optical and radio polarization maps share many similarities, and in both, the magnetic field is largely parallel to the jet, except in the “shock-like” knot regions (parts of HST-1, A, and C), where it becomes perpendicular to the jet. We do observe significant differences between the radio and optical polarized structures, particularly for bright knots in the inner jet, giving us important insight into the radial structure of the jet. Unlike in the radio, the optical magnetic field position angle becomes perpendicular to the jet at the upstream ends of knots HST-1, D, E and F. Moreover, the optical polarization appears to decrease markedly at the position of the flux maxima in these knots. In contrast, the magnetic field position angle observed in the radio remains parallel to the jet in most of these regions, and the decreases in radio polarization are smaller. More minor differences are seen in other jet regions. Many of the differences between optical and radio polarimetry results can be explained in terms of a model whereby shocks occur in the jet interior, where higher-energy electrons are concentrated and dominate both polarized and unpolarized emissions in the optical, while the radio maps show strong contributions from lower-energy electrons in regions with B parallel, near the jet surface.

Photoemission electron microscope for the study of magnetic materials

Abstract: The design of a high resolution photoemission electron microscope (PEEM) for the study of magnetic materials is described. PEEM is based on imaging the photoemitted (secondary) electrons from a sample irradiated by x rays. This microscope is permanently installed at the Advanced Light Source at a bending magnet that delivers linearly polarized, and left and right circularly polarized radiation in the soft x-ray range. The microscope can utilize several contrast mechanisms to study the surface and subsurface properties of materials. A wide range of contrast mechanisms can be utilized with this instrument to form topographical, elemental, chemical, magnetic circular and linear dichroism, and polarization contrast high resolution images. The electron optical properties of the microscope are described, and some first results are presented.

Synchrotron radiation X-ray fluorescence at the LNLS : Beamline instrumentation and experiments

Abstract: The x-ray fluorescence heamline of the Laboratorio Nacional de Luz Sincrotron (LNLS) is described. The main optical component of the beamline is a silicon (111) channel-cut monochromator, which can tune energies between 3 and 14 keV. A general description of two experimental stations is given. Beam characterization was done by measuring experimental parameters such as vertical profile and monochromatic flux. These results show that the photon flux at 8 keV in an area of 20 mm 2 is 4.2 × 10 9 photons s -1 . The possibility of achieving fine tuning of energies (high resolution) was confirmed. This paper presents some original results derived from the commissioning of the beamline, such as a comparison of detection limits in different experimental conditions, and a novel mechanical system to align capillaries together with a semi-automatic adjustment procedure. So far, there have been several proposals to perform a variety of experiments at this beamline, covering different fields, such as physics, chemistry, geology and biology.

Performance of the grating-crystal monochromator of the ALOISA beamline at the Elettra Synchrotron

Abstract: The new beamline ALOISA, now operational at the Elettra Synchrotron, is designed for surface studies by means of several experimental techniques: surface x-ray diffraction and reflectivity, photoemission spectroscopy, photoelectron diffraction, e−-Auger coincidence spectroscopy. A new monochromator has been specifically designed and realized for this multipurpose beamline: it makes use of a channel-cut Si crystal dispersive element for the 3–8 keV range and of a plane mirror-plane grating element for the 200–2000 eV range. Both dispersive elements share the same optical system. In the low energy range (200–900 eV) the spectral resolving power exceeds 5000 while maintaining a throughput higher than 1010 photons/s/200 mA/0.02% BW. In the case of the N2 1s→π* and Ne 1s→3p transitions, the extremely high signal-to-noise ratio of the absorption spectra allowed a very accurate determination of the corresponding natural linewidth (116±2 and 250±10 eV, respectively). Moreover, the vibrational structure of the CO–...

Modelling of the non-thermal flares in the Galactic microquasar GRS 1915+105

Abstract: The microquasars recently discovered in our Galaxy offer a unique opportunity for a deep insight into the physical processes in relativistic jets observed in different source populations. We study the temporal and spectral evolution of the radio flares detected from the relativistic ejecta in the microquasar GRS 1915+105, and propose a model that suggests that these flares are caused by synchrotron radiation of relativistic electrons suffering radiative, adiabatic and energy-dependent escape losses in fast-expanding plasmoids (radio clouds). Analytical solutions to the kinetic equation for relativistic electrons in the expanding magnetized clouds are found, and the synchrotron radiation of these electrons is calculated. Detailed comparison of the calculated radio fluxes with the ones detected from the prominent flare of GRS 1915+105 during 1994 March/April provides conclusive information on the basic parameters in the ejecta, such as the absolute values and temporal evolution of the magnetic field, the speed of expansion, the rates of continuous injection of relativistic electrons into and their energy-dependent escape from the clouds, etc. The data from radio monitoring of the pair of resolved ejecta enable unambiguous determination of the parameters of the bulk motion of the counter-ejecta and the degree of asymmetry between them, and also contain important information on the prime energy source for accelerated electrons. These data allow us, in principle, to distinguish between the scenarios of bow-shock powered and relativistic magnetized wind powered plasmoids. Assuming that the electrons in the ejecta can be accelerated up to very high energies, we also calculate the fluxes of the synchrotron and inverse Compton components of the radiation that could be expected during the flares in the broad band from radio frequencies to very high-energy γ-rays.

Hard x-ray phase imaging using simple propagation of a coherent synchrotron radiation beam

Abstract: Particularly high coherence of the x-ray beam is associated, on the ID19 beamline at ESRF, with the small angular size of the source as seen from a point of the sample (0.1-1 µrad). This feature makes the imaging of phase objects extremely simple, by using a `propagation' technique. The physical principle involved is Fresnel diffraction. Phase imaging is being simultaneously developed as a technique and used as a tool to investigate light natural or artificial materials introducing phase variations across the transmitted x-ray beam. They include polymers, wood, crystals, alloys, composites or ceramics, exhibiting inclusions, holes, cracks, ... . `Tomographic' three-dimensional reconstruction can be performed with a filtered back-projection algorithm either on the images processed as in attenuation tomography, or on the phase maps retrieved from the images with a reconstruction procedure similar to that used for electron microscopy. The combination of diffraction (`topography') and Fresnel (`phase') imaging leads to new results.

Refraction-enhanced x-ray imaging of mouse lung using synchrotron radiation source.

Abstract: The low divergent x-ray beam from the third-generation synchrotron radiation source such as SPring-8 enables us to observe refraction of x rays that is in the range of microradians. Under an experimental condition for which ray optics is a good approximation, we found that the refraction produces a high-contrast projection image of a mouse when it was recorded at 6.5 m behind the specimen. Especially, the lung is visualized far better than with the conventional imaging which utilizes absorption of x rays. This is a promising new technique for the diagnosis of diseases in the human lung with a low radiation dose.

Multicomponent X-Ray Emissions from Regions near or on the Pulsar Surface

Abstract: We present a model of X-ray emission from rotation-powered pulsars, which in general consist of one nonthermal component, two hard thermal components, and one soft thermal component. The nonthermal X-rays come from synchrotron radiation of e ± pairs created in the strong magnetic field near the neutron star surface by curvature photons emitted by charged particles on their way from the outer gap to the neutron star surface. The first hard thermal X-ray component results from polar-cap heating by the return current in the polar gap. The second hard thermal X-ray component results from polar-cap heating by the return particles from the outer gap. Because of cyclotron resonance scattering, most of the hard thermal X-rays will be effectively reflected back to the stellar surface and eventually reemitted as soft thermal X-rays. However, some of the hard thermal X-rays can still escape along the open magnetic field lines, where the e+/e− pair density is low. Furthermore, the characteristic blackbody temperatures of the two hard X-ray components emitted from the polar-cap area inside the polar gap and the polar-cap area defined by the footprints of the outer-gap magnetic field lines are strongly affected by the surface magnetic field, which can be much larger than the dipolar field. In fact, the strong surface magnetic field can explain why the effective blackbody radiation area is nearly 2 orders of magnitude larger than that deduced from the dipolar field for young pulsars (2 orders of magnitude less for old pulsars). Our model indicates how several possible X-ray components may be observed, depending on the magnetic inclination angle and viewing angle. Using the expected X-ray luminosity and spectra, we explain the observed X-ray spectra from pulsars such as Geminga, PSR B1055-52, PSR B0656+14, and PSR B1929+10.

Hard x-ray quantitative non-interferometric phase-contrast microscopy

Abstract: We report the results of quantitative phase-contrast imaging experiments using synchrotron radiation, in-line imaging geometry and a non-interferometric phase retrieval technique. This quantitative imaging method is fast, simple, robust, does not require sophisticated x-ray optical elements and can potentially provide submicron spatial resolution over a field of view of the order of centimetres. In the present experiment a spatial resolution of approximately 0.8 m has been achieved in images of a polystyrene sphere using 19.6 keV x-rays. We demonstrate that appropriate processing of phase-contrast images obtained in the in-line geometry can reveal important new information about the internal structure of weakly absorbing organic samples. We believe that this technique will also be useful in phase-contrast tomography.

Coherent γ-ray optics

Abstract: With the advent of high brightness synchrotron radiation sources, an important new field has been opened up involving coherent nuclear excitations induced by synchrotron radiation pulses traversing a piece of matter. We review the theory of coherent resonant γ-ray optics, including some of the interesting new phenomena which occur when systems of nuclei are excited by synchrotron radiation pulses, such as the creation of nuclear exciton states, superradiant and subradiant decay, spatially coherent quantum beats, and temporal Pendellosung. We also discuss the relation between the nuclear exciton states and multi-photon Dicke superradiance and γ-ray lasers, and comment on neutron phasors and neutron superradiance in resonant neutron optics. The interesting features of coherent enhancement, superradiant decay, and dynamical beats are discussed from the fundamental perspective of the radiative normal modes of a system of nuclear resonators.

Characterization of nanocrystalline γ-Fe2O3 with synchrotron radiation techniques

Abstract: A variety of synchrotron radiation techniques like soft X-ray magnetic circular dichroism, near-edge as well as extended X-ray absorption fine structure, and synchrotron radiation X-ray diffraction have been used to characterise the crystal and electronic structure as well as the magnetic properties of superparamagnetic nanocrystalline samples with the nominal composition γ-Fe2O3. The results are compared with data from bulk reference samples like Fe3O4, Fe3—δO4, α-Fe2O3, and FeO.

Strongly Polarized Optical Afterglows of Gamma-Ray Bursts.

Abstract: The optical afterglows of the gamma-ray bursts can be strongly polarized, in principle up to tens of percents, if (1) the afterglow is synchrotron radiation from an ultrarelativistic blast, (2) the blast is beamed during the afterglow phase, i.e., the shock propagates within a narrow jet, (3) we observe at the right time from the right viewing angle, and (4) magnetic fields parallel and perpendicular to the jet have different proper strengths.

Domains in the low-temperature phase of magnetite from synchrotron-radiation x-ray topographs

Abstract: Domains in the low-temperature, ferrimagnetic, and ferroelectric, phase of magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ were investigated in the bulk, via their ferroelastic distortion, by means of synchrotron radiation x-ray topography, complemented by Nomarski optical microscopy on the surface After cooling through the Verwey transition under a magnetic field parallel to [001], the images show a hierarchy of domains Domains associated with a small difference in lattice distortion provide a clear indication that the symmetry is triclinic They appear as a substructure of monoclinic domains separated by walls creating no long-range stress The domain structure deduced from the observation, under the experimentally self-tested assumption that the walls involved are stress free, is favorable from the elastic, magnetostatic, and electrostatic points of view

Pure nuclear bragg reflection of a periodic 56fe/57fe multilayer

Abstract: Grazing incidence nuclear multilayer diffraction of synchrotron radiation from a periodic stack of alternating 56Fe and 57Fe layers was observed. Resonant layer fraction, substrate size, flatness, and surface roughness limits were optimized by previous simulations. The isotopic multilayer (ML) sample of float glass/57Fe(2.25 nm)/[56Fe(2.25 nm)/57Fe(2.25 nm)]×15/Al(9.0 nm) nominal composition was prepared by molecular beam epitaxy at room temperature. Purity structure and lateral homogenity of the isotopic ML film was characterized by magnetometry, Auger electron, Rutherford backscattering, and conversion electron Mossbauer spectroscopies. The isotopic ML structure was investigated by neutron and synchrotron Mossbauer reflectometry. Surface roughness of about 1 nm of the flat substrate (curvature radius >57 m) was measured by scanning tunneling microscopy and profilometry. A pure nuclear Bragg peak appeared in synchrotron Mossbauer reflectometry at the angle expected from neutron reflectometry while no ele...

Quantitative texture analysis of small domains with synchrotron radiation X-rays

Abstract: Quantitative analysis of crystallographic preferred orientation (texture) of very small volumes in fine-grained polycrystalline materials has been carried out with a monochromatic X-ray microbeam (≤30 µm) at the microfocus beamline of the European Synchrotron Radiation Facility (ESRF). The experimental procedure is described and illustrated with textures of rolled aluminium, aluminium and steel wires, polymer fibers and natural bone material (apatite).

Photoelectron bremsstrahlung spectrum in synchrotron radiation excited total reflection x-ray fluorescence analysis of silicon wafers

Abstract: When synchrotron radiation is used as an excitation source, the total reflection x-ray fluorescence analysis of surface contamination on silicon wafer has an extremely low background intensity that determines the minimum detection limit. In this article, the background spectrum originating from the photoelectron bremsstrahlung is calculated using the Monte Carlo method. The doubly differential electron bremsstrahlung cross sections obtained from the Born approximation modified by the Elwert factor and with the use of the form factor approach for screening are used instead of empirical formulas. In addition to the bremsstrahlung spectrum produced from the silicon wafer, the bremsstrahlung intensity that photoelectrons, which escape from the silicon wafer, produce in the filter attached to the detector is also calculated in accordance with the usual synchrotron radiation excited total reflection x-ray fluorescence experimental conditions. The calculated photoelectron bremsstrahlung spectra are compared with...

Imaging of magnetic structures by photoemission electron microscopy

Abstract: Photoemission electron microscopy (PEEM) has proven to be a powerful analytical tool in surface science. In this contribution, a status report is given on the application of the PEEM technique in the investigation of surface and interface magnetism. Owing to its fast parallel image acquisition and its wide zoom range, allowing fields of view from almost 1 mm down to a few micrometres, combined with a high base resolution of the order of 20 nm, the method offers a unique access to many aspects in surface and thin-film magnetism on the mesoscopic length scale. There are three basically different modes of magnetic imaging using PEEM. The first one exploits the magnetic x-ray circular dichroism (MXCD) for contrast formation. It offers the important advantage of selecting the magnetic contrast of a certain element via the corresponding x-ray absorption edges using a tuneable x-ray source such as synchrotron radiation. This mode gives access to magnetic structures and coupling phenomena with a sensitivity in the submonolayer range and the capability to image the signal of buried layers with an information depth up to more than 5 nm. The two other modes work with simple UV light sources and are therefore highly attractive for standard laboratory applications. The magnetic stray-field-induced changes of the electron trajectories close to the sample surface lead to a Lorentz-type contrast. A third type of contrast arises as a consequence of the Kerr rotation of the dielectric vector inside a magnetic material, a phenomenon which is also responsible for the well known magneto-optical Kerr effect. Examples and typical applications of magnetic imaging using PEEM are discussed.

Optical constants of amorphous for photons in the range of 60-3000 eV

Abstract: Optical constants of amorphous have been calculated by the Kramers-Kronig analysis in the continuous spectral region including , O K and Si K absorption edges from the reflection spectra measured using synchrotron radiation. It is the first experiment carried out in such a wide energy region with the same sample. It has been found that for extrapolation of the experimental curve in the high energy region, where the photon energy exceeds the ionization potential of the Si K edge, the relation can be used for energies such that at small grazing angles.

New high temperature furnace for structure refinement by powder diffraction in controlled atmospheres using synchrotron radiation

Abstract: A low thermal gradient furnace design is described which utilizes Debye–Scherrer geometry for performing high temperature x-ray powder diffraction with synchrotron radiation at medium and high energies (35–100 keV). The furnace has a maximum operating temperature of 1800 K with a variety of atmospheres including oxidizing, inert, and reducing. The capability for sample rotation, to ensure powder averaging, has been built into the design without compromising thermal stability or atmosphere control. The ability to perform high-resolution Rietveld refinement on data obtained at high temperatures has been demonstrated, and data collected on standard Al2O3 powder is presented. Time-resolved data on the orthorhombic to rhombohedral solid state phase transformation of SrCO3 is demonstrated using image plates. Rietveld refinable spectra, collected in as little as 8 s, opens the possibility of performing time-resolved structural refinements of phase transformations.

Nuclear resonant scattering of synchrotron radiation

Abstract: Nuclear resonant scattering of synchrotron radiation from the 25.6 keV level of 161 Dy is studied with two techniques: nuclear forward scattering (NFS) and nuclear incoherent scattering (NIS). NFS time spectra of Dy metal are measured at temperatures ranging from 15 K to 80 K. They reveal electron spin relaxation in ferromagnetic dysprosium with long relaxation times of 10-30 ns. NIS energy spectra of Dy2O3 at room temperature are measured with meV resolution. Scattering accompanied by phonon excitation is observed.

Coherent pulse propagation through resonant media

Abstract: Resonant pulse propagation (RPP) is reviewed with special emphasis on the propagation of synchrotron radiation (SR) pulses through nuclear single-resonance media. The most remarkable feature in the time evolution of RPP is the dynamical beat (DB), a pronounced modulation with periods increasing with time and decreasing with increasing sample thickness. A comparison of RPP at α-wavelengths (SR and Mossbauer radiation) with RPP in the infrared and visible regimes in case of molecular, atomic and excitonic resonances reveals an astonishing universality of the observed phenomena. The DB is described within the double-hump picture and the group-velocity picture, and is finally attributed to the energy exchange between radiation field and oscillator system in multiple scattering.