Hard X-ray Region Research Articles

Brilliant opportunities across the spectrum

Third generation synchrotron light sources provide stable, tuneable light of energy up to the hard X-ray region. The gain of a trillion in brightness as compared to a conventional laboratory X-ray source transforms the opportunities for establishing structure-function relationships. The light may be quasi-continuous or pulsed, have controllable polarisation and have coherence lengths larger than the sample size. The high brightness provides a basis for adding time and spatial resolution to X-ray scattering and spectroscopy. It may also be used to identify very specific information about the magnetic properties of atoms within materials, element specific vibrations, and local structural descriptions identified with chemical speciation. More demanding scattering and diffraction problems can be solved such as weakly scattering materials, large unit cells and structural entities. The high collimation of the source also provides enhanced spectroscopic and diffraction resolution that gives more insight into molecular, extended and supramolecular structures. The length scales can be bridged from the atomic up to that of visible light microscopy and buried features within materials can be observed with the appropriate energy. With an increased emphasis on ease of use, such capabilities are open to exploitation for chemical challenges.

High energy X-ray observations of NGC 5506

We have a continuing programme of spectral and temporal studies of a variety of galactic and extragalactic sources in the 20–200 keV band using a large area scintillation counter telescope. The observations of NGC 5506, a Seyfert type 2 galaxy, taken on three occasions during the 1998–2000 period suggest that not only source luminosity varies by a factor of ∼10, but also there is a marked change in the spectral index. The data is consistent with a systematic long-term periodic variation. Our analysis of the all sky monitor light curve suggests a 41.12-day period in the source intensity. This paper presents the spectral parameters in the hard X-ray region and proposes that the observed behaviour of the source can be explained on the basis of a binary black hole model for the core activity of the source.

Kirkpatrick-Baez type x-ray focusing mirror fabricated by the bent-polishing method

Microfocusing optics with total reflection mirrors has been constructed, and a performance test in the hard x-ray region has been carried out. The material of the mirror is fused quartz, and parabolic surface figures are fabricated by the bent-polishing method. The reflective surface is coated with platinum. By configuring two parabolic mirrors in the Kirkpatrick-Baez optics, a focused beam size of 150nm in the vertical direction ×110nm in the horizontal direction has been achieved at an x-ray energy of 12keV. A focused beam size of smaller than 300nm is obtained within an x-ray energy range of 8–28keV. In a performance test of the scanning microscope, fine periodic structures with a line pitch of 140nm are clearly resolved. Diffraction limit and modulation transfer function of these focusing optics are discussed.

Hard x-ray holographic microscopy using refractive prism and Fresnel zone plate objective

An optics for hard x-ray holographic microscopy has been developed and preliminary experiments have been done at SPring-8 undulator beamline 20XU. The optical system consists of an x-ray objective lens (Fresnel zone plate) and a wave front-division-type interferometer with prism optics. The illuminating x-ray beam is coherent with parallel radiation, and the spatially coherent area is much larger than the aperture of the objective lens. The refractive prism is placed behind the back focal plane of the objective lens in order to configure the wavefront-dividing interferometer. Half of the illuminating radiation is used for illuminating an object, and the other half is used for forming a reference wave. The magnified image of the object is generated at an image plane, and the reference wave is superimposed on the magnified image of the object. The recorded interferogram includes both amplitude and phase information of the object. The spatial resolution is determined by the numerical aperture of the objective lens. Therefore, in principle, this method enables holographic imaging at nanometer scale to be carried out in the hard x-ray region.

Luminescence of CaWO4, CaMoO4, and ZnWO4 scintillating crystals under different excitations

The luminescence spectra of CaWO4, CaMoO4, and ZnWO4 scintillating crystals were investigated in the temperature range 8–400K. The excitation photon energy was varied from the ultraviolet (4.5eV) to the hard x-ray region (35keV). It is found that as the excitation energy decreases the relative intensity of the low-energy luminescence band, attributed to the extrinsic emission of defect centers in CaWO4 and CaMoO4 crystals, increases. This observation is interpreted in terms of the total absorption of incident radiation, i.e., the variation of the mean penetration depth of the photons with their energy. It indicates that the centers responsible for the extrinsic emission in the crystals with scheelite structure are mainly localized in a thin (∼100nm) surface layer. On the other hand no noticeable changes with the excitation energy were found in the emission spectra of ZnWO4 crystals with wolframite structure. The possible implication of this finding is discussed. The light yield of the crystals is compared at low temperature using monochromatic x-ray excitation and it is shown that ZnWO4 has ∼10% higher light yield than CaWO4, while this parameter has a factor of 4 lower in CaMoO4.

Performance Test of Fresnel Zone Plate with 50 nm Outermost Zone Width in Hard X-ray Region

A microfocusing experiment for hard X-rays has been performed to evaluate the performance of Fresnel zone plate optics. A tantalum Fresnel zone plate with an outermost zone width of 50 nm and a thickness of 0.5 µm has been fabricated by electron-beam lithography. The focused beam size measured by a knife-edge scan is 58 nm in full-width at half-maximum for the first-order diffraction at an X-ray energy of 8 keV. It can be concluded that this zone plate has nearly diffraction-limited resolution in the hard X-ray region. The measured diffraction efficiency is 5% at 8 keV. The spot size using the third-order focus of the zone plate is measured to be approximately 30 nm.

Application of CdTe for the NeXT mission

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and γ -ray detection. The high-atomic number of the materials ( Z Cd = 48 , Z Te = 52 ) gives a high quantum efficiency in comparison with Si. The large band-gap energy ( E g = 1.5 eV ) allows to operate the detector at room temperature. Based on recent achievements in high-resolution CdTe detectors, in the technology of ASICs and in bump-bonding, we have proposed the novel hard X-ray and γ -ray detectors for the NeXT mission in Japan. The high-energy response of the super mirror onboard NeXT will enable us to perform the first sensitive imaging observations up to 80 keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector, will provide spectra and images in the wide energy range from 0.5 to 80 keV. In the soft γ -ray band up to ∼ 1 MeV , a narrow field-of-view Compton γ -ray telescope utilizing several tens of layers of thin Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The continuum sensitivity will reach several × 10 - 8 photons - 1 keV - 1 cm - 2 in the hard X-ray region and a few × 10 - 7 photons - 1 keV - 1 cm - 2 in the soft γ -ray region.

Temperature dependence of x-ray magnetic circular dichroism in rare earth iron garnets (rare earth = Gd, Dy and Sm)

We studied the feasibility of magnetic circular dichroism (MCD) in the hard x-ray region for determining 4f magnetic moments by measuring the temperature dependence of MCD spectra at rare earth L2-edge in systems of rare earth iron garnets (R-IG) for R = Gd, Dy and Sm. Two components arising from the magnetization of the Gd and Fe sublattices, which were combined into the MCD spectrum of Gd-IG, were separatively deduced. For Dy- and Sm-IG, we successfully obtained the temperature dependence of the MCD component that comes from only R sublattice magnetization under the assumption that the MCD spectra of the series of R-IG contain the same contribution of the iron sublattice.

Highly Charged Ion Research at the Livermore Electron Beam Ion Traps

Spectroscopy performed with the three Livermore electron beam ion traps is reviewed, which is continuing and complementing the innumerable contributions to atomic physics provided over the years by heavy-ion accelerators. Numerous spectrometers were developed that cover the spectral bands from the visible to the hard x-ray region. These enabled exhaustive line surveys useful for x-ray astrophysics and for systematic studies along iso-electronic sequences, such as the 4s–4p, 3s–3p, and 2s–2p transitions in ions of the Cu-I, Na-I, and Li-I sequences useful for studying QED and correlation effects as well as for precise determinations of atomic-nuclear interactions. They also enabled measurements of radiative transition probabilities of very long-lived (milli- and microseconds) and very short-live (femtosecond) levels. Because line excitation processes can be controlled by choice of the electron beam energy, the observed line intensities are used to infer cross sections for electron-impact excitation, dielectronic recombination, resonance excitation, and innershell ionization. These capabilities have recently been expanded to simulate x-ray emission from comets by charge exchange. Specific contributions to basic atomic physics, nuclear physics, and high-temperature diagnostics are illustrated.

Superbend upgrade on the Advanced Light Source

The Advanced Light Source (ALS) is a third generation synchrotron light source at Lawrence Berkeley National Laboratory (LBNL). There was an increasing demand for additional high brightness hard X-ray beamlines in the 7–40 keV range, so in August 2001, three 1.3 T normal conducting bending magnets were removed from the storage ring and replaced with 5 T superconducting magnets (Superbends). The radiation produced by these Superbends is an order of magnitude higher in photon brightness and flux at 12 keV, making them excellent sources of hard X-rays for protein crystallography and other hard X-ray applications. The Superbends did not compromise the performance of the facility in the VUV and soft X-ray regions of the spectrum. The Superbends will eventually feed 12 new beam lines, greatly enhancing the facility's capability and capacity in the hard X-ray region. The Superbend project is the biggest upgrade since the ALS storage ring was commissioned in 1993. In this paper we present an overview of the Superbend project, its challenges and the resulting impact on the ALS.

Evaluation of a cadmium–zinc–telluride focal plane detector for hard X-ray astronomy

Because the Earth has a protective atmosphere that is opaque to many wavelengths, X-ray astronomy cannot be performed from the ground. It was not until the 1960s that astronomers had the technological capability to put their experiments at high enough altitudes to directly observe cosmic X-rays (Giacconi et al., 1962). Since that time a multitude of high-altitude balloons, rockets, and satellites have been launched to study the Universe in this wavelength region. Of the 108 missions flown, the majority of them concentrate on studying the Universe below 20 keV (Slavis, 2001). As a result, the hard X-ray region (20–100 keV) is still relatively unexplored. One instrument that will study the Universe in the hard X-ray region is the high energy replicated optics (HERO) balloon borne telescope being developed at Marshall Space Flight Center and the National Space Science and Technology Center. The HERO telescope consists of nested grazing incidence optics and several focal plane detectors. The full balloon payload will consist of 16 mirror modules each containing 15 nested shells (Ramsey et al., 2004). Each focal plane detector must exhibit a high photopeak quantum efficiency, good energy resolution, and good spatial resolution (B200mm). To sample the full mirror response the focal plane detector must have 128 128 pixels (Ramsey, 2001), although the sensitive portion of the mirrors’ response can be covered by 64 64 pixels. These focal plane requirements can be optimally met with a many-pixel cadmium–zinc–telluride (CdZnTe) array.

X-ray imaging microscopy using Fresnel zone plate objective and quasimonochromatic undulator radiation

An imaging microscopy experiment in a hard x-ray region has been performed at the undulator beamline 40XU of SPring-8. The helical undulator at the BL40XU provides quasimonochromatic radiation with a bandwidth of 1.2% at 8.34 keV. A Fresnel zone plate with an outermost zone width of 0.25 μm and a Fresnel zone number of 100 is used as an objective. The natural bandwidth of the undulator radiation of BL40XU is nearly equal to the monochromaticity required for the zone plate objective. Therefore, compared with conventional beamlines with a crystal monochromator, the available photon flux is higher by two orders. Fine structure of test patterns with 0.25 μm line and space was clearly observed at an x-ray energy of 8.34 keV within an exposure time of 1.5 ms.

The use of the X-ray absorption near edge spectrum in materials characterisation

The increasing availability and use of synchrotron radiation has opened up new horizons in materials research. These sources offer very high brightness photon probes in an unparalleled wavelength range from the UV to the hard X-ray region. Although the medium to hard energy extended X-ray absorption fine structure (EXAFS) has been widely used in materials characterisation, the near edge absorption spectrum (XANES), especially in the soft X-ray region below 2 kV, has been exploited to only a limited degree. This relates to both the difficulties and limited availability of soft X-ray monochromators and the complexity of the spectra relative to the EXAFS region. We have used XANES in studies of semiconductor oxide and nitride materials, the speciation of sulfur in industrial smelting anodes and the development of oxide based electrode materials for Li ion batteries. Although of limited quantitative applicability, the chemical detail available in these spectra is considerably superior to that in the X-ray photoelectron (XPS) spectrum and demonstrates the considerable power of the method. � 2003 Elsevier B.V. All rights reserved.

Investigating Actinyl Oxo Cations by X‐Ray Absorption Spectroscopy

AbstractFor Abstract see ChemInform Abstract in Full Text.

Hard x-ray imaging using free-standing spherically bent crystals

We report the attempt to prepare free-standing spherically bent Si crystals with radii of curvature as small as 30 cm and large uniform working areas up to several cm2. We also report on the use of these crystals in the Laue geometry to record two-dimensional high spatial resolution x-ray images in the hard x-ray region (λ∼0.5 Å). We discuss how these bent crystals were made, and show how these spherically bent crystals were used in the Laue geometry to obtain hard x-ray images of test objects at the silver K-alpha wavelength at two different magnifications. The mean spatial resolution of this x-ray imaging scheme, determined from the recorded image traces, was found to be better than 10 μm over a field of view of 1.5×1.5 mm. Contributions of possible focusing mechanisms (dynamical and polychromatic) are discussed. Additional theoretical understanding of how such a scheme is working is needed.

Antiparallel ruthenium coupling in doped La1.2Sr1.8Mn2−xRuxO7

X-ray magnetic circular dichroism measurements in the hard and soft x-ray region have been performed at Mn K and L2,3 edges of the ruthenium (with x=0.05, 0.1 and 0.5) doped Ruddlesden–Popper-phase La1.2Sr1.8Mn2−xRuxO7. In combination with magnetometry, clear evidence of unexpected antiferromagnetic sublattice coupling between manganese and ruthenium was found. A simple coupling model of Mn+3/Mn+4 and Ru+3/Ru+4 ions in the high-spin state is proposed which explains the antiparallel alignment and the technological interesting increase of the Curie temperature.

The Goos-Hänchen effect at Bragg diffraction.

The strong incident-angle dependence of the phase of complex reflectivity causes a shift of the reflected beam from the geometrically expected path. This effect, known as the Goos-Hänchen effect in the visible region, was observed for Bragg-case diffraction in the hard X-ray region. The shift was found to be in good agreement with the theory.

ASCAView of the Supernova Remnant γ Cygni (G78.2+2.1): Bremsstrahlung X‐Ray Spectrum from Loss‐flattened Electron Distribution

With the ASCA X-ray satellite, we perform spatial and spectral studies of the shell-type supernova remnant (SNR) γ Cygni that is associated with the brightest EGRET unidentified source 3EG J2020+4017. At energies below 3 keV, the bulk of the X-ray flux from the remnant is well described by the emission of thermal plasma with characteristic temperature kTe 0.5-0.9 keV. In addition to this thermal emission, we found an extremely hard X-ray component from several clumps localized in the northern part of the remnant. This component, which dominates the X-ray emission from the remnant above 4 keV, is described by a power law with a photon index of Γ 0.8-1.5. Both the absolute flux and the spectral shape of the nonthermal X-rays cannot be explained by the synchrotron or inverse-Compton mechanisms. We argue that the unusually hard X-ray spectrum can be interpreted naturally in terms of nonthermal bremsstrahlung from Coulomb-loss-flattened electron distribution in dense environs with the gas density about 10-100 cm-3. For a given spectrum of the electron population, the ratio of the bremsstrahlung X- and γ-ray fluxes depends on the position of the in the electron spectrum. Formally, the entire high-energy γ-ray flux detected by EGRET from γ Cygni could originate in the hard X-ray regions. However, it is more likely that the bulk of γ-rays detected by EGRET comes from the radio-bright and X-ray-dim cloud at the southeast, where the very dense gas and strong magnetic field would illuminate the cloud in the radio and γ-ray bands but suppress the bremsstrahlung X-ray emission due to the shift of the Coulomb break in the electron spectrum toward higher energies.

Bulk-sensitive XAS characterization of light elements: from X-ray Raman scattering to X-ray Raman spectroscopy

X-Ray absorption spectroscopy (XAS) is a powerful tool for element-specific characterization of local structure and chemistry. Although application of XAS in the hard X-ray region is now routine, the soft X-ray region (containing light-element K-edges) presents a number of experimental problems. Most of the difficulties, including surface sensitivity, restricted sample environments, and radiation damage, stem from the submicron path lengths of soft X-rays and/or electrons. X-Ray Raman scattering (XRS) provides a means for obtaining the information content of soft X-ray spectra while maintaining the experimental benefits of hard X-ray techniques. In the XRS process, an incident photon is inelastically scattered and part of its energy is transferred to excite an inner shell electron into an unoccupied state. Under the dipole approximation, the resulting features are identical to the corresponding XAS spectrum. In the past, the extremely low cross-section of XRS has made this technique impractical, but intense new X-ray facilities and improvements in X-ray optics have helped to put XRS on the brink of becoming a routine spectroscopic tool. At present, high-quality X-ray Raman spectra can be obtained in minutes to hours. X-Ray Raman spectroscopy thus represents a hard X-ray alternative to conventional XAS techniques in the study of systems with light elements, including C, N and O. In this review we describe the technique, present examples of recent work, and discuss the prospects for the future.

A unique polarized x-ray facility at the Advanced Photon Source

To use the unique element-specific nature of polarized x-ray techniques to study a wide variety of problems related to magnetic materials, we have developed a dual-branch sector that simultaneously provides both hard and soft x-ray capabilities. This facility, which is located in sector 4, is equipped with two different insertion devices providing photons in both the intermediate (0.5–3 keV) and hard x-ray regions (3–100 keV). This facility is designed to allow the simultaneous branching of two undulator beams generated in the same straight section of the ring.