A. Snigirev, I. Snigireva, V. Kohn, S. Kuznetsov, I. Schelokov

Rev. Sci. Instrum. Volume: 66, Issue: 12 Pages: 5486-5492 Published: 1995

Coherent properties of the x-ray beam delivered at the ESRF allow the observation of very weak perturbations of the wave front, resulting in the phase contrast. A straightforward experimental setup for phase contrast imaging is proposed and used to record holographic images from organic samples of 10-100 µm at energy 10-50 keV with the contrast up to 50%-100%. The theory of phase contrast imaging is considered and some theoretical estimations are made to reveal the performance of the proposed technique in terms of resolution, sensitivity, geometrical requirements, and energy range applicability. It is found that for carbon-based fibers a detectable size with 2% contrast is 0.1 µm for 10 keV and -1 µm for 100 keV. It is demonstrated that the fine interference structure of the image is very sensitive to the shape, density variation, and internal structure of the sample. Some prospects for the practical use and future development of the new coherent techniques such as phase contrast microscopy, microtomography, holography, and interferometry at high energies are also discussed.

Cited by: 581

A. Snigirev, V. Kohn, I. Snigireva, B. Lengeler

Nature Volume: 384 Issue: 6604 Pages: 49-51 Published: 1996

The development of techniques for focusing X-rays has occupied physicists for more than a century. Refractive lenses, which are used extensively in visible-light optics, are generally considered inappropriate for focusing X-rays, because refraction effects are extremely small and absorption is strong. This has lead to the development of alternative approaches based on bent crystals and X-ray mirrors, Fresnel and Bragg-Fresnel zone plates, and capillary optics (Kumakhov lenses). Here we describe a simple procedure for fabricating refractive lenses that are effective for focusing of X-rays in the energy range 5-40 keV. The problems associated with absorption are minimized by fabricating the lenses from low-atomic-weight materials. Refraction of X-rays by one such lens is still extremely small, but a compound lens (consisting of tens or hundreds of individual lenses arranged in a linear array) can readily focus X-rays in one or two dimensions. We have fabricated a compound lens by drilling 30 closely spaced holes (each having a radius of 0.3 mm) in an aluminium block, and we demonstrate its effectiveness by focusing a 14-keV X-ray beam to a spot size of 8 µm.

Cited by: 453

Yu. Kagan, A. M. Afanasev, V. G. Kohn

Journ. of Phys. C: Solid State Phys. Volume: 12 Issue: 3 Pages: 615-631 Published: 1979

The time dependence of delayed radiation is investigated when a pulse of synchrotron radiation undergoes resonance Bragg scattering by the nuclei in a crystal that have an isomeric level with a Mossbauer transition. Instead of the usual exponential law exp(-t/ tau 0) characteristic of an isolated nucleus, a time dependence in the form ( tau 0/t)2 exp(-t/ tau 0) was discovered at small deviations alpha from the Bragg angle. The acceleration of the decay is connected with the collective nature of the excitation of the nuclei in the crystal-the nuclear exciton formation. The exponential law remains at large alpha but the intensity decreases as 1/ alpha 2. In the case of large divergence of the incident beam the law ( tau 0/t) exp(-t/ tau 0) is obeyed. The frequency distribution of the reflected pulse and the possibility of formation of a resonance structure with sufficient resolution are analysed in detail.

Cited by: 161

A. M. Afanasev, V. G. Kohn

Acta Cryst. Volume: A27 Issue: 5 Pages: 421-430 Published: 1971

Cited by: 102

C. Raven, A. Snigirev, I. Snigireva, P. Spanne, A. Souvorov, V. Kohn

Appl. Phys. Lett. Volume: 69 Issue: 13 Pages: 1826-1828 Published: 1996

Cross-sectional information on low electron density materials can be obtained by probing a sample with a 60 keV coherent synchrotron x-ray beam in an in-line holography setup. Such objects are practically transparent to high energy x-rays and create a phase shift of the wave front only. Images of a 100 µm diameter boron fiber were recorded in the extreme near field region, where contrast occurs only at interfaces between regions with different decrements of refractive index. Theoretical simulations are in good agreement with the measured intensities. In a tomographic reconstruction the 15 µm diameter core of the fiber is clearly visible, demonstrating the possibility of reconstructing three dimensional interfaces between low density materials.

Cited by: 95

А. М. Афанасьев, В. Г. Кон

ЖЭТФ Volume: 74 Issue: 1 Pages: 300-313 Published: 1978

Cited by: 90

A. Snigirev, V. Kohn, I. Snigireva, A. Souvorov, B. Lengeler

Applied Optics Volume 37 Issue: 4 Pages: 653-662 Published: 1998

Compound lenses made from low-Z materials (e.g., Be, B, C, and Al) set up as a linear array of refractive lenses are proposed for submicrometer focusing of high-energy x rays (ɱ keV) in one or two dimensions. A theory of focusing based on Maxwell's equation and the Fresnel-Kirchhoff approach is presented. Compound refractive lenses were manufactured by drilling into an Al block a linear array of 200 closely spaced holes 0.5 mm in diameter for linear focusing and two crossed arrays of 100 holes each for point focusing. Focal spots of 3.7 µm and 8 µm × 18 µm were obtained at 30 keV for linear and two-dimensional lenses, respectively. Different technologies of manufacturing and possible applications of the proposed lenses are discussed.

Cited by: 74

A. M. Afanasev, M. V. Kovalchuk, E. K. Kovev, V. G. Kohn

Phys. Status Solidi (a) Volume: 42 Issue: 1 Pages: 415-422 Published: 1977

A method of analysis of curves for diffraction reflection from crystals with a disturbed surface layer is developed. It allows to determine, directly from experimental data, the integral characteristics of the disturbed layer, such as the thickness of the disturbed layer, Leff, mean change in the interplanar spacings along the layer delta(a) and so on. This method is illustrated by an example of analysis of curves of reflection from diffuse layers of silicon for different orders of reflection.

Cited by: 67

V. G. Kohn., A. I. Chumakov, R. Ruffer

Phys. Rev. B Volume: 58 Issue: 13 Pages: 8437-8444 Published: 1998

The Singwi and Sjölander theory of nuclear resonant inelastic absorption of x rays is extended to the general case of an anisotropic single crystal. The energy dependence of nuclear inelastic absorption for the specific direction of the x-ray quantum relative to the crystal lattice is described using the density of phonon states, weighted by the projection of the phonon polarization vectors to the wave vector of the x-ray quantum. An applicability of the sum rules in the case of the anisotropic crystal is analyzed. The method of calculation of the phonon projected density of states from experimental data is proposed, where deconvolution of the data with the instrumental function of the monochromator and the subtraction of the multiphonon absorption is handled using the Fourier transformation. The results are illustrated by processing the experimental data of nuclear inelastic absorption of x rays in the anisotropic ferric borate FeBO₃ single crystal

Cited by: 61

G. V. Smirnov, V. G. Kohn

Phys. Rev. B Volume: 52 Issue: 5 Pages: 3356-3365 Publoshed: 1995

A general theory of the time dependence of nuclear resonant forward scattering (spatially coherent) as well as 4А scattering (incoherent) is presented for a system where resonant nuclei are moving diffusively. As in Mцssbauer absorption spectroscopy the diffusive motion is taken into account by employing the Van Hove correlation function. However, in the case of scattering the role of correlations turns out to be more complicated. The general formulas are obtained and used to analyze the case of free diffusion.

Cited by: 56