Kouhei Okitsu

Polarization-dependent six-beam X-ray pinhole topographs.

X-ray six-beam pinhole topograph images were obtained for a silicon crystal with incident synchrotron X-rays. The polarization state of X-rays incident on the sample crystal was controlled by using a four-quadrant phase-retarder system [Okitsu et al. (2002). Acta Cryst. A58, 146-154] that can be rotated around the transmitted beam axis to generate arbitrarily polarized X-rays. Quantitative agreement was found between the experimental and computer-simulated topograph images based on the n-beam Takagi-Taupin dynamical theory under the assumption that the polarization state of the incident X-rays was identical with the experiment. This result confirmed the validity of the computer algorithm to solve the n-beam dynamical theory and the proper operation of the rotating four-quadrant phase-retarder system simultaneously.

X-ray double phase retarders to compensate for off-axis aberration

An X-ray double phase retarder system composed of two transmission-type phase retarders is proposed and developed in order to compensate for off-axis aberration (phase-shift inhomogeneity due to angular divergence of incident X-rays). The scattering planes of the two phase retarders are set to be inclined by 45 degrees with respect to the plane of incident polarization, but the two phase retarders give Bragg reflections in opposite directions. By using this X-ray optical system, vertically polarized X-rays with a 0.99 degree of linear polarization were obtained from horizontally polarized synchrotron radiation with a horizontal beam divergence of 20 arcsec (0.1 mrad). This value is favorably compared with the value of 0.87 which was obtained using a conventional single phase retarder of identical total thickness, 627 microns. The comparison was made at the nickel K-absorption edge (8333 eV) with the condition that 47% of incident X-rays were transmitted through the two phase retarder crystals. The crystals were (100)-oriented diamond plates giving asymmetric 111 Laue reflections.

Polarization-dependent X-ray six-beam pinhole topographs for a channel-cut silicon crystal

It was pointed out in a previous paper [Okitsu et al. (2006), Acta Cryst. A62, 237-244] that an n-beam Takagi-Taupin (T-T) equation can be solved for a crystal of arbitrary shape. The procedure to integrate the n-beam T-T equation is to let all the Fourier coefficients of the electric susceptibility be zero at positions inside the Borrmann pyramid but outside the crystal. The efficiency of this simple procedure is verified in the present paper by showing qualitative and quantitative agreements between experimentally obtained and computer-simulated X-ray six-beam pinhole topographs for a channel-cut silicon crystal.

On the polarization state of X-rays generated using a rotating four-quadrant X-ray phase retarder system

A method of calculating the polarization state of X-rays transmitted through a rotating four-quadrant X-ray phase retarder system is described using Jones calculus. This method was applied to correcting the polarization states of X-rays incident on a channel-cut silicon crystal. Details and results of this experiment are given in a separate paper [Okitsu et al. (2011), Acta Cryst. A67, 550-556].

Tunable X-ray polarization reflector with perfect crystals

The possibility of a transmission-type X-ray linear polarizer is investigated using a thin Bragg reflector as a polarizing filter. In this device, the transmitted beam (rather than the Bragg-reflected beam) is the useful output of the device. Consequently, the position and the direction of the transmitted beam are unchanged as the energy is changed, or even when the polarization direction to be filtered out is changed. Theoretical considerations as well as preliminary transmissivity measurements are presented. The use of perfect crystals of silicon and diamond is examined. A polarization ratio, defined as I(H)/I(V), higher than 10(5) was observed in experiments to measure the performance characteristics of the proposed X-ray polarizing reflector. The transmission-type X-ray linear polarizer is well suited for spectroscopic measurements with polarized X-rays.

Microdefects in an As-Grown Czochralski Silicon Crystal Studied by Synchrotron Radiation Section Topography with Aid of Computer Simulation

Grown-in microdefects of a Czochralski (CZ) silicon crystal grown at a slow growth rate were studied by section topography using high energy synchrotron radiation. Images of the microdefects in the section topographs were analyzed quantitatively using computer simulation based on the Takagi-Taupin type dynamical diffraction theory of X-rays, and reproduced successfully by the simulation when the microdefects were assumed to be spherical strain centers. Sizes and positions of the microdefects were able to be determined by detailed comparison between the experiments and the computer simulations. The validity of the computer simulation in an analysis of the section topographs is discussed.

Numerically Simulated and Experimentally Obtained X-Ray Section Topographs of a Spherical Strain Field in a Floating Zone Silicon Crystal

An undoped floating zone (FZ) silicon crystal has been investigated by synchrotron X-radiation section topography with high-order reflections up to 14 14 0. Numerically simulated topographs based on the Takagi-Taupin equations were in good agreement with experimental distorted patterns when a spherical strain field was assumed in the crystal. The volume change of the lattice caused by the strain center was estimated to correspond to a sphere with a radius of 10 µm.

X-Ray N-Beam Takagi-Taupin Dynamical Theory and N-Beam Pinhole Topographs Experimentally Obtained and Computer-Simulated

In the field of X-ray crystal structure analysis, while the absolute values of structure factors are directly observed, phase information is lost in general. However, this problem (phase problem) has been overcome mainly by the direct method developed by Hauptmann and Karle except for protein crystals. In the case of protein crystal structure analysis, the isomorphous replacement method and/or anomalous dispersion method are mainly used to solve the phase problem. Phasing the structure factors is sometimes the most difficult process in protein crystallography.

Three-beam X-ray rocking curves calculated from computer-simulated pinhole topographs.

X-ray rocking curves are reported which have been obtained by fast-Fourier-transforming X-ray amplitudes in three-beam pinhole topographs. The topographs were computer-simulated based on the Takagi-Taupin equation with the condition of spherical-wave X-ray incidence. This is another strategy for calculating three-beam rocking curves, which are usually calculated based on the Ewald-Laue dynamical theory.

X-ray magnetic circular dichroism imaging with hard X-rays

X-ray polarization-contrast images resulting from X-ray magnetic circular dichroism (XMCD) in the hard X-ray region have been successfully recorded for the first time. The apparatus used consisted of an X-ray polarizer, double X-ray phase retarders, and a high-spatial-resolution X-ray charge-coupled-device detector. The sample used was a hexagonal-close-packed cobalt polycrystal foil having a thickness of about 4 microns. The X-ray polarization-contrast image resulting from XMCD was observed at a photon energy of 10 eV above the cobalt K-absorption edge (7709 eV). The observed contrast in the image was reversed by inversion of the magnetic field. Furthermore, the contrast was reversed again at a photon energy of 32 eV above the cobalt K-absorption edge.