Yu. V. Shvyd'ko

X-ray FEL with a meV bandwidth

Abstract A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL was proposed by Feldhaus et al. (Opt. Commun. 140 (1997) 341) and named “two-stage SASE FEL”. The scheme consists of two undulators and an X-ray monochromator located between them. For the Angstrom wavelength range the monochromator can be realized using Bragg reflections from crystals. We propose a scheme of monochromator with a bandwidth of 20xa0meV for the 14.4 keV X-ray SASE FEL being developed in the framework of the TESLA linear collider project. The spectral bandwidth of the radiation from the two-stage SASE FEL ( 20 meV ) is determined by the finite duration of the electron pulse. The shot-to-shot fluctuations of energy spectral density are dramatically reduced in comparison with the 100% fluctuations in a SASE FEL. The peak and average brilliance are three orders of magnitude higher than the values which could be reached by a conventional X-ray SASE FEL.

Precise measurement of the lattice parameters of α-Al2O3 in the temperature range 4.5–250 K using the Mössbauer wavelength standard

The lattice parameters of α-Al2O3 have been measured in a temperature range from 4.5 to 250u2005K with a relative accuracy of better than 6 × 10−6. The experimental method uses Bragg backscattering and the recently proposed Mossbauer wavelength standard, i.e. the wavelength λM = 86.025474u2005(16)u2005pm of the nuclear resonance radiation of 57Fe (Shvydko et al., 2000), which has previously been applied successfully to measure the lattice parameters of α-Al2O3 at temperatures between 286 and 374u2005K (Shvydko et al., 2002). The experimental data in the range from 4.5 to 374u2005K are consistent with the Debye model of thermal expansion. At 4.5u2005K, the thermal expansion coefficient is as low as 1.2u2005(9) × 10−10u2005K−1 in the a direction.

Measuring wavelengths and lattice constants with the Mossbauer wavelength standard

The newly proposed atomic-scale length standard, the wavelength of the 57Fe Mossbauer radiation [Shvydko et al. (2000). Phys. Rev. Lett. 45, 495–498], is used to measure the wavelengths of the Mossbauer radiation of 151Eu, 57.556185u2005(27)u2005pm, 119Sn, 51.920811u2005(39)u2005pm, and 161Dy, 48.334336u2005(19)u2005pm, with a relative accuracy of ∼0.5u2005p.p.m. Also, the lattice constants of Al2O3 are measured in a temperature range from 286u2005K to 374u2005K. At room temperature, T = 295.65u2005K, their values are a = 4.759213u2005(8)u2005A, c = 12.991586u2005(4)u2005A.

Backscattering mirrors for X-rays and Mössbauer radiation

Observation of exact backscattering of X-rays and studies of its energy and angular dependences; test of the validity of the dynamical theory of diffraction in the extreme case of exact backscattering; backscattering high-energy-resolution monochromators; backscattering interferometers, in particular of the Fabry–Pérot interferometer type; and precise, up to 5·10—9 Å, measurements of crystal lattice parameters: these are central topics of the paper. Special attention is paid to the selection of crystals to be used as backscattering mirrors. Noncubic crystals like Al2O3, SiC, etc., allow backscattering for X-rays with practically any energy above 10 keV. Feasibility of backscattering mirrors for Mössbauer radiation of 57Fe (14.4 keV), 151Eu (21.5 keV), 119Sn (23.9 keV), and 161Dy (25.6 keV) nuclei is demonstrated by Al2O3 crystals. A concrete design of a sapphire Fabry–Pérot–Bragg étalon is presented.

On the theory of an X-ray Fabry-Perot interferometer

The theory of an X-ray Fabry-Perot interferometer (FPI) is developed as a particular case of the dynamical theory of X-ray Bragg diffraction in layered crystalline systems. Mathematical expressions are derived for the transmissivity and reflectivity of the X-ray FPI built as a system of two perfect crystal plates parallel to each other. The performance of the X-ray FPI is similar to that of the optical FPI. Both show fine interference structure in the transmission and reflection dependences. However, for the X-ray FPI this occurs only inside the region of the Bragg back diffraction peak. The influence of possible imperfections, such as the roughness of the crystal plate surfaces and the error in the parallelism of the atomic planes are discussed. It is shown that both factors may significantly deteriorate the performance of the X-ray FPI. Numerical estimations are given.

Magnetic Nature of the 500 meV peak in La2−xSrxCuO4 Observed with Resonant Inelastic X-ray Scattering at the Cu K-edge

We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at q = ({pi},0) in resonant inelastic x-ray scattering (RIXS) experiments on La{sub 2}CuO{sub 4}. The intensity of this peak persists above the Neel temperature (T{sub N} = 320 K), but decreases gradually with increasing temperature, reaching zero at around T = 500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon B{sub 1g} Raman peak in La{sub 2}CuO{sub 4} and, with suitable rescaling, agrees with the Raman peak shifts in EuBa{sub 2}Cu{sub 3}O{sub 6} and K{sub 2}NiF{sub 4}. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin-correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.

Quality Assessment of Sapphire Wafers for X-Ray Crystal Optics Using White Beam Synchrotron X-Ray Topography

The white beam Synchrotron X-Ray Topography (SXRT) technique was used to assess the quality of sapphire wafers grown by the Heat-Exchanger Method (HEM) and the Modified Czochralski Method (MCM). Sapphire is a potential new material for X-ray crystal optics, especially for use as Bragg backscattering mirrors for X-rays and Mossbauer radiation. The dislocation distribution, dislocation density and Burgers vector of selected dislocations and stacking faults in the sapphire wafers were studied. A correlation between the sapphire quality and its performance as an X-ray backscattering mirror was established in this paper. The results reveal the high quality of the inspected HEM sapphire wafers and their subsequently improved performance as Bragg backscattering mirrors.

Beryllium and aluminium refractive collimators for synchrotron radiation

Be and Al refractive lenses with long focal lengths provide a simple and efficient method of collimating synchrotron radiation. The divergence of an undulator beam at SPring-8 is reduced from >11u2005µrad full width at half maximum without the collimators to <3u2005µrad downstream of the collimators. The Be collimators have almost no losses (∼90% transmission) while the Al collimators reduce the flux by a factor of two (∼45% transmission). Data are shown at 14.4 and 18.5u2005keV.

Hybrid forms of beat phenomena in nuclear forward scattering of synchrotron radiation

In nuclear forward scattering (NFS) of synchrotron radiation, inter-resonance interference leads to a quantum beat (QB), and intra-resonance interference to a dynamical beat (DB). In general both interference processes determine the time evolution of NFS. Only in the case of far distant resonances the resulting interference pattern can be interpreted as a well distinguishable combination of QB and DB. Multiple scattering by near neighbouring resonances, by contrast, leads to a hybridisation of QB and DB. In particular, asymmetrical continuous distributions of resonances make QB and DB blend into a fast hybrid beat with thickness dependent period and distribution sensitive modulation.

Fast Switching of Nuclear Bragg Scattering of Synchrotron Radiation by a Pulsed Magnetic Field

The time dependence of perturbed nuclear Bragg scattering of synchrotron radiation by a 57FeBO3 crystal was studied. The perturbation was an abrupt rotation by 90° of the hyperfine fields in the crystal effectuated by fast switching of an external magnetic field. This perturbation did not destroy the coherence. It transformed the nuclear transitions and caused, therefore, dramatic changes in the time evolution. A new quantum beat pattern of different frequency was immediately established after switching. The intensity and decay rate of the re-emitted radiation sensitively depended on the moment of switching. The frequency of the original beat pattern was restored by switching back.