RESUMO
Time-delayed, narrow-band echoes generated by forward Bragg diffraction of an X-ray pulse by a perfect thin crystal are exploited for self-seeding at hard X-ray free-electron lasers. Theoretical predictions indicate that the retardation is strictly correlated to a transverse displacement of the echo pulses. This article reports the first experimental observation of the displaced echoes. The displacements are in good agreement with simulations relying on the dynamical diffraction theory. The echo signals are characteristic for a given Bragg reflection, the structure factor and the probed interplane distance. The reported results pave the way to exploiting the signals as an online diagnostic tool for hard X-ray free-electron laser seeding and for dynamical diffraction investigations of strain at the femtosecond timescale.
RESUMO
Total variation minimization is applied to the particular case of X-ray tomography in a scanning electron microscope. To prove the efficiency of this reconstruction method, noise-free and noisy data based on the Shepp & Logan phantom have been simulated. These simulations confirm that Total variation minimization-reconstruction algorithm better manages data containing low number of projections with respect to simultaneous iterative reconstruction technique or filtered backprojection, even in the presence of noise. The algorithm has been applied to real data sets, with a low angular sampling and a high level of noise. Two samples containing micro-interconnections have been analyzed and 3D reconstructions show that Total variation minimization-based algorithm performs well even with 60 projections in order to properly recover a 500 nm diameter void inside a copper interconnection.
RESUMO
We investigated an amorphous Se(90)Te(10) alloy produced by mechanical alloying using two different approaches. First, we used extended x-ray absorption fine structure (EXAFS) spectroscopy and the cumulant expansion method using the Einstein model for the temperature dependence of the cumulants to obtain the cumulants C(*)(1), C(*)(2), and C(*)(3). From these, we found information about the structure of the alloy as well as the thermal and structural disorder, anharmonicity of the effective interatomic pair potentials, thermal expansion of the Se-Se and Se-Te bonds and asymmetry of the partial distribution functions g(Se-Se)(r) and g(Se-Te)(r). The cumulants C(*)(1), C(*)(2), and C(*)(3) also allowed us to reconstruct the g(EXAFS)(ij)(r,T) functions from EXAFS. Then, we made reverse Monte Carlo (RMC) simulations using the total structure factor S(K) obtained from synchrotron x-ray diffraction and the EXAFS oscillations χ(k) on the Se K edge as input data to obtain the g(RMC)(ij)(r) functions. Both methods furnished very similar g(ij)(r) functions, and the structural data obtained from them were also very similar. The results obtained from both methods showed the presence of Se-Te pairs indicating that there is alloying at the atomic level. In addition, we could not find any evidence of the presence of Te clusters in the alloy.
RESUMO
The local atomic order of an amorphous Se(0.90)S(0.10) alloy produced by mechanical alloying was studied by x-ray diffraction and extended x-ray absorption fine structure (EXAFS) data obtained at three temperatures, T = 300, 200 and 30 K. From the cumulant analysis of the EXAFS data, structural properties such as average interatomic distances, average coordination numbers, Debye-Waller factors and anharmonicity, given by the third cumulant, were obtained. The results found indicate that there is alloying at an atomic level, and Se-S pairs are more disordered and distorted than Se-Se ones due to the milling process.
