arXiv subjects

Ari-Pekka Honkanen

Publications and source records attributed to Ari-Pekka Honkanen.

8 recordsLinked to original sources

arXiv2023

Inner product regularized multi-energy X-ray tomography for material decomposition

Multi-energy X-ray tomography is studied for decomposing three materials using three X-ray energies and a classical energy-integrating detector. A novel regularization term comprises inner products between the material distribution functions, penalizing any overlap of different materials. The method is tested on real data measured of a phantom embedded with Na$_2$SeO$_3$, Na$_2$SeO$_4$, and elemental selenium. It is found that the two-dimensional distributions of selenium in different oxidation states can be mapped and distinguished from each other with the new algorithm. The results have applications in material science, chemistry, biology and medicine.

cond-mat.mtrl-sci↗

arXiv2022

Monochromatic computed tomography using laboratory-scale setup: proof-of-concept

In this article, we demonstrate the viability of highly monochromatic full-field X-ray absorption near edge structure based tomography using a laboratory-scale Johann-type X-ray absorption spectrometer based on a conventional X-ray tube source. In this proof-of-concept, by using a phantom embedded with elemental Se, Na$_2$SeO$_3$, and Na$_2$SeO$_4$, we show that the three-dimensional distributions of Se in different oxidation states can be mapped and distinguished from the phantom matrix and each other with absorption edge contrast tomography. The presented method allows for volumetric analyses of chemical speciation in mm-scale samples using low-brilliance X-ray sources, and represents a new analytic tool for materials engineering and research in many fields including biology and chemistry.

physics.ins-det↗

arXiv2021

Redox oscillations in 18650-type lithium-ion cell revealed by in operando Compton scattering imaging

Compton scattering imaging using high-energy synchrotron x-rays allows the visualization of the spatio-temporal lithiation state in lithium-ion batteries probed in-operando. Here, we apply this imaging technique to the commercial 18650-type cylindrical lithium-ion battery. Our analysis of the lineshapes of the Compton scattering spectra taken from different electrode layers reveals the emergence of inhomogeneous lithiation patterns during the charge-discharge cycles. Moreover, these patterns exhibit oscillations in time where the dominant period corresponds to the time scale of the charging curve.

physics.app-ph↗

arXiv2020

General method to calculate the elastic deformation and X-ray diffraction properties of bent crystal wafers

Curved single crystals are widely employed in spectrometer designs in the hard X-ray regime. Due to their large solid angle coverage and focusing properties, toroidally bent crystals are extremely useful in applications where the output of photons is low. Spherically bent crystals, a subgroup of toroidally bent crystals, particularly have found their way in many instruments at synchtrotrons and free electron laser lightsource end-stations but also in the re-emerging field of high-resolution laboratory-scale X-ray spectroscopy. A solid theoretical understanding of the diffraction properties of such crystals is essential when aiming for optimal spectrometer performance. In this work, we present a general method to calculate the internal stress and strain fields of toroidally bent crystals and how to apply it to predict their diffraction properties. Solutions are derived and discussed for circular and rectangular spherically bent wafers due to their prevalence in contemporary instrumentation.

physics.ins-det↗

arXiv2019

Johann-type laboratory-scale X-ray absorption spectrometer with versatile detection modes

We present a low-cost laboratory X-ray absorption spectrometer that uses a conventional X-ray tube source and bent Johann-type crystal monochromators. The instrument is designed for XAS studies in the 4-20 keV range which covers most K edges of 3d transition metals and L edges of 5d transition metals and actinides. The energy resolution is typically in the range of 1-5 eV at 10 keV depending on the crystal analyser and the Bragg angle. Measurements can be performed in transmission, fluorescence, and imaging modes. Due to its simple and modular design, the spectrometer can be modified to accommodate additional equipment and complex sample environments required for e.g. in situ studies. A showcase of various applications is presented.

physics.ins-det↗

arXiv2017

A finite element approach to dynamical diffraction problems

A finite element approach to solve numerically the Takagi-Taupin equations expressed in a weak form is presented and applied to simulate X-ray reflectivity curves, spatial intensity distributions and focusing properties of bent perfect crystals in symmetric reflection geometry. The proposed framework encompasses a new formulation of the Takagi-Taupin equations, which appears to be promising in terms of robustness and stability and supports the Fresnel propagation of the diffracted waves. The presented method is very flexible and has the potential of dealing with dynamical X-ray or neutron diffraction problems related to crystals of arbitrary shapes and deformations. The reference implementation based on the commercial COMSOL Multiphysics software package is available to the relevant user community.

physics.optics↗

arXiv2017

Resonant X-ray Emission Spectroscopy With a Standing Wave Excitation

The Borrmann effect is the anomalous transmission of x rays in perfect crystals under diffraction conditions. It arises from the interference of the incident and diffracted waves, which creates a standing wave with nodes at strongly absorbing atoms. Dipolar absorption of x rays is thus diminished, which makes the crystal nearly transparent for certain x-ray wave vectors. Indeed, a relative enhancement of electric quadrupole absorption via the Borrmann effect has been demonstrated recently. Here we show that the Borrmann effect has a significantly larger impact on resonant x-ray emission than is observable in x-ray absorption. Emission from a dipole forbidden intermediate state may even dominate the corresponding x-ray spectra. Our work extends the domain of x-ray standing wave methods to resonant x-ray emission spectroscopy and provides means for novel spectroscopic experiments in d- and f-electron systems.

cond-mat.mtrl-sci↗

arXiv2016

A computationally efficient method to solve the Takagi-Taupin equations for a large deformed crystal

We present a treatise on solving the Takagi-Taupin equations in the case of a strain field with an additional, spatially slowly varying component (owing to \emph{e.g.}~heat expansion or angular compression). We show that the presence of such a component in a typical case merely shifts the reflectivity curve as a function of wavelength or incidence angle, while having a negligible effect on its shape. On the basis of the derived result, we develop a computationally efficient method to calculate the reflectivity curve of a large deformed crystal. The validity of the method is demonstrated by comparing computed reflectivity curves with experimental ones for bent silicon wafers. A good agreement is observed.

physics.optics↗