Photon-photon interaction in axial channeling

We investigate the possibility that high-energy photons are channeled, when passing through an oriented single crystal, due to Delbrück scattering. For this purpose the exact electron propagator for the single-string model is constructed. Starting from a separation of variables, we solve the Dirac equation for a cylindrical electrostatic potential. The propagator for such external fields is constructed from solutions of the radial Dirac equation. This propagator is applied to a calculation of the S matrix for Delbrück scattering. We specify the conditions under which photon channeling takes place. Unfortunately these conditions are only matched for a very small fraction of those photons being produced by channeled electrons

Magnetic neutrino scattering by crystals

The magnetic dipole scattering of neutrinos by the electrostatic potentials of single atoms as well as crystals is investigated. It is shown that scattering by a rigid cubic lattice can amplify the neutrino-atom cross section by a factor of N1/3, N being the number of scatterers. However, comparing the results with typical weak-interaction cross sections, the effect seems to be not observable in experiment

Quantum-mechanical treatment of high-energy channeling radiation

An alternative theoretical description of axial electron channeling in the multi-GeV region has been developed. We solve a kinetic equation to evaluate an electron distribution function in axially oriented single crystals. Based on the single-string model, the required matrix elements for radiation and scattering by lattice vibrations are calculated employing solutions of the Dirac equation in cylindrical coordinates. Results obtained for 150-GeV electrons propagating along the axis of germanium are in good agreement with experimental observations

The mechanism of oxygen isotope fractionation during N2O production by denitrification

The isotopic composition of soil-derived N2O can help differentiate between N2O production pathways and estimate the fraction of N2O reduced to N2. Until now, δ18O of N2O has been rarely used in the interpretation of N2O isotopic signatures because of the rather complex oxygen isotope fractionations during N2O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H2O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N2O production by denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the δ18O values of the produced N2O. We performed several soil incubation experiments. For the first time, ∆17 O isotope tracing was applied to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found bacterial denitrification to be typically associated with almost complete oxygen isotope exchange and a stable difference in δ18O between soil water and the produced N2O of δ18O(N2O / H2O) = (17.5±1.2) ‰. However, some experimental setups yielded oxygen isotope exchange as low as 56 % and a higher δ18O(N2O / H2O) of up to 37‰. The extent of isotope exchange and δ18O(N2O / H2O) showed a very significant correlation (R2 = 0.70, p < 0.00001). We hypothesise that this observation was due to the contribution of N2O from another production process, most probably fungal denitrification. An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification the isotope exchange occurs prior to the isotope branching and that the mechanism of this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0±0.9) ‰; in contrast to fungal denitrification where higher values of up to 30‰ have been reported previously. This suggests that δ18O might be used as a tracer for differentiation between bacte- 5 rial and fungal denitrification, due to their different magnitudes of branching isotope effect

Oxygen isotope fractionation during N2O production by soil denitrification

The isotopic composition of soil-derived N₂O can help differentiate between N₂O production pathways and estimate the fraction of N₂O reduced to N₂. Until now, <i>δ</i>¹⁸O of N₂O has been rarely used in the interpretation of N₂O isotopic signatures because of the rather complex oxygen isotope fractionations during N₂O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H₂O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N₂O production by soil denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the <i>δ</i>¹⁸O values of the produced N₂O. <br><br> In our soil incubation experiments Δ¹⁷O isotope tracing was applied for the first time to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found that N₂O formation in static anoxic incubation experiments was typically associated with oxygen isotope exchange close to 100 % and a stable difference between the ¹⁸O ∕ ¹⁶O ratio of soil water and the N₂O product of <i>δ</i>¹⁸O(N₂O ∕ H₂O)  =  (17.5 ± 1.2) ‰. However, flow-through experiments gave lower oxygen isotope exchange down to 56 % and a higher <i>δ</i>¹⁸O(N₂O ∕ H₂O) of up to 37 ‰. The extent of isotope exchange and <i>δ</i>¹⁸O(N₂O ∕ H₂O) showed a significant correlation (<i>R</i>² = 0.70, <i>p</i> &lt;  0.00001). We hypothesize that this observation was due to the contribution of N₂O from another production process, most probably fungal denitrification. <br><br> An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification, isotope exchange occurs prior to isotope branching and that this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0 ± 0.9) ‰, in contrast to fungal denitrification where higher values of up to 30 ‰ have been reported previously. This suggests that <i>δ</i>¹⁸O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects

Microplastic fibers affect dynamics and intensity of CO2 and N2O fluxes from soil differently

Microplastics may affect soil ecosystem functioning in critical ways, with previously documented effects including changes in soil structure and water dynamics; this suggests that microbial populations and the processes they mediate could also be affected. Given the importance for global carbon and nitrogen cycle and greenhouse warming potential, we here experimentally examined potential effects of plastic microfiber additions on CO2 and N2O greenhouse gas fluxes. We carried out a fully factorial laboratory experiment with the factors presence of microplastic fibers (0.4% w/w) and addition of urea fertilizer (100 mg N kg− 1) using one target soil. The conditions in an intensively N-fertilized arable soil were simulated by adding biogas digestate at the beginning of the incubation to all samples. We continuously monitored CO2 and N2O emissions from soil before and after urea application using a custom-built flow-through steady-state system, and we assessed soil properties, including soil structure. Microplastics affected soil properties, notably increasing soil aggregate water-stability and pneumatic conductivity, and caused changes in the dynamics and overall level of emission of both gases, but in opposite directions: overall fluxes of CO2 were increased by microplastic presence, whereas N2O emission were decreased, a pattern that was intensified following urea addition. This divergent response is explained by effects of microplastic on soil structure, with the increased air permeability likely improving O2 supply: this will have stimulated CO2 production, since mineralization benefits from better aeration. Increased O2 would at the same time have inhibited denitrification, a process contributing to N2O emissions, thus likely explaining the decrease in the latter. Our results clearly suggest that microplastic consequences for greenhouse gas emissions should become an integral part of future impact assessments, and that to understand such responses, soil structure should be assessed

Кластер как инструмент повышения качества высшего образования

В статье рассматривается место и роль университета при его вхождении в инновационный кластер на базе научно-исследовательского университета. Представлены основные преимущества данной форы интеграции образования и бизнеса, а также доказано, что кластер является инструментом повышения качества высшего образования

An assessment of discretizations for convection-dominated convection-diffusion equations

The performance of several numerical schemes for discretizing convection-dominated convection-diffusion equations will be investigated with respect to accuracy and efficiency. Accuracy is considered in measures which are of interest in applications. The study includes an exponentially fitted finite volume scheme, the Streamline-Upwind Petrov--Galerkin (SUPG) finite element method, a spurious oscillations at layers diminishing (SOLD) finite element method, a finite element method with continuous interior penalty (CIP) stabilization, a discontinuous Galerkin (DG) finite element method, and a total variation diminishing finite element method (FEMTVD). A detailed assessment of the schemes based on the Hemker example will be presented

An assessment of discretizations for convection-dominated convection-diffusion equations

The performance of several numerical schemes for discretizing convection-dominated convection-diffusion equations will be investigated with respect to accuracy and efficiency. Accuracy is considered in measures which are of interest in applications. The study includes an exponentially fitted finite volume scheme, the Streamline-Upwind Petrov--Galerkin (SUPG) finite element method, a spurious oscillations at layers diminishing (SOLD) finite element method, a finite element method with continuous interior penalty (CIP) stabilization, a discontinuous Galerkin (DG) finite element method, and a total variation diminishing finite element method (FEMTVD). A detailed assessment of the schemes based on the Hemker example will be presented