During the production of the radiation source that used in radiotherapy may occur nuclear reactions are very important in terms of human health. ^{137}Cs is used in radiotherapy that consists of fission the ^{235}U core. Because of physical half-live of ^{137}Cs is 30 years, it is advantage for the radioactive half-lives. In this study, radionuclide production reaction cross section for ^{235}U (n,f)^{137}Cs is calculated with TALYS 1.6 nuclear simulation code that based on Monte Carlo.
The electron transfer rates at the steady state are evaluated in terms of the Gaussian wave packet motion on free energy curves in the two- and three-surface models in the presence of inertial effects. The autocorrelation functions of the solvent polarization coordinate are fitted to the results of recent molecular dynamics simulations. It is found that the inertial effects are particularly important for the electron transfer processes in acetonitrile and water. They constitute an impeding factor in the wave packet motion. The neglect of the inertial part of the solvent autocorrelation function gives underestimation of the electron transfer rate coefficient.
Kinetics of processes, in which the reaction rate increases with conversion, is discussed and illustrated with an example of the chemical reaction of isomerization of an azobenzene derivative in a liquid crystalline matrix. A simple phenomenological model is put forward explaining the effect by dynamic changes of interactions between the reacting species and the matrix.
The chemical etching of intrinsic and n-type polycrystalline silicon in Br_2 ambient is considered. The theoretically calculated dependences of silicon etching rates on pressure of Br_2 molecules at different temperatures are compared with experimentally measured ones. The reaction and desorption activation energies are evaluated. It is found that activation energy of Si + Br_2 → SiBr_2 reaction for intrinsic silicon is equal to (1.82 ± 0.24) eV, and decreases to (1.45 ± 0.24) eV when n-type silicon films are used. Desorption activation energy of SiBr_2 molecules for intrinsic silicon is equal to (1.94 ± 0.17) eV, and decreases to (1.51 ± 0.17) eV when n-type silicon films are used. Desorption of SiBr_2 molecules is an etching-rate limiting process at high pressure of Br_2 molecules.
A standard technique of electroless gold deposition was modified by changing the composition of Ag-based activation solution. This allows preparation of two types of poly(ethyleneterephthalate) (PET) track etched membranes coated entirely with gold after 1, 5, and 24 h of reaction at 4°C. After dissolving the polymer template, gold nanotubes with outer diameter of 70-80 nm were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Reduction of p-nitrophenol to p-aminophenol by sodium borohydride was used to investigate the catalytic activity of as-prepared Au/PET membrane. All experiments were carried out for five consecutive cycles and rate constant of the pseudo-first-order reaction was calculated. It was found that Au/Ag/PET composites prepared after activation for 3 min in Ag-based solution (with potassium sodium tartrate as reducing agent) more effective catalyst (k=0.087 min^{-1}) was obtained when the Au/PET samples activated with ammonia silver nitrate solution (k=0.041 min^{-1}).
Electroless deposition has been used to coat finely porous polyethylene terephthalate (PET) track-etched membranes with silver, forming silver nanotubes within the pores with inner and outer diameters of 60 and 100 nm. The sample's X-ray diffraction pattern shows a face-centered cubic crystalline phase of silver with the lattice constant 4.0838 nm. The average size of silver nanoclusters, as obtained from the scanning electron microscopy analysis is about 30 nm which is consistent with the X-ray diffraction results. The temperature dependent catalytic activity of prepared composites is demonstrated for two model reactions such as reduction of 4-nitrophenol (4-NP) and decomposition of hydrogen peroxide. Apparent constant rates and activation energy as well as reusability of catalysts were determined. The developed composite catalyst could be used consecutively for several runs without any damages for 4-NP reduction. For hydrogen peroxide reaction decomposition the reaction rate of the second cycle is reduced 2.4 times. Moreover, the second reuse reduced conversion of H₂O₂ to 54.7% suggests removal of active Ag centers during the first cycle of testing.
Two-dimensional exchange ^{35}Cl nuclear quadrupole resonance spectroscopy for studies of the CCl_3-group reorientation processes in hexachloroethane and chloral hydrate has been applied. Experimental results were interpreted on the basis of the 2D exchange nuclear quadrupole resonance theory, which takes into account the off-resonance irradiation. It has been demonstrated that 2D nuclear quadrupole resonance exchange spectroscopy is appropriate for quantitative studies of exchange processes in molecular crystals containing quadrupolar nuclei. The method is of particular value for the detection of exchange networks in systems with many sites. Thus, detailed information on the exchange pathways within a network of structural isomers can be deduced and a proper assignment of the nuclear quadrupole resonance lines can be made. Temperature dependence of the exchange rate was studied. The mixing dynamics by exchange and the expected cross-peak intensities have been derived.
The sol-gel method, which is among the methods used for the production of ThO₂-UO₂ mixed oxide fuel pellets, allows a high degree of micro-homogeneity of uranium and thorium in the solution stage. The present study seeks to develop an alternative method in order to produce mixed thorium-uranium dioxide fuel pellets, prepared by internal gelation technique of sol-gel process. The study also aims to investigate shrinkage behavior and first-stage sintering kinetics of pellets compacted in the Ar-5%H₂ atmosphere using a dilatometer. The chemical and physical properties such as density, homogeneity, O/M ratio, metallic impurities, unit cell parameters, phase contents and surface area of powders, that have different compositions of thoria-urania, were studied. The influences of sintering parameters on the characteristics of microspheres and on the properties of sintered pellets are discussed.
Analysis of the o-Ps lifetimes concentration dependences in water-propanol mixtures (with and without CoCl_2 additive) shows that this mixture looks like an emulsion of alcohol micelles in water at propanol mole fractions 0.1-0.4.
In this paper we present the transport coefficients in Ar/CF_4 mixtures with realistic abundances of CF_x radicals, F atoms, and F_2 molecules that are standard products of plasma chemistry in plasma etching systems and are present in large abundances of the order of few percent in realistic plasma etching devices. It was found that, although radicals make a minimum impact on distribution function and mean energy, the effect on drift velocity is significant and the effect on rates of attachment is large and may change the mode of operation of plasmas.
Thermal decomposition behavior of bis (4-nitrophenol)-2,4,6-triamino-1,3,5-triazine monohydrate (BNPM) has been studied by means of thermogravimetric analysis at three different heating rates 10, 15 and 20°C min¯¹. Non-isothermal studies of BNPM have revealed that the decomposition occurs in three stages involving dehydration and decomposition. The values of effective activation energy (E_{a}), pre-exponential factor (A) of each stage of thermal decomposition for all heating rates were calculated by model free methods: Arrhenius, Flynn-Wall, Friedman, Kissinger and Kim-Park method. A significant variation of effective activation energy (E_{a}) with conversion (α) indicates that the process is kinetically complex. The linear relationship between the A and E_{a} values was well established (compensation effect). Dehydration stage was governed by the Avrami-Erofeev model (A2) and decomposition stages were governed by the Avrami-Erofeev model (A4).
A theoretical model for calculation of equilibrium concentrations of isomeric butane molecule forms, and also the forms of hydrocarbons of the alkene series, is proposed. We considered the changes of internal rotational potential energy in respect of reaction coordinate, of rapid conversion of butane isomers from one form to another at room temperature. This is very important for better understanding of the nature of chemical reactions in butane, because the changes of internal rotational potential energy reflect conformational transitions of butane and changes of its chemical and physical properties. The presented model is based on Gribov's system of kinetic equations for isomer-isomer transitions. The canonical ensemble is used to calculate the room temperature probabilities of the rotoisomer conformational states as functions of dihedrial angle, and the obtained results are in good agreement with available ones. Also, in a sense of calculated lifetime of unstable rotoisomers (<10^{-10} s), our results are comparable with coupled cluster and density functional calculations on alkene derivatives.
The thermal decomposition kinetics of melaminium bis(trichloroacetate) dihydrate (MTCA) has been studied by thermogravimetry and derivative thermogravimetry techniques using non-isothermal experiments at three different heating rates 10, 15, and 20°C min^{-1}. Non-isothermal studies of MTCA revealed that the decomposition occurs in three stages involving dehydration and decomposition. The apparent activation energy (E_{a}) and the pre-exponential factor (ln A) of each stage of thermal decomposition at various linear heating rates are calculated using Flynn-Wall, Friedman, Kissinger, and Kim-Park method. A significant variation of effective activation energy (E_{a}) with conversion (α) indicates that the process is kinetically complex. The linear relationship between the A and E_{a} values is well established (compensation effect). Isothermal kinetics of thermal decomposition of MTCA was found to obey Avrami-Erofeev's (A4) and power law (P3) equations. In addition to the above, mechanical properties have been estimated by Vicker's microhardness test for the grown crystal.
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