Steady Irradiation Research Articles

Radiolytic synthesis and characterization of PVA/Au nanocomposites: The influence of pH values

Nanocomposites which consist of polymer matrix and nanoparticles of noble metal are a very important class of nanomaterials. The optical properties of these materials are strongly dependent on the nanoparticles size as well as on the synthesized procedure of nanocomposites preparation. In this work the Au nanoparticles were synthesized by the simple radiolytic procedure, using steady state gamma irradiation, under different pH values. PVA/Au nanocomposite films were obtained by solvent evaporation from Au colloids. UV-Vis absorption spectra of Au colloids show absorption in the range of 450 to 650 nm with the maximum around 520 nm, which is the typical plasmon band of Au nanoparticles. Absorption spectra of PVA/Au nanocomposites are red shifted compared to corresponding primary Au colloids. This red shift is not result of any change in particle size distribution and can be explained by the change of dielectric property of surrounding medium. IR spectra indicated anchoring the OH groups of PVA molecule at the cluster surface. This effect limits the growing of the metal clusters and confirmed that the PVA is a god matrix for in situ synthesis of Au nanoparticles. The size of Au nanoparticles is dependent on the experimental conditions. Obtained results indicated that the pH value of solution played an important role. The values of absorption maximum (?max) were not dependent on the concentration of Au cations, but show strong dependence on the pH value of the initial solution. The increase of pH value reduced the mean diameters of nanoparticles. The lowest value of ?max was obtained for colloid with pH 7, which theoretically corresponds to the nanoparticles with minimum dimension. These results are in agreement with the XRD measurement of PVA/Au nanocomposite films and theoretical calculations. According to obtained results of radiolytic synthesis of Au nanoparticles in PVA solution can be conclude that it is possible to control the particle size by tuning the pH value of solution.

Reversible cyclisation of a sulfonated arylazo compound containing an o-acetylamino substituent

A sulfonated o-acetylamino arylazo compound unexpectedly cyclised to a cationic benzo-1,2,4-triazinium species under thermal or acid conditions, reversible with base. The same substrate underwent trans to cis-azo photoisomerisation, observable by NMR spectroscopy, under steady state laser irradiation conditions.

Modelling of grain face bubbles coalescence in irradiated UO 2 fuel

Two different mechanisms of grain face bubbles coalescence in irradiated UO 2 fuel are critically analysed and further developed. Coalescence of face bubbles due to their random migration is reconsidered on the base of the general theory for evolution of two-dimensional array of bubbles and shown to be effective for steady irradiation conditions and apparently insufficient for description of high-temperature annealing regimes. On the contrary, the improved mechanism of coalescence by growth and impingement of randomly distributed bubbles predicts rather weak decrease of face bubbles concentration under steady state irradiation conditions, however, becomes effective for simulation of high-temperature annealing tests with noticeable gas release. On this base, the general coalescence model which includes superposition of the two complementary mechanisms is formulated and proposed for implementation in the MFPR code.

Effect of complexation with lanthanide metal ions on the photochromism of (1,3,3‐trimethyl‐5′‐hydroxy‐6′‐formyl‐ indoline‐spiro2,2′‐[2h]chromene) in different media

Complexation of lanthanide ions {Ln(III) ions [Tb(III), Eu(III), or Sm(III)]} with the spiropyran‐derived merocyanine obtained in dark and under steady irradiation of indoline spiropyran (1,3,3‐trimethyl‐5′‐hydroxy‐6′‐formyl‐indoline‐spiro‐2,2′‐[2H]chromene) induces a noticeable hypsochromic shift of about 10–110 nm of its visible absorption band concomitant with hypochromic effect and influences its thermal bleaching in the dark. The effect of lanthanide ions and medium on photochromic, spectral‐and‐kinetic, and luminescence properties of the spiropyran and its complexes in solution and polymer matrix of polymethylmethacrylate (PMMA) is studied. Efficient energy transfer from the spiropyran moiety results in efficient typical luminescence from the Ln(III) ion that becomes more pronounced in polar nonalcoholic solvents and PMMA solid matrix. Moreover, luminescence mappings for pattern recognition analysis have been obtained from which the nature of the solvent and/or the ligand is clearly identified.

Development of the mechanistic code MFPR for modelling fission-product release from irradiated UO 2 fuel

The models of the mechanistic code MFPR developed in collaboration between IBRAE (Moscow, Russia) and IRSN (Cadarache, France) are described. Exhaustive description of fission-gas behaviour in grain and out of grain is given in relation with individual validation results on analytical experiments under various conditions (steady irradiation, transient, post-irradiation annealing). It is shown that microscopic defects in the UO 2 crystal structure can strongly influence fission-gas transport out of grains and release from fuel pellets. These defects include point defects such as vacancies, interstitials and fission atoms, and extended defects such as bubbles, pores and dislocations. A model for the dislocation generation and evolution in irradiated UO 2 fuel has been developed and implemented in the mechanistic code MFPR along with a fuel densification model. Being combined with the set of equations describing evolution of point defects (vacancies and interstitials) and their interactions with fission-gas bubbles, a completely self-consistent consideration of the whole system of point and extended defects in irradiated UO 2 fuel has been obtained. The mechanistic description of chemically active-elements behaviour is also presented. It is based on complex association of diffusion–vaporisation mechanism involving multi-phase and multi-component thermo-chemical equilibrium at the grain boundary with accurate calculation of fuel oxidation. Examples of application to the VERCORS 4 and 5 experiments show the possibilities of the code in the frame of severe-accident interpretation.

A new model of grain growth kinetics in UO 2 fuel pellets. Part 1: Grain growth kinetics controlled by grain face bubble migration

A new mechanism of the lenticular grain face bubble migration which controls the bubble mobility and determines the drag force exerted on the grain boundary, is developed. It is shown that besides a more complicated (so called ‘lenticular’) shape of grain face bubbles, the migration mechanism of these bubbles might be essentially different from the intragranular bubbles, owing to their specific location on and interaction with a grain boundary. The model is validated against tests on grain growth kinetics during steady irradiation exposure and during post-irradiation annealing of UO 2 fuel samples, and allows explanation of a strong retarding effect of irradiation on the grain growth observed in these tests.

A new formulation of sink strengths under steady irradiation: recombination and interference effects

The rate theory of irradiation effects in crystalline solids rests on the balance between the production and the annihilation of defects. The annihilation takes place either by mutual recombination or by absorption on sinks. The absorption rate is a function of the sink strength, a factor mainly of geometrical origin attached to each type of sink. Following Doan and Martin [Phys. Rev. B 67 134107 (2003)], an alternative definition of the sink strength is used which, at variance with the classical one, depends on the irradiation conditions and ensures, by construction, a correct partitioning of defect elimination between recombination and absorption on sinks. The correction factors for individual sink strengths can reach several orders of magnitude. Interference effects between sinks can also be introduced and calculated analytically.

Effect of complexation on the photochromism of 5′-(benzothiazol-2-yl)spiroindolinonaphthooxazines in polar solvents

Complexation of Ni(II), Co(II) and Zn(II) with the photomerocyanines obtained under steady irradiation of spiroindolinonaphthooxazines containing benzothiazolyl substituent in the naphthooxazine moiety induces a slight hypsochromic shift of its visible absorption band and drastically slows down its thermal bleaching in the dark. As a result, the relaxation time at 20°C ranges from 2 to 42 min, the actual value depending on the metal ion and compound. High ‘colourabilities’ up to 5 × 10 4 M −1 cm −1 ( ɛ = 6 × 10 4 M −1 cm −1) are calculated from the absorbance values obtained by extrapolation to t = 0 of the room temperature linearly descending kinetic curves ln A/ t. Complexation with Ni(II), Co(II) and Zn(II) allows the spiroindolinonaphthooxazines to isomerize to their open-coloured form even under dark conditions giving a complex spectroscopically identical to the photoinduced product and greatly increasing its lifetime to several days.

Thermally activated and light-induced metal ion complexation of 5′-(hydroxy)spiroindolinonaphthooxazines in polar solvents

Chelation with Al(III), Fe(II) or Cu(II) of the open photomerocyanine form obtained under steady irradiation of spiroindolinonaphthooxazines, with a hydroxyl group at the 5′ position in the naphthooxazine moiety, induces a slight hypsochromic shift of its visible absorption band and increases the lifetime of this form, slowing down its thermal bleaching in the dark (rate constant ∼ 10 −3 s −1). Complexation with Al(III), Fe(II) or Cu(II) allows the spiroindolinonaphthooxazines to isomerize to their open coloured form even under dark conditions giving a complex spectroscopically identical to the photoinduced product. The activation energy of thermal complexation is independent of the metal ion which implies the ring opening as the rate determining step.

Varying temperature effects on mechanical properties of vanadium alloys during neutron irradiation

The varying temperature irradiation experiment in the HFIR was carried out in order to investigate the performance of vanadium alloys subject to temperature variation during operation. No significant differences of irradiation hardening between steady 340 °C irradiation and variable 225/340 °C irradiation could be seen in any alloys. In the case of the irradiation at 520 °C, the temperature variation to 360 °C influenced the formation process of voids in the unalloyed and dilute vanadium alloys compared to isothermal irradiation at 520 °C. It is contributed to the change of irradiation hardening behavior between steady irradiation and temperature variable irradiation at 520 °C. It could not be seen any effect of varying irradiation temperature in vanadium alloys containing >1 wt% titanium in this study. It is caused by the insensitivity of formation process of Ti(OCN) precipitates against the effect of irradiation temperature variation.

Transport of ionizing radiation in terrestrial-like exoplanet atmospheres

The propagation of ionizing radiation through model atmospheres of terrestrial-like exoplanets is studied for a large range of column densities and incident photon energies using a Monte Carlo code we have developed to treat Compton scattering and photoabsorption. Incident spectra from parent star flares, supernovae, and gamma-ray bursts are modeled and compared to energetic particles in importance. Large irradiation events with fluences of 10 6–10 9 erg cm −2 at the conventional habitable zone can occur at a rate from many per day (flares from young low-mass parent stars) to ∼100 per Gyr (supernovae and gamma-ray bursts). We find that terrestrial-like exoplanets with atmospheres thinner than about 100 g cm −2 block nearly all X-rays, but transmit and reprocess a significant fraction of incident γ-rays, producing a characteristic, flat surficial spectrum. Thick atmospheres (≳100 g cm −2) efficiently block even γ-rays, but nearly all the incident energy is redistributed into diffuse UV and visible aurora-like emission, increasing the effective atmospheric transmission by many orders of magnitude. Depending on the presence of molecular UV absorbers and atmospheric thickness, up to 10% of the incident energy can reach the surface as UV reemission. For the Earth, between 2×10 −3 and 4×10 −2 of the incident flux reaches the ground in the biologically effective 200–320 nm range, depending on O 2/O 3 shielding. For atmospheres thicker than ∼50 g cm −2 in the case of pure Rayleigh scattering and ∼100 g cm −2 in the case of O 2/O 3 absorption, the UV reemission exceeds the surficial transmitted ionizing radiation. We also discuss the effects of angle of incidence and derive a modified two-stream approximation solution for the UV transfer. Finally, we suggest that transient atmospheric ionization layers can be frequently created at altitudes lower than the equilibrium layers that result from steady irradiation and winds from the parent star. We suggest that these events can produce frequent fluctuations in atmospheric ionization levels and surficial UV fluxes on terrestrial-like planets.

Effects of the environment on the photochromic behaviour of a novel indeno-fused naphthopyran.

The photochromism of a new chromene-type compound, an indeno-fused naphthopyran, was studied under steady irradiation in solvents of different polarity and/or proticity, in microheterogeneous systems (micelles and gel) and in a nematic liquid crystal. The solutions change from colourless to coloured upon UV irradiation, due to cleavage of the carbon-oxygen pyran bond. The photoreaction is thermally reversible. Spectra, molar absorption coefficients of the colourless and coloured forms, quantum yield of photocolouration and kinetic parameters of the thermal bleaching (rate constant and activation energy) were determined. Compared with other chromenes, the spectra of both the coloured and colourless forms are red-shifted and the coloured form exhibits a marked positive solvatochromism. The photocolourability is good, even at ambient temperature, and is better in a polar and/or protic medium where the entropy loss due to solvent reorganisation around the transition state decreases the rate of the bleaching process, Thus, the best media for colouration are ethanol and the liquid crystals, In the gel (Aerosol-OT [sodium bis(2-ethylhexyl)sulfosuccinate] isooctane-animal gelatin-water) and microemulsion (AOT-isooctane-water) the non-polar ground state molecules (in both the open and closed forms) occupy the hydrophobic sites and therefore the behaviour is similar to that observed in the pure hydrocarbon.

Role of starch in carbon translocation and partitioning at the plant level

Endogenous regulation of translocation and of carbon partitioning, major factors for integrating plant function, depend on diurnal regulation of starch synthesis and mobilization. Regulated diurnal cycling of transitory starch provides a steady carbon supply to sink growth and avoids potentially adverse high sugar levels. Carbon availability from starch affects development and alters carbon availability with respect to nitrogen. Along with sugar sensing, the level and turnover of starch are involved in endogenous regulation in response to carbohydrate status. Despite their key roles in plant metabolism, mechanisms for endogenous regulation of starch synthesis and degradation are not well characterized. Time course studies with labeled carbon reveal endogenous diurnal regulation of starch metabolism, by which sucrose synthesis from starch and newly-fixed carbon are mutually regulated in support of translocation at night, under low light, and during periods of water stress. Even under steady irradiance, which supports photosynthesis at midday levels, starch synthesis begins gradually and slows under an end-of-day circadian regulation that anticipates the dark period. Studies with Arabidopsis mutants identified two requisite components of starch mobilization, endoamylase, and glucose transport across the chloroplast inner envelope. Time course studies of carbohydrate levels and labeling studies of plant-level carbon metabolism in mutant plants with impaired ability to mobilize starch identified steps in starch mobilization that support diurnal regulation of translocation. Endogenously regulated exit of glucose across the chloroplast membrane appears to regulate starch mobilization.

On the theory of fission gas bubble evolution in irradiated UO 2 fuel

The standard approaches for modelling of the intra- and intergranular bubbles evolution in the UO 2 fuel are critically analysed on the basis of available experimental data. It is demonstrated that the main disadvantage of the simplified treatment of the problem by the standard models can be associated with underestimation of the radiation effects at low temperatures (below 1500°C) and thermal effects at high temperatures (above 1500°C). The presented analysis allows a quantitative description of the bubble nucleation mechanism, adequate modelling of the bubble diffusion growth, and evaluation of the intragranular bubble number density and stable size attained under steady irradiation conditions.

Spectrokinetic study of 2,2-diphenyl-5,6-benzo(2H)chromene: a thermoreversible and photoreversible photochromic system

In this paper the photochromic reaction of the 2,2-diphenyl-5,6-benzo(2H)chromene is investigated by spectrophotometric methods using monochromatic irradiation wavelengths in the UV and visible ranges. Two coloured species are produced upon irradiation, one is thermoreversible, the other is photoreversible. The colour-forming kinetics were followed under steady irradiation using UV light (357 nm). The kinetic parameters (rate coefficient and activation energy) of the thermal ring-closure reaction were determined. The photochemical bleaching was induced by exciting with visible light (422 nm). To determine reaction quantum yields and molar absorption coefficients of the metastable species, both photostationary and photokinetic methods were employed. Based on the results obtained, a plausible reaction mechanism is proposed

Models of Matter Transport: Their Verification and Use for Dilute Systems with the Aid of Statistical Theories

AbstractStatistical expressions for the Onsager phenomenological coefficients are now available for several models of matter transport in systems which are dilute in solutes, vacancies and interstitials. They describe transport by unpaired and paired defects, as in the familiar five‐frequency model. The role of atomistic calculations of the parameters of such models in validating, simplifying or improving them is illustrated. In general it may be difficult to convincingly determine from experiment all the parameters of a given model or to distinguish between closely related models, such as between those retaining first or first and second neighbour binding between a solute and a vacancy. Nevertheless, there is particular utility in knowledge of all the Onsager coefficients for models suggested by atomistic calculations and other information, when one interprets phenomena involving coupled fluxes. This is illustrated by the example of coupled fluxes of a solute, vacancies and interstitials in a dilute alloy under steady irradiation by high energy particles. Here, essential contributions to modelling solute segregation to grain boundaries can be made; striking qualitative differences between closely related models can occur in this case. Finally, the limitations of the current statistical results are examined for systems where the solutes and defects carry effective charges. Limitations are suggested at low temperatures by the existence of a phase transition in the restricted primitive model of dilute aqueous electrolytes. The range of concentration and temperature within which the current procedures, stemming from the early work of Teltow and Lidiard, have been properly validated is limited. In the absence of recent theoretical advances Monte Carlo simulation is becoming the method of choice in investigating such limitations and in studying the transition to higher concentrations where there are larger clusters of defects.

Kinetic Analysis of the Photochromic Behavior of a Naturally Occurring Chromene (Lapachenole) Under Steady Irradiation

The photokinetic behavior of a naturally occurring chromene, lapachenole (2,2-dimethyl-6-methoxy-7,8-benzo(2H)chromene, Ch), has been investigated by using monochromatic irradiation in the UV and visible ranges. Two colored photoproducts were identified: one is thermoreversible, the other is thermostable and photoreversible. The quantum yields of the color-forming and color-bleaching photoreactions were determined by a spectrokinetic method. The kinetic parameters of the thermal steps were measured (285 – 325 K). The sensitized color-forming reaction was also investigated, using thioxantone as a triplet sensitizer. A plausible reaction mechanism is proposed.

Prediction of spur overlap time, radical yield profiles, and decomposition of trichloroethylene induced by various pulse types of electron beam

A kinetic model was suggested to compute the yield profiles of primary radicals generated from water radiolysis. For various cases including pulse radiolysis and steady irradiation time of spur overlap was computed in order to ensure homogeneity over the entire system. As a result, consistency to roughly first order kinetics was resulted for decomposition of 1 ppm trichloroethylene (TCE) and slight deviation from the linear model was predicted for 10 ppm TCE.

Photosensitization of photochromism of spiro-indoline-oxazines by camphorquinone

The thermoreversible photochromic reaction spiro-indoline-oxazine ⇌ photomerocyanine was photosensitized by triplet energy transfer using camphorquinone ( E T = 215 kJ mol −1) as sensitizer and visible light excitation ( λ exe = 436 nm). From absorbance/time data, obtained during steady irradiation, the quantum yield of the photosensitized reaction was determined. The rate parameter for the donor-acceptor interaction was obtained from quenching measurements of the sensitizer phosphorescence and from the effect of the acceptor concentration on the reaction yield. The results obtained for six photochromes (spiro-indoline-oxazines differently substituted in the indoline and/or oxazine moiety) show that energy transfer occurs at a diffusion-controlled rate and that the limiting quantum yield of the sensitized reaction, approximately unity, is greater than the yield obtained by direct excitation of the photochrome. An upper limit to the triplet energies of the spiro-oxazines was established. Direct evidence for the triplet state was only obtained for a nitro-spiro-oxazine from low temperature phosphorescence emission of both the closed and open forms.

Photoinduced reduction of W 10O 324− by organic compounds in aqueous solution

The photoreduction of decatungstate anon (W 10O 32 4−, DTA), a representative polyoxotungstate, was studied in the presence of several types of organic substrate in aqueous solution. The complete conversion of DTA to DTA 2− was observed on steady irradiation in the presence of certain organic substrates in aqueous solution. Colour-selective irradiation showed that the formation of DTA 2− is due to the photoinduced reduction of excited DTA − by organic molecules. Laser flash photolysis with benzyl alcohol, phenol and terephthalaldehyde exhibited different time profiles of electron transfer. Prompt formation and second-order decay of DTA − at the rate of the diffusion limit were observed. The quantum yield of reduction changed as a function of the amount of substrate, indicating the formation of a complex with organic molecules before photoexcitation. Proton nuclear magnetic resonance (NMR) measurements supported complex formation. DTA was suggested to play a major role in the photoinduced reactions of aqueous colloidal WO 3.