Absorption Coefficient Of Radiation Research Articles

Monitoring the temperature of an external object from the optical absorption of a solid-state sample cooled by laser radiation

This paper proposes a method of determining the temperature of an external object from the optical absorption in a solid-state sample cooled by laser radiation. A system of kinetic equations is obtained that describes the process of laser cooling of an impurity-containing solid body and its solution in the high-temperature approximation. An expression is obtained for the final cooling temperature of the sample in terms of the temperature of the vacuum chamber that surrounds it, which can be thermally coupled with the external sample. It is shown that, by measuring the absorption coefficient of the pump radiation in the process of laser cooling, it is possible to determine the temperature of the cooled sample, and consequently the temperature of the external object.

Nanodipoles for an optical phased array

Two realizations of an optical electric dipole emitter are considered in the application to a dipole optical phased array project. Realization 1 is a dielectric disc in which a leaky wave of the E0m0 type is excited. Realization 2 is a piece of nanowire of a well-reflecting metal (Ag) coated with a dielectric layer (ZnO). In the second case, a surface plasmon is excited. Excitation of the surface plasmon is shown to be also possible in the case where the permittivity of the coating exceeds the absolute value of the permittivity of the metal ɛ. The permittivity of the surrounded medium is less than |ɛ|. There is a restriction to the thickness of the ZnO coating. Losses in the disc and coated nanowire are calculated. In the former case, the losses are due to radiation; in the latter case, to losses in the metal and to radiation. The radiation absorption coefficients are assessed to be rather high, ∼105 cm−1.

Artificial neural network approach for evaluation of temperature and density profiles of salt gradient solar pond

AbstractThe purpose of this study is to evaluate temperature and density profiles of an experimentally investigated salt gradient solar pond (SGSP) by using artificial neural network (ANN). The input parameters of the ANN are solar pond depth, ambient temperature, radiation absorption coefficient of salty solution in the pond, initial density values of the pond and time of day. The output parameters of the ANN are temperature and density profiles in the pond. The experimental data set consists of 168 values. These divided into two groups, of which the 134 values were used for training/learning of the network and the rest of data (34 values) for testing/validation of the network performance. According to the ANN predicted results compared to the experimental results, the mean relative error (MRE) is 2·30% for temperature and 0·63% for density. The correlation coefficients (R2) between the experimentally measured and the ANN predicted results are 0·9632 for temperature and 0·9855 for density in the test/val...

Instantaneous radiant emittance of the accelerating non-stationary black hole

Using entropy density near event horizon of the accelerating non-stationary black hole, the instantaneous radiant emittance is calculated, and we arrive at the conclusion that the instantaneous radiant emittance of black hole in a direction at any time is always proportional to the quartic power of temperature of the event horizon of black hole in that direction. It is found that the generalized Stefan-Boltzmann coefficient is no longer a constant, but a coefficient dynamically related to the rate of change of event horizon, the structure of space-time near event horizon and the radiation absorption coefficient of the black hole. It shows that an intrinsic relation between the gravitational field around the black hole and its thermal radiation must exist.

Radiative Ignition of Fine‐Ammonium Perchlorate Composite Propellants

AbstractRadiative ignition of quasi‐homogeneous mixtures of ammonium perchlorate (AP) and hydroxyterminated polybutadiene (HTPB) binder has been investigated experimentally. Solid propellants consisting of fine AP (2 μm) and HTPB binder (~ 76/24% by mass) were ignited by CO2 laser radiation. The lower boundary of a go/no‐go ignition map (minimum ignition time vs. heat flux) was obtained. Opacity was varied by adding carbon black up to 1% by mass. Ignition times ranged from 0.78 s to 0.076 s for incident fluxes ranging from 60 W/cm2 to 400 W/cm2. It was found that AP and HTPB are sufficiently strongly absorbing of 10.6 μm CO2 laser radiation (absorption coefficient ≈250 cm−1) so that the addition of carbon black in amounts typical of catalysts or opacitymodifying agents (up to 1%) would have only a small influence on radiative ignition times at 10.6 μm. A simple theoretical analysis indicated that the ignition time‐flux data are consistent with in‐depth absorption effects. Furthermore, this analysis showed that the assumption of surface absorption is not appropriate, even for this relatively opaque system. For broadband visible/near‐infrared radiation, such as from burning metal/oxide particle systems, the effects of in‐depth absorption would probably be even stronger.

QED effects in a cavity with a time-dependent thin semiconductor slab excited by laser pulses

We consider the problem of the creation of quanta of an electromagnetic field from the initial vacuum (or thermal) state in a closed high-Q cavity due to periodical variations of conductivity of a thin semiconductor boundary layer excited by short laser pulses. Fast changes of conductivity from practically a zero value to a high one and then again to zero simulate periodical displacements of the cavity wall. This scheme has been chosen to model the non-stationary Casimir effect in the experiment which is under preparation at Padua University. We provide analytical and numerical evaluations for the number of photons which could be created under realistic experimental conditions. We show the importance of taking into account intrinsic losses in the semiconductor slab caused by the finite conductivity during the intermediate part of the excitation–recombination cycle. We analyse the influence of different parameters, such as the diffusion and mobility coefficients of carriers, surface recombination velocity, absorption coefficient of laser radiation, thickness of the slab and geometry of the cavity. We conclude that a significant amount (>103) of ‘Casimir photons’ with a frequency of 2.5 GHz can be produced from vacuum in a cavity with dimensions of the order of 10 cm, if one can arrange several thousand strongly periodical laser pulses with a duration of the order of 1 ps, periodicity close to 200 ps and energy ∼10−3 J, illuminating the semiconductor slab of thickness ∼1 mm and the mobility ∼1 m2 V−1 s−1, provided the recombination time can be reduced below the critical value ∼30 ps.

Growth of mercuric iodide crystals

Mercuric Iodide (HgI2) is a semiconductor candidate for the construction of X- and gamma-ray detectors for digital medical imaging due to its high atomic number (Z Hg = 80, Z I = 53). Also, HgI2 has a wide optical band-gap (2.13 eV) and high photon absorption coefficient for high-energy radiation. Different structures can lead to varying electrical and optical properties of the final material. In this work, HgI2 crystals were produced by the solvent evaporation technique. The solvents used were ethanol (solubility around 20 mg/ml at 25ºC), ether (solubility around 3.5 mg/ml at 25ºC) and acetone (solubility around 24mg/ml at 25ºC). The evaporation conditions were varied in order to produce different final crystals. The Bérnard cells are responsible for crystallites formation due to the Bérnard-Maragoni convection in the liquid. Millimeter-sized crystals can be obtained as seen by Scanning Electron Microscopy.

Optical and physicochemical properties of silica‐supported TiO2photocatalysts

AbstractCommercial applications of photocatalysis in slurry reactors employing titanium dioxide particles present the disadvantage of the additional cost associated with the downstream catalyst separation. In past years a significant effort has been made to develop supported titania photocatalysts on particles of larger size in order to facilitate the sedimentation recovery process. In this work, two different silica materials have been used for preparing immobilized catalysts. Their physicochemical characteristics have been compared with those corresponding to two commercially available unsupported titanium dioxide catalysts. However, for reaction kinetics and reactor design purposes, three additional properties must be known: (i) the spectral specific radiation absorption coefficient, (ii) the spectral specific radiation scattering coefficient, and (iii) the asymmetry factor of a model for the angular distribution of the scattered photons (the phase function). They have been determined with specially designed spectrophotometer transmission, diffuse transmittance, and diffuse reflectance experiments and constitute the most significant addition to this contribution. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Prediction of theStandard AtmosphereProfiles of Temperature, Pressure, and Density with Height for the Lower Atmosphere by Solution of the (S−S) IntegralEquations of Transferand Evaluation of the Potential for Profile Perturbation by Combustion Emissions

This analytical solution, believed to be original here, to the 1D formulation of the (1905−1906) integral (S−S) Equations of Transfer, governing radiation through the atmosphere, is developed for future evaluation of the potential impact of combustion emissions on climate change. The solution predicts, in agreement with the Standard Atmosphere experimental data, a linear decline of the fourth power of the temperature, T4, with pressure, P, and, at a first approximation, a linear decline of T with altitude, h, up to the tropopause at about 10 km (the lower atmosphere). From these two results, with transformation using the Equation of State, the variations of pressure, P, and density, ρ, with altitude, h, are also then obtained, with the predictions again, separately, in substantial agreement with the Standard Atmosphere data up to 30 km altitude (1% density). The analytical procedure adopts the standard assumptions commonly used for numerical solutions of steady state, one dimensionality, constant flux directional parameter (μ), and a gray-body equivalent average for the effective radiation absorption coefficient, k, for the mixed thermal radiation-active gases at an effective (joint-mixture) concentration, p. Using these assumptions, analytical closure and validation of the equation solution is essentially complete. Numerical closure is not yet complete, with only one parameter at this time not independently calculated but not required numerically for validation of analytical closure. This is the value of the group-pair (kp)o representing the ground-level value of (kp), the product of the effective absorption coefficient and concentration of the mixed gases, written as a single parameter but decomposable into constituent gases and/or gas bands. Reduction of the experimental value of (kp)o to values of k for a comparison with relevant band data for water and CO2 shows numerical magnitudes substantially matching the longest wavelength bands for each of the two gases. Allowing also for the maximum absorption percentages, α°, of these two bands for the two gases, respectively, 39% for water and 8.5% for CO2, these values then support the dominance of water (as gas and not vapor) at about 80%, compared with CO2 at about 20%, as the primary absorbing/emitting (“greenhouse”) gas in the atmosphere. These results provide a platform for future numerical determination of the influence on the T, P, and ρ profiles of perturbations in the gas concentrations of the two primary species, carbon dioxide and water, and it provides, specifically, the analytical basis needed for future analysis of the impact potential from increases in atmospheric carbon dioxide concentration, because of fossil-fuel combustion, in relation to climate change.

DETERMINATION OF THE INFLUENCE OF UNCERTAIN MODEL PARAMETERS IN PRESSURIZED GASIFICATION OF BLACK LIQUOR USING A FACTORIAL DESIGN

ABSTRACT Introduction of pressurized gasification of black liquor in the pulping industry has the potential to give a significant increase in energy efficiency. However, uncertainties about the reliability and robustness of the technology are preventing large-scale market introduction. One important step toward a greater trust in the process reliability is the development of a better understanding of the sensitivity of the process to parameter variations. A computational fluid dynamics model for pressurized gasification of black liquor in an entrained-flow gasifier is presented and used for investigation of the effects of uncertainties in the specific heat capacity of black liquor, the radiation absorption coefficient, and the volatile devolatilization rate using factorial design methodology. It is found that all main factor effects, but none of the interaction effects, influence the considered responses: char conversion, maximum temperature, and outlet temperature. However, the main effects are found to be relatively small and the uncertainties in the examined model parameters would not invalidate the results from a design optimization with the presented model.

G231 フォトクロミックダイを利用した選択レーザー加熱手法の検討(OS-13 材料加工における熱工学関連技術III)

This paper deals with a local laser heating technique using photochromic-dye. He-Cd laser was used as excitation light source to actively control the radiation absorption coefficient of the dye added polymer material, and Ar^+ laser was used as heating light source. Both experimental and numerical simulation studies were carried out to elucidate the characteristics of the present technique. The results showed that the radiation absorption coefficient distribution in the sample thickness direction and the color attenuation effect of the dye during the heating had large effect on the temperature increase. According to the obtained knowledge, laser transmitting welding process for transparent polymer material was successfully realized.

Infrared Quenching of Electroluminescence in ZnS : Mn Thin-Film Electroluminescent Structures

Electroluminescence from thin-film electroluminescent devices is found to be quenched after IR irradiation of the devices in the interval between exciting voltage pulses. The IR irradiation decreases the emission intensity in the spectral range 530–540 nm, while increasing it between 640 and 690 nm. These effects are explained by IR-induced charge exchange between the deep centers due to V S 2+ and V S + sulfur vacancies, an increase in the concentration of the latter vacancies, and the redistribution of the channels of impact excitation of Mn2+ and V S + centers in favor of V S + centers. The cross section and rate of impact excitation of V S + centers, the photoexcitation cross section for V S 2+ centers, the IR radiation absorption coefficient, the internal quantum efficiency of electroluminescence, and the probability of radiative relaxation of Mn2+ centers, as well as the electron multiplication factor in the phosphor layer, are evaluated.

Plasma behind the front of a damage wave and the mechanism of laser-induced production of a chain of caverns in an optical fibre

The properties of the plasma behind the front of a damage wave generated by laser radiation in an optical fibre are considered. A plasma with a low degree of ionisation but a relatively high electron density is shown to emerge. However, the high absorption coefficient of laser radiation at a temperature of the order of 2000 K cannot be attributed to the presence of bremsstrahlung. The production of a chain of uniformly spaced caverns during the laser damage of the optical fibre is qualitatively explained. It is shown that this effect cannot be explained by the capillary Rayleigh instability because of the high viscosity of the glass. It is found that the fibre core deformation by a high pressure leads to an increase of the fibre volume sufficient to account for the emergence of the caverns after cooling. It is assumed that the periodicity of caverns is caused by the instability of a new type. A high-density double electrical-charge layer is produced at the plasma—liquid interface. Due to the repulsion of similar charges, the surface tends to increase, resulting in the instability development responsible for the production of the chain of caverns.

Measurement of radiative absorption coefficient for a vegetal medium

The radiative heat flux received by vegetation is a leading process of the propagation of forest fires. This flux is a function of the radiative absorption coefficient. We have measured this coefficient for Quercus coccifera. The variation of the values of the absorption coefficient as a function of the load (surface density of vegetation) has been studied. We have shown that the absorption coefficient is proportional to the load; the proportionality coefficient depends upon the nature of the vegetation.

Radiation absorption coefficients in arc plasmas

In the present paper, attention is given to the calculation of spectral coefficients of absorption and their comparisons with other sources of data for various types of the are plasmas. The coefficients of absorption have been calculated for various kinds of are plasmas (sulphur hexafluoride, hydrogen, oxygen) at different plasma temperatures and pressures above 1 bar. Photon absorption cross sections have been calculated using an approximation method of quantum defect. For the contributions from the line radiation, oscillator line strengths, widths and shifts of neutral and ionized atoms have been calculated using semi-empirical formulas. Curves are given for the absorption coefficients of radiation for wavelengths of 10 to 3000 nm.

A new concept of active temperature control for an injection molding process using infrared radiation heating

AbstractThis paper deals with an innovative injection molding process that uses infrared radiation heating for active temperature control of the molded polymer. The feasibility of the proposed technique was investigated through numerical simulation and experimental study. By directly heating the molded polymer with radiation energy, precise and rapid temperature control and a small effect on the cooling duration were expected. Zinc‐selenide and a CO2 laser were used as a transparent mold window and radiation energy source, respectively. In the numerical simulation, temperature distribution both in the molded polymer and the mold wall was evaluated for various molding conditions. The results showed that the heating extent of the molded polymer varied with radiation intensity, radiation absorption coefficient, and melt flow rate. It was also estimated that extension of the cooling duration with the present technique was much shorter than that of a conventional conductive heating technique. In the experimental study, the present technique was applied to actively control the melt temperature and to improve the quality of molded products. As a result, apparent reduction of the residual molecular orientation and considerable improvement of the surface transcription were successfully realized.

Photoinduced Localization of Orientationally Ordered Polymer Networks at the Surface of a Liquid Crystal Host

We demonstrated liquid crystalline template-directed and photoinduced localization of orientationally ordered polymer networks in a liquid crystal host. In the limit of a diffusion-controlled polymerization process, the location of these networks in a liquid crystal (LC) cell can be controlled by varying the wavelength-dependent absorption coefficient of the UV radiation used to initiate photopolymerization. Specific results are presented for a bulk polymer network (obtained when λ = 365 nm) and a micrometer-thick polymer surface layer (for λ = 322 nm), both of which are produced in similar LC cells, using the same 5 wt % initial concentration of a liquid crystalline diacrylate, and for both planar and homeotropic orientations of the liquid crystal director. The surface-localized networks have minimal impact on the desirable electrooptic properties of the host, such as low operating voltage and minimal scattering loss. We discuss predictive models for surface localization in the opposite limits of “infinite” and zero monomer diffusion constant.

The band gap in silicon nanocrystallites

The gap in semiconductor nanocrystallites has been extensively studied boththeoretically and experimentally over the last two decades. We have compared arecent ‘state-of-the-art’ theoretical calculation with a recent ‘state-of-the-art’experimental observation of the gap in Si nanocrystallite. We find that the two arein substantial disagreement, with the disagreement being more pronounced atsmaller sizes. Theoretical calculations appear to overestimate the gap.To reconcile the two we present two scenarios. (i) Recognizing that theexperimental observations are for a distribution of crystallite sizes, we proffer aphenomenological model to reconcile the theory with the experiment. We suggestthat similar considerations must dictate comparisons between the theoryand experiment vis-à-vis other properties such as radiative rate, decayconstant, and absorption coefficient. (ii) Either surface passivation orsurface orientation may also resolve the conflict between the theory and theexperiment. We have carried out tight-binding calculations on siliconclusters to study the role of surface passivation and surface orientation.

Inverse bremsstrahlung of hot, weakly coupled plasmas

The free–free absorption coefficient for radiation in hot, weakly coupled plasmas is determined from a systematic approach to the dynamical conductivity. Based on a generalized linear-response theory, it is expressed in terms of determinants of equilibrium correlation functions permitting a perturbative treatment. Within a Gould–DeWitt-type approach, dynamical screening is accounted for by a partial summation of loop diagrams, while strong collisions are treated by a ladder approximation. Known expressions for the absorption coefficient are reproduced when considering certain limits. A comparison is made with simulation results.

Collective versus single-particle effects in the optical spectra of finite electronic quantum systems

We study optical spectra of finite electronic quantum systems at frequencies smaller than the plasma frequency using a quasiclassical approach. This approach includes collective effects and enables us to analyze how the nature of the (single-particle) electron dynamics influences the optical spectra in finite electronic quantum systems. We present an analytical expression for the low-frequency absorption coefficient of electromagnetic radiation in a finite quantum system with regular ballistic electron dynamics; a two-dimensional electron gas confined to a strip of width a with specular reflection at the boundaries (our approach is not restricted to systems with regular electron dynamics; it applies equally in the case of diffusive or classically chaotic electron motion). By comparing with results of numerical computations using the random-phase approximation we show that our analytical approach provides a qualitative and quantitative understanding of the optical spectrum.