Re-emission Processes Research Articles

Hydrogen permeability of amorphous and recrystallized iron alloys

The hydrogen permeability of amorphous and recrystallized Fe alloys is investigated. It is found that the surface layers of both samples are enriched with metalloids, which interfere with the penetration of hydrogen from the molecular and atomic phases. The kinetics of curves of flow transition to a steady state upon ion irradiation in an amorphous alloy points to processes of reversible capture. The penetrating flow demonstrates Arrhenius dependence on temperature in the case of recrystallized samples and nonmonotonic dependence on temperature in the case of amorphous samples. A model of hydrogen penetration is suggested, which includes the reemission and diffusion processes; and estimates are obtained of the energy of activation of thermodesorption and diffusion.

Mössbauer optics of synchrotron radiation at an isotopic boundary

The inelastic coherent Mossbauer scattering (ICMS) of synchrotron radiation at an isotopic boundary—a flat interface between two regions of matter which have different concentrations of the Mo ssbauer isotope—is investigated theoretically. Attention is focused primarily on the ICMS component for which the absorption of a synchrotron radiation photon by a nucleus occurs with recoil, i.e., with the creation or annihilation of lattice phonons, and the subsequent process of reemission of a photon by the Mossbauer nucleus occurs without recoil, as a result of which radiation is pumped from the wide synchrotron radiation line into the narrow Mo ssbauer line. Formulas similar to the Fresnel formulas, well known in optics, for the transmission and reflection of light at a dielectric boundary are obtained for ICMS at an isotopic boundary. Specifically, it is shown that the angle of reflection for ICMS at an isotopic boundary is different from the angle of mirror reflection of a synchrotron radiation beam, and the direction of the ICMS transmitted through the isotopic boundary depends on the deviation of its frequency from the exact value of the Mossbauer resonance frequency and in general is different from the direction of propagation of the synchrotron radiation beam. The suppression of ICMS at grazing angles of incidence of the synchrotron radiation beam is analyzed. A similar problem is solved for a plate-shaped sample containing a Mossbauer isotope. It is shown that the specific nature of the ICMS at an isotopic boundary could be helpful in the problem of Mo ssbauer filtering of synchrotron radiation.

An analytical approach to the description of absorption and reemission effects in large scintillation counters

It is well known that the absorption–reemission process in scintillators plays an important role in determining the features of the detected photons. In this paper a methodology capable of leading to a quantitative assessement of such an effect is described, extending the usual simplified approach to the complex case of large size detectors. In particular, the mathematics describing the phenomenon is thoroughly developed for the special geometry of spherical detectors.

Electron spreading effects in quantum well pixelless imagers

The performance of pixelless imaging device based on an integrated quantum well photodetector and light-emitting diode is studied theoretically. It is shown that the photon reabsorption and reemission processes substantially influence the electron spatial distribution and limit the quality of output image.

On the possible enhancement of the magnetic field by neutrino reemission processes in the mantle of a supernova

URCA neutrino reemission processes under the conditions in the mantle of a supernova with a strong toroidal magnetic field are investigated. It is shown that parity violation in these processes can be manifested macroscopically as a torque that rapidly spins up the region of the mantle occupied by such a field. Neutrino spin-up of the mantle can strongly affect the mechanism of further generation of the toroidal field, specifically, it can enhance the field in a small neighborhood of the rigid-body-rotating core of the supernova remnant.

Evoked otoacoustic emissions arise by two fundamentally different mechanisms: a taxonomy for mammalian OAEs.

Otoacoustic emissions (OAEs) of all types are widely assumed to arise by a common mechanism: nonlinear electromechanical distortion within the cochlea. In this view, both stimulus-frequency (SFOAEs) and distortion-product emissions (DPOAEs) arise because nonlinearities in the mechanics act as "sources" of backward-traveling waves. This unified picture is tested by analyzing measurements of emission phase using a simple phenomenological description of the nonlinear re-emission process. The analysis framework is independent of the detailed form of the emission sources and the nonlinearities that produce them. The analysis demonstrates that the common assumption that SFOAEs originate by nonlinear distortion requires that SFOAE phase be essentially independent of frequency, in striking contradiction with experiment. This contradiction implies that evoked otoacoustic emissions arise by two fundamentally different mechanisms within the cochlea. These two mechanisms (linear reflection versus nonlinear distortion) are described and two broad classes of emissions--reflection-source and distortion-source emissions--are distinguished based on the mechanisms of their generation. The implications of this OAE taxonomy for the measurement, interpretation, and clinical use of otoacoustic emissions as noninvasive probes of cochlear function are discussed.

Study on Efficiency of Multi-Wire Tungsten Moderator for Slow Positron Source on SPring-8 Hard Synchrotron Radiation

The paper provides results of numeric simulations of in-target positron production process, processes of moderation, thermalization, diffusion, and reemission of positrons in high efficiency multi-wire moderator made of tungsten monocrystalline wire with regular wire spacing. The paper looks into dynamics of slow positrons in the moderator's vacuum gaps taking into account of external fields. The possibility for using multi-wire moderator with non-regular structure - multi-layer “wire felt” moderator is discussed. According to maximal estimate the multi-wire moderators can reach very high efficiency of fast-slow positron transformation ∼10-2. Using such moderator the intensity of slow positron source on hard synchrotron radiation of SPring-8 can reach the level of ∼1011 e+/s.

Evaluation of Sink Effects on VOCs from a Latex Paint

ABSTRACT The sink strength of two common indoor materials, a carpet and a gypsum board, was evaluated by environmental chamber tests with four volatile organic compounds (VOCs): propylene glycol, ethylene glycol, 2-(2-butoxyethoxy)ethanol (BEE), and Texanol. These oxygenated compounds represent the major VOCs emitted from a latex paint. Each chamber test included two phases. Phase 1 was the dosing/sorption period during which sink materials (pieces of carpet and gypsum board samples) were exposed to the four VOCs. The sink strength of each material tested was characterized by the amount of the VOCs adsorbed or absorbed. Phase 2 was the purging/de-sorption period during which the chambers with the dosed sink materials were flushed with purified air. The reemission rates of the adsorbed VOCs from the sinks were reflected by the amount of the VOCs being flushed. Phase 1 results indicated that the sink strength for the four target compounds is more than 1 order of magnitude higher than that for other VOCs previously tested by the U.S. Environmental Protection Agency (EPA). The high sink strength reflected the unusually high sorption capacity of common indoor materials for the four VOCs. Phase 2 results showed that reemission was an extremely slow process. If all the VOCs adsorbed were reemittable, it would take more than a year to completely flush out the VOCs from the sink materials tested. The long reemission process can result in chronic and low-level exposure to the VOCs after painting interior walls and surfaces.

Indirectly driven target design influence on energetic balance of inertial confinement fusion power plant

Abstract A detailed energy flow diagram for indirect drive ICF was elaborated. Using driver pulse energy as input, this diagram undergoes various energy processes of conversion, absorption, re-emission and transfer, until the thermonuclear energy delivered by target fusion is reached as output. Each process was identified with an energetic parameter (efficiency or gain). Starting from this diagram it was possible to analyse the influence of each energetic parameter on the energetic balance of ICF power plant. In this paper special attention was put on the influence of target design (materials and dimensions) on the transfer efficiency and on capsule illumination uniformity. The analysis led to the evaluation of optimal target materials and dimensions to achieve a positive energetic balance of an ICF power plant.

Profile simulation of conformality of chemical vapor deposited copper in subquarter-micron trench and via structures

Copper profile evolution in ultralarge scale integration via and trench structures was investigated for thermal low pressure, low temperature, chemical vapor deposition (LPCVD) from CuI(tmvs)(hfac). The investigation examined copper profiles in specialized cantilever structures as a function of systematic changes in key processing conditions, namely, substrate temperature, precursor flux, and hydrogen reactant flow. Resulting experimental observations from cross section scanning electron microscopy were incorporated in a fast analytical simulator, using a two-dimensional adsorption/re-emission model, to simulate copper profile evolution. The deposition profiles were simulated using a single rate limiting precursor model. A comparison of simulation results and actual experimental profiles for thermal LPCVD copper showed that species re-emission within the via and trench structures play a critical role in achieving conformal step coverage and complete filling. In additional, precursor flux and substrate temperature were identified as the dominant parameters in the species re-emission process, with the probability for re-emission being inversely proportional to substrate temperature and directly proportional to precursor flux. The results of this study were employed in the development of an optimum LPCVD process window for complete copper filling of aggressive via and trench structures at growth rates above 2000 Å/min and as-deposited resistivity below 2.0 μΩ cm.

Aspects of postcollision interactions near the ArLshell

In the present work we are interested in near-threshold photoionization experiments involving postcollision effects related to the Auger decay of a vacancy in the Ar $L$ shell. In particular, we have measured the photoelectron energy spectrum resulting from the above postcollision interaction effects and have observed electrons produced by the process of electron capture and reemission.

Retention and re-emission processes of hydrogen isotopes in graphite and beryllium

The mass balance equations and the rate constants of elementary processes related to retention and re-emission of hydrogen isotopes have been described for evaluation of their transient recycling fluxes from targets irradiated simultaneously with H + and D +. The D/H ratios of the concentrations of H and D retained at steady state calculated under three different conditions are compared with the experimental values for graphite and beryllium measured by means of the elastic recoil detection technique. The retention and re-emission processes of hydrogen isotopes in graphite and beryllium are discussed.

Measurements and modeling of tritium reemission rates after HTO depositions at sunrise and at sunset

In a series of small-scale field studies, two experiments were carried out on reemission after 1-h HTO deposition phases, one at sunrise (7.00 a.m.) and one at sunset (9.00 p.m.), on two subsequent days. In both cases, wind speeds were extremely low to absolutely calm during the observed 12-h reemission periods. Both sunrise and sunset experiments showed the same reemission rate of 28% h −1 during the first hour of reemission, in spite of the differences of the atmosphere/soil conditions. Time-of-day specific deviations of the reemission rates appeared only during the subsequent hours, leading to total losses of the deposited HTO of 84% during the 12-h day-phase after the sunrise deposition and of 57% during the 12-h nightphase after the sunset deposition. The experimental conditions were used to model the time courses of the reemission rates by the appropriate module of the UFOTRI code, which is characterized by a coupling of HTO reemission to H 2O evaporation and a set of default values, where more detailed information is not available. The comparison shows a rather poor agreement, as neither the high first-hour values nor the subsequent strong reductions can be reproduced. Agreement is improved, however, after modification of the default values. The findings suggest that at least the initial reemission rate should be modeled independently from the H 2O evaporation rate, and the spatial resolution of multilayer soil models should be of the same order of magnitude as the scaling length of the deposition profile, i.e. some 10 −3 m, characterizing the penetration depth of HTO after 1h of deposition and approximating the thickness of the uppermost soil layer that releases tritium during the first hour of the reemission process.

Photon transport in luminescent solar concentrators

The theory of radiative energy transfer in the stationary state in a reflecting luminescent layer is applied to solve the problem of photon transport in planar single dye luminescent solar concentrators. Efficiency yields of reabsorption and reemission processes are investigated. The optical efficiency of the collector is increased by secondary and higher reemission processes, but decreased by the reabsorption of photons transported through the concentrator. The average optical path length of luminescent photons is introduced, and the transported spectral photon density at the edges is calculated and determined experimentally. The results permit the conclusion that the mean photon transport length is proportional to the square root of surface area of the concentrator for all geometries.

The importance of gas scattering processes on the stoichiometry deviations of laser deposited films

Laser ablation of a BiSrCaCuO (2212) target is performed in vacuum, both in reactive (O2) and inert (Ar) atmospheres (0.1 mbar). Two substrates are properly located in the chamber in order to collect material from the scattering of the ablated species by the foreign atmosphere and from the re-emission and/or re-sputtering processes occurring in the growing films. The results show that the latter processes are very weak and may affect to 3% of the deposited material at the most. Whereas films deposited in vacuum are nearly stoichiometric, significant deviations are observed in the composition of films grown in the presence of a foreign gas, that are qualitatively similar in Ar and in O2 environments. The results show clearly that these deviations are related to scattering processes prior to deposition and not to chemical reactions in the gas phase or re-emission processes from the growing film. These scattering processes are more important for the lighter species, thus explaining the Bi enrichment of the films in the presence of a foreign gas.

Comprehensive numerical simulation of defect density and temperature-dependent transport properties in hydrogenated amorphous silicon.

In this work the mobility-lifetime products of electrons and holes, (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{e}}$ and (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{h}}$, the response time of the photocurrent ${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$ and the electron drift mobility ${\mathrm{\ensuremath{\mu}}}_{\mathit{d}\mathit{e}}$ have been numerically simulated as a function of the dangling bond density ${\mathit{N}}_{\mathit{d}}$ and temperature T in hydrogenated amorphous silicon. We have considered all possible recombination and reemission processes occurring between extended and localized states. The simulated results are in good agreement with experimental data. (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{h}}$ in undoped a-Si:H is insensitive to ${\mathit{N}}_{\mathit{d}}$ in the low-${\mathit{N}}_{\mathit{d}}$ range in contrast to (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{e}}$. This asymmetric ${\mathit{N}}_{\mathit{d}}$ dependence of (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{e}}$ and (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{h}}$ is attributed to the inherent asymmetry of the density-of-states distribution of the conduction- and valence-band tails. The \ensuremath{\mu}\ensuremath{\tau} products and ${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$ decrease with the generation rate, whereas ${\mathrm{\ensuremath{\mu}}}_{\mathit{d}\mathit{e}}$ increases. The effect of thermal broadening of the band-tail states must be taken into account in the simulation, especially the increase of the characteristic energy of the conduction band-tail states with T. Apart from the defect states, the band-tail states play a very important role in the determination of the photocarrier lifetimes. In the low-${\mathit{N}}_{\mathit{d}}$ range, recombination via the band-tail states dominates over that via dangling bonds, while dangling bonds become the predominant recombination centers in the high-${\mathit{N}}_{\mathit{d}}$ range. The transition from the tail-state-dominated to the defect-state-dominated recombination process depends essentially on the defect density, the temperature, the generation rate, and the Fermi-level position.

Investigations and Modeling of Tritium Reemission from Soil

Although the process of tritium reemission from soils after HT or HTO deposition is principally understood, the prediction of initial values and time courses of reemission rates on the basis of readily available data is still insufficient, especially when high time resolutions are required. Theoretical and experimental investigations discussed here show that for a yet limited number of environmental conditions good model performance is reached. In the case of evaporation conditions the initial reemission rate after HT deposition can be coupled to the evaporation rate, when the tritium profile in soil is of exponential shape, characterized by a mean scaling length. After HTO deposition the ideal profile can be described by an error function with a scaling length about 1/3 of that of the HT case. Hence, the initial reemission rate is 3 times higher. Time courses can be modeled by a diffusion approach applying the same diffusion coefficient as for the deposition process.

The role of film re-emission and gas scattering processes on the stoichiometry of laser deposited films

Laser ablation of a BiSrCaCuO target is performed both in vacuum and in an oxygen pressure of 0.1 mbar. Two substrates are located in the chamber in order to study the role of re-emission processes from the growing film and/or the scattering of the ablated species by an oxygen atmosphere. The results indicate that re-emission processes from the growing film are very weak (they may affect up to 1% of the deposited material at most) and are not related to the re-sputtering of the growing film. Films grown in vacuum are found to have the correct cation composition, whereas those grown in an oxygen environment show significant variations which are clearly related to gas scattering processes.

Monte Carlo analysis of the carrier relaxation processes in linear- and parabolic-GRINSCH quantum well laser structures

The carrier relaxation process is widely acknowledged to have a strong bearing on the modulation limit of quantum well lasers. As a first and crucial step toward achieving a better understanding of this phenomenon, we have developed a numerical technique to study such processes in graded-index separate confinement heterostructure quantum well laser structures having an arbitrary grading profile. We base our approach on ensemble Monte Carlo simulation of the carrier transport in the 3-D graded-index region and in the 2-D quantum well. We also introduce a technique to handle the carrier capture and re-emission processes within the Monte Carlo method. The results obtained from our calculations for a number of structures with quantum well sizes 50-100 /spl Aring/ indicate that the overall carrier capture time is about 5-7.5 ps under low injection condition for the linearly graded structures, and significantly longer for the parabolically graded structures. On the other hand, the carrier capture efficiency is found to be higher for the parabolic graded-index structures. We also compare our calculations to published experiments and find good agreement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Low energy positron scattering from atoms in and on solids observed with low energy positron diffraction and reemission microscopy

The present status of low energy positron diffraction (LEPD) and positron reemission microscopy (PRM) is reviewed in the context of unanswered questions regarding the elastic scattering of positrons in the 1–300 eV energy range from atoms in solids and at solid surfaces. Recent LEPD studies yield an agreement between theoretical and experimental diffraction intensities for semiconductors that has never been equalled in electron diffraction studies. This situation is discussed in terms of the repulsive interaction between the positron and the embedded atomic potential and the lack of exchange with the nonspherically distributed valence electrons. The scattering of re-emitted positrons in PRM from atoms chemisorbed or physisorbed on the re-emitting surface has not yet received the same theoretical attention as scattering from embedded atoms in LEPD. Possible ways in which positron scattering from overlying atomic structures manifests itself in PRM as well as positron re-emission holography are discussed, both from the practical viewpoint of observing these structures and in the context of fundamental questions regarding the positron re-emission process itself.