Light Element Impurities Research Articles

The calculation of light element impurity ([formula omitted], n) yield curves in a PuO[formula omitted] matrix and associated specific yield coefficients: Influence of the reaction cross sections

Most of the Pu separated from irradiated commercial nuclear fuel is stored as PuO 2. The primary quantitative nondestructive measurement technique used to verify the amount of Pu in storage containers is passive neutron correlation counting. An important physical property of the oxide material is the ratio, α, of the rate of (α, n) neutrons produced inside the item to the rate of neutrons produced by spontaneous fission. This ratio influences the precision of the correlated counting method and affects the interpretation of the data because of how it changes both the primary total neutron production rate and the rate of induced fission events taking place inside the item. In addition to the main O(α, n) contribution, additional contributions come from α-particle interactions with light element impurities that are inevitably present. In this work, we calculate specific (α, n) yield coefficients, expressed in units of neutrons per second per gram of α-emitting nuclide per part per million by mass of the specified impurity element distributed in a pure PuO 2 matrix, for some key α-emitting actinides commonly present in reprocessed Pu (238–242Pu+241Am). These coefficients are directly applicable to nuclear safeguards verification work in which the α ratio is often calculated from the Pu-isotopic composition and chemical information obtained by other means. They also provide a convenient up-to-date reference set against which values generated by other methods can be compared. Results are presented for impurities with atomic number from 3 to 17 inclusive, plus K and Fe. In most cases, these coefficients are not expected to change by more than 5%–10% at any time in the future. However, as new data become available, changes as large as 20% may be needed for some targets (e.g., F). The present yield calculations are limited by the general shortage of quality experimental total (α, n) reaction cross section data, which, together with unexplained variation between determinations, means that an objective and coherent evaluation is not possible. The situation is even less satisfactory for the partial differential cross section needed to calculate neutron spectra.

Effect of purity on the vacancy defects induced in self–irradiated tungsten: A combination of PAS and TEM

Vacancy defects in tungsten induced by self-ion irradiation were characterized by positron annihilation spectroscopy (PAS) and transmission electron microscopy (TEM). Taking advantage of their complementarity, defects ranging from single vacancies up to vacancy clusters were detected, and the effects of sample purity (99.95 wt.% and 99.9999 wt.%) on their formation and evolution under low damage dose (0.01 to 0.02 dpa) irradiation with 20 and 1.2 MeV-W ions were studied at room temperature (RT), 500 and 700 °C respectively. When irradiation was performed at RT, PAS probed mainly single vacancies. At high irradiation temperatures, larger vacancy clusters were detected by both techniques. Both PAS and TEM revealed larger vacancy clusters in the purest samples after irradiation at 500 and 700 °C. A significant difference in the evolution of vacancy-type defects related to sample purity was observed. In view of the properties (migration, trapping) of some light-element impurities (LEs), present at high concentrations, determined by first-principles calculation, it is reasonable to assign the effect of purity during the irradiation response to the formation of vacancy-impurity complexes, which is critical for the understanding of the behavior of tungsten in the future fusion reactor.

Effect of Light Element Impurities on the Edge Dislocation Glide in Nickel and Silver: Molecular Dynamics Simulation

The influence of light element (C, N, O) impurities on the edge dislocation glide in fcc metals (Ni, Ag) is studied by molecular dynamics simulation. The introduction of impurity atoms is found to increase the dislocation glide threshold stress significantly, from 10 MPa in the pure metals at a temperature of 300 K to 1000–2000 MPa after the introduction of 10 at % impurity atoms. The increase in the threshold stress with the concentration of impurity atoms is shown to be caused by the Suzuki mechanism, i.e., the pinning of impurity atoms by the stacking fault between partial dislocations. The energies of binding of impurity atoms to stacking faults were determined for the metals under study. When temperature increases, the dislocation glide velocity in the pure metals decreases. After the introduction of impurity atoms, this dependence is reversed: the dislocation velocity increases gradually with temperature.

Effect of light elements impurity on process of nickel crystallization near the triple interface of grain boundaries: a molecular dynamics simulation

Molecular dynamics method was used to study the effect of impurities of light elements of carbon, nitrogen and oxygen on crystallization process near the triple interface of grain boundaries in nickel. Tilt boundaries with misorientation axis <111> were considered as the grain boundaries. Interactions of nickel atoms with each other were described by many-particle Clery-Rosato potential constructed within the framework of the tight binding model. To describe interactions of atoms of light elements impurities with nickel atoms and atoms of impurities with each other, Morse pair potentials were used. Calculation cell had a shape of cylinder, axis of which coincided with the line of triple interface and the axis of grain misorientation. Periodic boundary conditions were imposed along the cylinder axis, and the atoms on side surface of cylinder were motionless. To simulate crystallization, calculation cell was melted by heating to a temperature well above the melting temperature of nickel. After the simulated polycrystal become liquid, the thermostat was turned on and held at a constant temperature below the melting temperature. Rigid boundary conditions on the lateral surface of cylindrical calculation cell in this case simulated crystallization fronts from three crystallization centers. The area near the triple interface had crystallized the last. In this area, defects and free volume were concentrated. Presence of impurities led to a significant slowdown in the rate of crystallization. With introduction of 10 % of impurity atoms, the rate of motion of crystallization front decreased several times. The effect of impurities on crystallization rate was enhanced in C – N – O direction, which is due to difference in crystal lattice deformation caused by impurity atoms. The greater this deformation was, the stronger was impurity atoms inhibit crystallization front. Formation of aggregates at fairly high concentrations was typical for impurity carbon atoms. Crystallization front had impeded on these aggregates. The oxygen and nitrogen atoms did not form aggregates. However, due to distortions of crystal lattice caused by them, they also strongly slowed down the crystallization front.

Influence of Light Impurities on the Crystal–Melt Interface Velocity in Ni and Ag. Molecular Dynamics Simulation

The influence of impurities of light elements (C, N, and O) on the crystal–melt interface velocity in fcc metals (Ni and Ag) has been investigated by the molecular dynamics method. Dependences of the crystallization rate on the impurity concentration are obtained. It is shown that the presence of impurities reduces significantly the crystal–melt interface velocity in metals. Impurity atoms induce local lattice strain, which leads to deceleration of the crystal–melt interface; as a rule, the stronger this strain is, the more significant the deceleration of the crystal–melt interface by impurity atoms is.

Fundamental Analysis for Electrochemical Extraction and Monitoring of Impurities from Lead Lithium and Lithium with Chloride Molten Salt

Liquid Li-Pb eutectic alloy and lithium, assumed to be used in a liquid blanket system for fusion reactors, can easily contain light element impurities. It is thought that these impurities have bad influences on compatibility with structural materials. However, an effective reducing method for impurities has not yet been established. In this work, an electrochemical method is focused on this application. The experimental results of our work suggest the transfer of hydrogen and nitrogen impurities. From the results, a possible way to remove the impurities by electrochemical method is shown.

Влияние примесей легких элементов на скорость движения фронта кристаллизации в Ni и Ag: молекулярно-динамическое моделирование

The effect of impurities of light elements C, N, O on the velocity of motion of the crystallization front in fcc metals Ni and Ag was studied by the method of molecular dynamics. The dependences of the crystallization rate on the concentration of impurities are obtained. It is shown that the presence of impurities leads to a significant slowdown in the velocity of motion of the crystallization front in metals. Braking of the crystallization front by the impurity atoms is associated with the local deformation of the crystal lattice that they cause, and, as a rule, the greater this deformation, the stronger the impurity atoms inhibit the crystallization front.

Direct imaging of light-element impurities in graphene revealstriple-coordinated oxygen

Along with hydrogen, carbon, nitrogen and oxygen are the arguably mostimportant elements for organic chemistry. Due to their rich variety of possiblebonding configurations, they can form a staggering number of compounds. Here, wepresent a detailed analysis of nitrogen and oxygen bonding configurations in adefective carbon (graphene) lattice. Using aberration-corrected scanningtransmission electron microscopy and single-atom electron energy loss spectroscopy,we directly imaged oxygen atoms in graphene oxide, as well as nitrogen atomsimplanted into graphene. The collected data allows us to compare nitrogen and oxygenbonding configurations, showing clear differences between the two elements. Asexpected, nitrogen forms either two or three bonds with neighboring carbon atoms,with three bonds being the preferred configuration. Oxygen, by contrast, tends tobind with only two carbon atoms. Remarkably, however, triple-coordinated oxygen withthree carbon neighbors is also observed, a configuration that is exceedingly rare inorganic compounds.

(Invited) Proximity Gettering Design of Hydrocarbon Molecular Ion Implanted Silicon Wafers Using Direct Bonding Technique for Advanced CMOS Image Sensors: A Review

Complementary metal oxide semiconductor (CMOS) image sensors have become widely used ubiquitous devices such as smart phone and personal computer type tablets. Consumer markets strongly demands more high sensitivity imaging and more high speed image data processing for achieving advanced CMOS image sensor fabrication. However, there are some serious improvement technical issues in CMOS image sensor fabrication process. Extremely important technical issue is metallic impurity contaminated device active region during device fabrication process such as high temperature rapid thermal annealing, dry etching and plasma heat treatment. Metallic impurities formed deep energy level defects in the silicon band-gap. Thus, these deep energy level defects strongly affects electrical device characteristic such as pn-junction leakage current, recombination lifetime and gate oxide breakdown voltage. Another technical issue is oxygen impurity out-diffused to the device active region from a Czochralski (CZ) grown silicon substrate during device heat process. Oxygen impurity formed oxygen related deep energy level defects in the device active region of photo-diode space charge region and transfer transistor gate channel region. These oxygen related defects strongly influenced on electrical device performance such as image lag. Therefore, in recent years, CMOS image sensor manufacturers have strongly demanded silicon wafers with the highest metal and oxygen impurities gettering capability during CMOS device processes. In response, we have developed a novel gettering wafer production concept using a hydrocarbon molecular ion implantation technique for advanced CMOS image sensors [1][2][3]. This technique can implant a silicon wafer surface simultaneously with carbon and hydrogen elements that form the projection range using a hydrocarbon compound molecular gas source. In our previous study, it was found that a hydrocarbon molecular ion implanted silicon wafer had three unique characteristics for high performance of advanced CMOS image sensors. First, a hydrocarbon molecular ion projection range has high gettering capability of metallic impurities [1][2][4]. Second, this projection range also has a diffusion barrier effect for oxygen impurities out-diffusing to the device active region from a CZ grown silicon wafer substrate [5]. Third, it is expected that diffusing hydrogen to the device active region from the hydrogen of hydrocarbon molecular ions gettered in the projection range during the device fabrication process will have a passivation effect on unreconstructed surface dangling bonds and Si/SiO2 interface state densities such as local oxidation of silicon or shallow trench isolation [6][7]. Moreover, we achieved more improvement for gettering capability of hydrocarbon molecular ion implanted silicon wafer which formed epitaxial layer without epitaxial growth process using room temperature direct bonding technique. In this presentation, we demonstrate that a hydrocarbon molecular ion implanted silicon wafers using room temperature direct bonding technique has high gettering capability characteristics for transition metal and light element impurities such as oxygen, and hydrogen during the CMOS image sensor fabrication process.

Distribution of light-element impurities in Si crystals grown by seed-casting method

We investigated the distributions of interstitial oxygen (Oi) and substitutional carbon (Cs) in high-performance (HP) multicrystalline Si (mc-Si) and monocrystalline-like Si (mono-like Si) and compared them with those in conventional mc-Si, grown using the same furnace. The Oi concentration in mono-like Si grown using a Czochralski (Cz) silicon seed was the highest among the three crystals. On the other hand, the Oi and Cs concentrations in HP mc-Si grown using Siemens Si incubation seeds were the same as those in conventional mc-Si. Therefore, it is considered that Oi incorporated into the growing Si crystal originates not only from the quartz crucible wall but also from the seed. Additionally, Oi and Cs in HP mc-Si grown on the incubation seeds with adequately low Oi and Cs concentrations are distributed similarly to those in conventional mc-Si grown under the same conditions. We believe that it is important to consider the Oi and Cs concentrations in the feed stock materials both for the seed and whole ingots in the seed-casting method.

Structures and Stability of Iron Halides at the Earth’s Mantle and Core Pressures: Implications for the Missing Halogen Paradox

The terrestrial abundance of heavy halogens Cl, Br, and I is depleted by approximately one order of magnitude relative to those predicted on the basis of their volatilities. One plausible explanation for this missing halogen paradox is their sequestration into the Earth’s core. Therefore, heavy halogens in the core may combine with the dominant element, Fe, to form iron halides that potentially exert important effects on the properties and dynamic evolution of the Earth’s inner core. In this study, stable iron halide phases have been predicted from first-principles structural searches at four pressures corresponding to those at the Earth’s mantle and core. At 360 GPa (corresponding to the inner core), the most stable iron chloride is CsCl-type FeCl, supporting the hypothesis that light-element impurities can stabilize the body-centered cubic Fe structure. At pressures of the Earth’s core, it is also observed that the chemical nature of iodine changes from an electron acceptor to an electron donor. This ch...

Ground-state splitting of ultrashallow thermal donors with negative central-cell corrections in silicon

Ultrashallow thermal donors (USTDs), which consist of light element impurities such as carbon, hydrogen, and oxygen, have been found in Czochralski silicon (CZ Si) crystals. To the best of our knowledge, these are the shallowest hydrogen-like donors with negative central-cell corrections in Si. We observed the ground-state splitting of USTDs by far-infrared optical absorption at different temperatures. The upper ground-state levels are approximately 4 meV higher than the ground-state levels. This energy level splitting is also consistent with that obtained by thermal excitation from the ground state to the upper ground state. This is direct evidence that the wave function of the USTD ground state is made up of a linear combination of conduction band minimums.

Electrically active light-element complexes in silicon crystals grown by cast method

Electrically active light-element complexes called thermal donors and shallow thermal donors in silicon crystals grown by the cast method were studied by low-temperature far-infrared absorption spectroscopy. The relationship between these complexes and either crystal defects or light-element impurities was investigated by comparing different types of silicon crystals, that is, conventional cast-grown multicrystalline Si, seed-cast monolike-Si, and Czochralski-grown Si. The dependence of thermal and the shallow thermal donors on the light-element impurity concentration and their annealing behaviors were examined to compare the crystals. It was found that crystal defects such as dislocations and grain boundaries did not affect the formation of thermal or shallow thermal donors. The formation of these complexes was dominantly affected by the concentration of light-element impurities, O and C, independent of the existence of crystal defects.

Structural and chemical analysis of annealed plasma-enhanced atomic layer deposition aluminum nitride films

Plasma-enhanced atomic layer deposition was utilized to grow aluminum nitride (AlN) films on Si from trimethylaluminum and N2:H2 plasma at 200 °C. Thermal treatments were then applied on the films which caused changes in their chemical composition and nanostructure. These changes were observed to manifest in the refractive indices and densities of the films. The AlN films were identified to contain light element impurities, namely, H, C, and excess N due to nonideal precursor reactions. Oxygen contamination was also identified in the films. Many of the embedded impurities became volatile in the elevated annealing temperatures. Most notably, high amounts of H were observed to desorb from the AlN films. Furthermore, dinitrogen triple bonds were identified with infrared spectroscopy in the films. The triple bonds broke after annealing at 1000 °C for 1 h which likely caused enhanced hydrolysis of the films. The nanostructure of the films was identified to be amorphous in the as-deposited state and to become nanocrystalline after 1 h of annealing at 1000 °C.

Computational and Spectroscopic Study for Reaction Mechanism of Boric Acid Extraction in Micro-Channel Device

We have attempted to develop a novel process to produce solar-grade silicon (SOG) from diatomaceous earth using wet chemical processes, which have potential advantage to conventional process with long-term, high temperature reactions [1]. The key step of the process is purification of silica solution by solvent extraction with micro-channel device to eliminate light-element impurities such as boron [2], and in order to achieve higher purity, its reaction mechanism should be elucidated in detail. In the present work, we investigate the behavior of reactant species in the channel, using computational and spectroscopic approaches. In our previous study, 2,2,4-trimethyl-1,3-pentanediol (TMPD) was proposed as an efficient extractant for boron (boric acid) [3]. Boric acid is extracted in the form of ester with TMPD at the interface between aqueous solution containing silica and organic solvent with TMPD, whose reaction equation is shown in Fig. 1. Figure 2 shows schematic view of the micro-channel device, which has two channels to provide liquid-liquid interface for the reaction. In order to investigate the reaction mechanism, the following three issues should be considered; (i) local area of the reaction system should be measured precisely, (ii) mass transfer, including fluidic flow, should be simplified in the measured reaction system, (iii) reactant behavior during the extraction should be understood at molecular level. To satisfy these conditions, we used micro analysis system with simulation technique. For the local area measurement, Raman spectroscopy, which is capable to provide chemical information within less than 1μm range, was utilized. For the flow state control in the system, micro-channel device has great advantage, since laminar flow state is predominant in the device. For molecular level understanding of the reaction, density functional theory (DFT) calculation was performed. Rate constant of the reaction in Fig. 1 was estimated by transition state theoretically estimated from DFT. Using this rate constant, finite element method (FEM) was performed to calculate concentration profile of reactants within the channel, comparing to the result of the spectroscopic study. From the result of transition state optimization by DFT, the ester formation was considered to be enhanced by water molecule. Comparing the concentration profile obtained from FEM and that from Raman spectroscopy, the measurement system in this study was expected to have capability to analyze the detailed reaction mechanism of the process. Acknowledgements This work performed as part of the CREST Program, JST, Japan References [1] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim. Acta, 100, 236-241 (2013). [2] N. Matsuo, Y. Matsui, Y. Fukunaka, T. Homma, J. Electrochem. Soc., 161(5), E93-E96 (2014). [3] N. Matsuo, T. Ishihara, T. Oyanagi, K. Nakajima, M. Kunimoto, Y. Fukunaka, T. Homma, ECS Trans., submitted. Figure 1

Current Status of AIST X-ray-Absorption-Spectroscopy (XAFS) Instrument with 100-Pixel Superconducting-Tunnel-Junction Array Detector

We constructed a fluorescence yield X-ray absorption fine structure (XAFS) apparatus for the measurement of local structure around trace light element impurities in light element matrices. Our XAFS instrument with a 100-pixel STJ detector is now in the stage of routine operation and open to users at a synchrotron radiation facility (KEK PF). The average energy resolution of the 100 pixels is 11.8 $$\pm $$ 0.6 at 400 eV in a sensitive area of 1 mm $$^{2}$$ . As an example of functional materials, we successfully measured nitrogen K-edge XAFS spectra of a compound semiconductor, 4H-SiC, with a nitrogen dopant concentration of 300 ppm. The faint N-K line was clearly separated from the strong C-K line.

Light-Element Impurities and their Reactions in Multicrystalline Si

Vibrational spectroscopy has been used to investigate the properties of hydrogen in multicrystalline Si (mc-Si) and its interactions with carbon impurities that can be present with high concentration. The properties of point defects containing N and O have been investigated by way of their vibrational properties and their far-infrared electronic transitions.

Transmission electron microscopy analysis of extended defects in multicrystalline silicon using in‐situ EBIC/FIB sample preparation

AbstractThis paper reports results of microstructural investigations of block cast multicrystalline silicon materials deliberately contaminated with iron and copper impurities from the melt. Electron microscopy techniques have been used to study interactions of grain boundaries, light element impurities such as nitrogen and oxygen, and metal impurities. The key to get access to extended defects typically present in a small density is the recently developed in‐situ EBIC/FIB approach to sample preparation.Special attention is drawn to oxygen and nitrogen containing precipitates at grain boundaries and resulting secondary defects. Strong accumulation of copper impurities is observed which is related to reduced excess carrier lifetimes in these regions. It is shown that copper silicide precipitation mainly accounts for local dislocation networks with a high recombination activity. These observations provide evidence that decoration of grain boundaries with light element contaminants leads to efficient nucleation of metal impurity precipitates. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Using neutron angular anisotropy information to dynamically determine the ratio of the (α,n) rate to spontaneous fission rate for coincidence counting applications

Typically, when neutron coincidence or multiplicity counting is performed, there are three unknowns: the sample mass, the leakage self-multiplication, and the ratio of the (α,n) rate of the source to the spontaneous fission rate. For a given counting time, the strength of the source or the detector efficiency must be sufficiently high for the singles, doubles, and triples count rates to represent statistically meaningful quantities. Often, the strength of the source and the allotted counting time are such that only the singles and doubles count rates are statistically meaningful. In this latter case, the ratio of (α,n) to the spontaneous fission must be estimated through some other means. With a simulated (α,n) rate, the two equations related to the singles and doubles count rates can be used to determine the sample mass. In order to determine the ratio of (α,n) to spontaneous fission rate of the source, the isotopic composition of the sample as well as the light element impurities inside the source must be known. Ideally, there would be a way to dynamically determine the (α,n) rate of the source from count rate information. In this paper, it is shown that the (α,n) rate of the source can be determined by using information about the ratio of the number of coincident neutrons at 180° to the number at 90°. By using this information, the three aforementioned unknowns can be dynamically determined through the sole use of singles and doubles count rates.

Evaluation of Silicon Substrates Fabricated by Seeding Cast Technique

We evaluated the properties of crystalline defects in silicon substrate, and clarified the origin of small-angle grain boundaries. In order to eliminate the effects of grain boundaries, the ingot was fabricated by unidirectional solidification technique with seed crystal. In single-crystalline region, Σ3 twin boundaries and SiC precipitates were observed near the seed crystal. No obvious relationship between twin boundaries and precipitates was observed. These defect decreased once and the precipitations appeared again. The density of precipitates increased through the crystal growth procedure. These precipitates were consisted of Si, C, and N. After the precipitation density increased, the small-angle grain boundaries appeared and some precipitates were observed at the boundaries. We considered the precipitations consisted of light element impurities such as C and N were one of the major origins of the small-angle grain boundary generation.