Boron Density Research Articles

Revisit: High resolution charge density study of α-rhombohedral boron using third-generation SR data at SPring-8

Experimental charge density of α-rhombohedral boron (α-B12) by a Maximum entropy method (MEM) has been re-investigated using the high resolution powder diffraction data measured at third-generation synchrotron radiation (SR) source, SPring-8. The present MEM charge density has many discrepancies from the previous MEM charge densities reported by Fujimori et al. and Hosoi et al. The data-resolution dependence of the MEM charge density was investigated using the present data. We found that diffraction data with d > 0.4 Å resolution range were needed to reveal qualitative bonding nature of α-B12 at 100 K. The peculiar bonding natures, such as a bend B–B bond and a propeller-shaped bond, which were found in the previous studies have disappeared by using d > 0.4 Å data. The bonding nature of MEM charge density using the full data with d > 0.327 Å d-spacing range is well agreed with those of theoretical calculations. The present study suggests that resolution test is important for an accurate charge density study of boron related materials.

Determination of the Compound Biological Effectiveness (CBE) Factors Based on the <i>ISHIYAMA-IMAHORI</i> Deterministic Parsing Model with the Dynamic PET Technique

Purpose: In defining the biological effects of the 10B(n, α)7Li neutron capture reaction, we have proposed a deterministic parsing model (ISHIYAMA-IMAHORI model) to determine the Compound Biological Effectiveness (CBE) factor in Borono-Phenyl-Alanine (BPA)-mediated Boron Neutron Capture Therapy (BNCT). In present paper, we demonstrate a specific method of how the application of the case of application to actual patient data, which is founded on this model for tissues and tumor. Method: To determine the CBE factor, we derived the following new calculation formula founded on the deterministic parsing model with three constants, CBE0, F, n and the eigen value Nth/Nmax. (1), where, Nth and Nmax are the threshold value of boron concentration of N and saturation boron density and CBE0, F and n are given as 0.5, 8 and 3, respectively. In order to determine Nth and Nmax in the formula, sigmoid logistic function was employed for 10B concentration data, Db(t) obtained by dynamic PET technique. (2), where, A, a and t0 are constants. Results and Conclusion: From the application of sigmoid function to dynamic PET data, it is concluded that the Nth and Nmax for tissue and tumor are identified with the parameter constants in the sigmoid function in Equation (2) as: (3). And the calculated CBE factor values obtained from Equation (1), with Nth/Nmax.

A feasibility study on the criticality control method using radioactive vitrified forms for spent fuel storage

Vitrification form is an immobilization method for the radioactive wastes that are generated from various facilities such as nuclear power plants. Currently, the resultant vitrified forms are classified as radioactive wastes and therefore they should be disposed of in radioactive waste disposal facilities; but this disposal method is costly. In this study, a re-use method for various vitrified forms as the criticality control material was proposed for the replacement of the conventional neutron absorber in spent fuel storages. First, a nuclear fuel storage design was proposed for PLUS7 and WH 17×17 fuel assemblies. In addition, the criteria of the effective boron densities were established. For the verification of the criticality control ability, the multiplication factors in the storages were estimated with the 43 vitrified forms which have been developed in various countries. The results showed that most of the vitrified forms, except for a few cases, can be utilized for the criticality control materials. The proposed method uses the vitrified forms as neutron absorbers in the spent fuel storages without any chemical additions. Therefore, it can contribute to an increase in the efficient disposal of radioactive waste, as well as the providing economic benefits.

Expansion of laser-ablated two-component plume with disparate masses

Laser ablation of boron and tungsten is studied theoretically. The effect of disparate masses of boron and tungsten on parameters of the laser-ablated plasma plume is studied. A theoretical model is applied, which describes both the target heating and formation of the plasma and its expansion. Calculations are made for the third harmonic of a nanosecond Nd:YAG laser. The calculated distributions of plasma temperature, velocity and electron density in the early phase of expansion show that the plasma temperature is higher in the case of heavier tungsten, but the velocity is higher in the case of lighter boron, which is in agreement with experimental findings. In addition, the absorption coefficient of 355 nm radiation, the emission coefficient, the mass density and heat capacity of boron and tungsten plasma are compared.

A cavity ring-down spectroscopy sensor for real-time Hall thruster erosion measurements.

A continuous-wave cavity ring-down spectroscopy sensor for real-time measurements of sputtered boron from Hall thrusters has been developed. The sensor uses a continuous-wave frequency-quadrupled diode laser at 250 nm to probe ground state atomic boron sputtered from the boron nitride insulating channel. Validation results from a controlled setup using an ion beam and target showed good agreement with a simple finite-element model. Application of the sensor for measurements of two Hall thrusters, the H6 and SPT-70, is described. The H6 was tested at power levels ranging from 1.5 to 10 kW. Peak boron densities of 10 ± 2 × 10(14) m(-3) were measured in the thruster plume, and the estimated eroded channel volume agreed within a factor of 2 of profilometry. The SPT-70 was tested at 600 and 660 W, yielding peak boron densities of 7.2 ± 1.1 × 10(14) m(-3), and the estimated erosion rate agreed within ~20% of profilometry. Technical challenges associated with operating a high-finesse cavity in the presence of energetic plasma are also discussed.

Enhancement of the power factor in two‐phase silicon–boron nanocrystalline alloys

In previous publications it was shown that the precipitation of silicon boride around grain boundaries may lead to an increase of the power factor in nanocrystalline silicon. Such an effect was further explained by computational analyses showing that the formation of an interphase at the grain boundaries along with high boron densities can actually lead to a concurrent increase of the electrical conductivity σ and of the Seebeck coefficient S. In this communication we report recent evidence of the key elements ruling such an unexpected effect. Nanocrystalline silicon films deposited onto a variety of substrates were doped to nominal boron densities in excess of 1020 cm−3 and were annealed up to 1000 °C to promote boride precipitation. Thermoelectric properties were measured and compared with their microstructure. A concurrent increase of σ and S with the carrier density was found only upon formation of an interphase. Its dependency on the film microstructure and on the deposition and processing conditions will be discussed.

In–out impurity density asymmetry in the pedestal region of Alcator C-Mod

Measurements are presented of the boron (B5+) density in the pedestal region at the low-field side (LFS) midplane and the high-field side (HFS) midplane of Alcator C-Mod. In H-mode plasmas, a large (≳10x) in–out asymmetry in impurity density forms, with larger densities at the HFS. In contrast, there is no impurity density asymmetry in L-mode or I-mode plasmas. A comparison of pedestal parameters in H-mode plasmas shows that the HFS impurity density pedestal width and position remain fairly fixed over a range of plasma conditions, while the LFS pedestal width widens, and the pedestal position shifts in towards the core as the plasma current is lowered, indicating a change in the underlying transport.

Validation of gyrokinetic modelling of light impurity transport including rotation in ASDEX Upgrade

Upgraded spectroscopic hardware and an improved impurity concentration calculation allow accurate determination of boron density in the ASDEX Upgrade tokamak. A database of boron measurements is compared to quasilinear and nonlinear gyrokinetic simulations including Coriolis and centrifugal rotational effects over a range of H-mode plasma regimes. The peaking of the measured boron profiles shows a strong anti-correlation with the plasma rotation gradient, via a relationship explained and reproduced by the theory. It is demonstrated that the rotodiffusive impurity flux driven by the rotation gradient is required for the modelling to reproduce the hollow boron profiles at higher rotation gradients. The nonlinear simulations validate the quasilinear approach, and, with the addition of perpendicular flow shear, demonstrate that each symmetry breaking mechanism that causes momentum transport also couples to rotodiffusion. At lower rotation gradients, the parallel compressive convection is required to match the most peaked boron profiles. The sensitivities of both datasets to possible errors is investigated, and quantitative agreement is found within the estimated uncertainties. The approach used can be considered a template for mitigating uncertainty in quantitative comparisons between simulation and experiment.

Influence of boron on donor–acceptor pair recombination in type IIa HPHT diamonds

We report on the investigation of donor–acceptor pair (DAP) and free carrier recombination in HPHT IIa type diamonds and determination of boron concentration by differential transmittivity (DT) technique. Photoluminescence and photoluminescence excitation spectra were measured in 8–300K temperature range and provided a broad (~0.67eV) Gaussian DAP band which peaked at 2.2eV at low temperatures, while above 200K it sharply shifted to 2.5eV and became more intense. Thermoluminescence measurements also demonstrated a similar tendency. This peculiarity was explained by DAP recombination between the nitrogen and the boron, the latter being in the ground and the excited states at low and high temperatures, respectively. A zero phonon line position coincided with the calculated one, when using nitrogen and boron activation energies. Scanning of DT across the sample at different delay times revealed the fast (200–500ns) free carrier lifetime and the slow recovery time (of optically recharged boron to its initial state). The temperature dependence of the slow component decay time provided the boron activation energy of 360meV. Saturation of the boron-related DT signal in the samples and the determined boron ionization cross section at 1064nm (σB=3.3×10−17cm2) provided the boron density in 1014–16cm−3 range and revealed its strongly inhomogeneous distribution across the HPHT layers. The B density was found much lower than the density of nitrogen donors (~1017cm−3), which were distributed in the layers much more homogeneously.

Light penetration depth dependence of photocarrier life time and the Hall effect in phosphorous-doped and boron-doped homoepitaxial CVD diamond films

Photocurrent in phosphorous-doped CVD diamond film of the bandgap of 5.5eV with the density of 2×1018cm−3 decreases with increasing photon energy in the energy range higher than 5.8eV at room temperature (RT). The photocarrier life time is 0.3ms at the excitation energy of 5.8eV and decreases with increasing excitation energy. These show that the photocarriers, ascertained to be electrons by the Hall effect of the photocurrent, are trapped near the surface. The life time of photo-excited holes in Boron-doped CVD diamond film with the density of 9×1017cm−3 is 35ms at RT and decreases with decreasing Boron density, which is explained from the relation between the Fermi energy and the density.

Dimerization of boron dopant in diamond (100) epitaxy induced by strong pair correlation on the surface

Experiments have shown that boron incorporation in diamond epitaxies is orientation dependent. Our first-principles calculations reveal that at a (100) surface, the formation of the boron dimer is more favored than that of the monomer, indicating a high density of ineffective boron formed under heavy doping. The reconstructed surface layer of carbon dimers in which the electrons are strongly pair correlated provides the mechanism. Hydrogen adsorption affects the correlation and thus the favorability of boron dimer formation, while at a (111) surface, the formation of boron monomer is more favored due to the less correlated surface electrons and hydrogen adsorption has no effect on the favorability.

Classical confinement and outward convection of impurity ions in the MST RFP

Impurity ion dynamics measured with simultaneously high spatial and temporal resolution reveal classical ion transport in the reversed-field pinch. The boron, carbon, oxygen, and aluminum impurity ion density profiles are obtained in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] using a fast, active charge-exchange-recombination-spectroscopy diagnostic. Measurements are made during improved-confinement plasmas obtained using inductive control of tearing instability to mitigate stochastic transport. At the onset of the transition to improved confinement, the impurity ion density profile becomes hollow, with a slow decay in the core region concurrent with an increase in the outer region, implying an outward convection of impurities. Impurity transport from Coulomb collisions in the reversed-field pinch is classical for all collisionality regimes, and analysis shows that the observed hollow profile and outward convection can be explained by the classical temperature screening mechanism. The profile agrees well with classical expectations. Experiments performed with impurity pellet injection provide further evidence for classical impurity ion confinement.

Aging Tests of Neutron-Shielding Materials for Transport of Storage Casks

Dry storage cask materials must withstand severe temperature conditions coming from conservative ambient assumptions and from envelope values of residual heat from spent fuels. TN International (TNI) research and development has consequently developed and characterized neutron-shielding materials that show high shielding capabilities and correspond to the needs of a range of temperatures. Aging tests of specimens have been performed accordingly.This paper shows the results of aging tests on shielding material Vyal-B, a material patented by TNI.TNI Vyal-B shielding material is composed of a thermoset resin matrix (vinylester resin in solution of styrene) and two mineral fillers (alumina hydrate and zinc borate). The cross-linking of the polymer leads to a rigid three-dimensional lattice, solid and resistant to transport conditions, especially the temperatures.TNI Vyal-B’s shielding ability for neutron radiation is related to the atomic density of hydrogen and boron. Atomic densities can be calculated from the chemical composition and bulk density of samples, and these two parameters are checked initially just after manufacturing.Assessment of the long-term behavior of TNI Vyal-B shielding material has been carried out through exposition tests of samples at different temperatures (150°C to 170°C). The test results give the variations of chemical composition and density. After the aging tests, the new chemical compositions are taken into account in the shielding analysis of the package. Considering a degradation mechanism following Arrhenius behavior, a maximal temperature of long-term use, for example, for interim storage, was confirmed equal to 160°C.

Gyrokinetic modelling of electron and boron density profiles of H-mode plasmas in ASDEX Upgrade

Local gyrokinetic calculations of the logarithmic gradients at mid-radius of both electron and boron densities in ASDEX Upgrade H-mode plasmas are presented and compared with the experimental observations. The experimental results show that both the electron and the boron density profiles increase their peaking in response to the addition of central electron cyclotron heating over a background of neutral beam injection (NBI) heating. The boron density profiles are always less peaked than the electron density profiles in the confinement region, and are flat or even slightly hollow in the presence of NBI heating only. The experimental behaviours are well reproduced by the theoretical predictions. The agreement allows the identification, through theoretical modelling, of the transport mechanisms responsible for the observed dependences. In particular, the observed increase in the logarithmic electron density gradient with increasing central electron heating is explained by a concurrent reduction of the outward pure convection and an increase in the inward thermodiffusion. In addition, it is found that the plasma toroidal rotation velocity and its radial gradient play a non-negligible role in the turbulent boron transport, and allow the prediction of a decrease in boron peaking with increasing rotation velocity, which is consistent with the experimental observations.

Strain effect on transport properties of hexagonal boron–nitride nanoribbons

The transport properties of hexagonal boron–nitride nanoribbons under the uniaxial strain are investigated by the Green's function method. We find that the transport properties of armchair boron–nitride nanoribbon strongly depend on the strain. In particular, the features of the conductance steps such as position and width are significantly changed by strain. As a strong tensile strain is exerted on the nanoribbon, the highest conductance step disappears and subsequently a dip emerges instead. The energy band structure and the local current density of armchair boron–nitride nanoribbon under strain are calculated and analysed in detail to explain these characteristics. In addition, the effect of strain on the conductance of zigzag boron–nitride nanoribbon is weaker than that of armchair boron nitride nanoribbon.

Influence of boron-interstitials clusters on hole mobility degradation in high dose boron-implanted ultrashallow junctions

Hole mobility degradation has been studied in high-dose boron-implanted ultrashallow junctions containing high concentrations of boron-interstitial clusters (BICs), combining an empirical method based on the self-consistent interpretation of secondary-ion-mass spectrometry (SIMS) and Hall measurements and liquid-nitrogen (LN2) to room temperature (RT) hole mobility measurements. It has been found that BICs act as independent scattering centers which have a strong impact on hole mobility in addition to the other scattering mechanisms such as lattice and impurities scattering. A mobility degradation coefficient α has been introduced, which gives information on the mobility degradation level in the analyzed junctions. In the case of very high concentrations of BICs (containing a boron density up to 8×1014 cm−2), measured hole mobilities were found to be ∼40% lower than corresponding theoretical values. BICs dissolution through multiple Flash anneals at high temperature (1300 °C) reduces the observe mobility degradation.

Electrical and structural properties of p ‐type nanocrystalline silicon grown by LEPECVD for photovoltaic applications

Abstractp‐doped hydrogenated nanocrystalline silicon (p‐nc‐Si:H) is one of the most critical layers in thin film silicon solar cells. LEPECVD is a new technique for the growth of nc‐Si at high growth rate without compromising the layer quality, using a dense but low energy plasma. Thin p‐nc‐Si:H layers are grown on glass and ZnO:Al coated glass and their structural and electrical properties are investigated as a function of the silane dilution (d) and of the doping ratio (DR). The influence of the substrate on the structural properties is investigated and discussed. The incubation layer is clearly observed on both substrate types and its thickness is estimated. LEPECVD distinguishes itself from other high growth rate methods by a very low impurity distribution coefficient to obtain a comparable conductivity and boron density. The conduction path is shown to be dependent on the density of boron in the layer while a significant decrease of conductivity at high DR is not observed in the studied range (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

10Boron distribution measurement in laser ablated B 4C thin films using (n,α) reaction and LR-115 passive detector

Lateral distribution of the 10B isotope within a boron carbide film of 550 nm maximum thickness deposited on silicon wafer using pulsed laser deposition technique has been determined taking advantage of the high cross section for (n,α) reaction and nuclear tracks detectors (NTD – LR-115 Kodak Pathé). A radioisotope neutron source ( 252Cf, 20 μg) and a 60 × 60 × 80 cm 3 graphite cube as moderator produce a relatively uniform thermal neutron field. Details of the passive detector etching process and data processing are included. The track density reveals the boron density spatial distribution. A 3D picture is produced to visualize the boron-10 spatial distribution. The result suggests that a gradient in the boron distribution exists to almost a factor of three. The advantages of the technique are discussed.

Cross-section modelling effects on pressurised water reactor main steam line break analyses

Because of the strong asymmetric overcooling effects occurring during a PWR main steam line break (MSLB) event, an accurate analysis of this transient requires the use of 3-D kinetics methods. An assessment has been made of the relative performance of the two kinetics solvers currently employed at PSI for such analyses, viz. CORETRAN and SIMULATE-3 K. For the purpose, the simulation of a hypothetical MSLB in a real operated PWR MOX cycle has been considered, employing consistent 3-D core models with specified thermal-hydraulic boundary conditions at the lower and upper plenums. Although the employed cross-section library is in both codes based on the same set of homogenised 2-group cross-sections prepared with CASMO-4, significant differences are shown to occur due to the smaller moderator reactivity coefficient calculated in CORETRAN. It is found that this stems largely from differences in the cross-section formalism, i.e. the manner in which feedback dependencies are modelled and interpolated for the cross-section sets. In particular, the CORETRAN cross-section formalism induces an inadequate treatment of coupled feedback effects, principally between boron density and moderator temperature, which renders the MSLB dynamics predictions quite sensitive to the methodology employed during the cross-section preparation. As such, transient-specific cross-section libraries need to be produced for reliable MSLB analysis in this case. The cross-section model for SIMULATE-3 K, on the other hand, is shown to be adequate for accurately capturing the coupled reactivity effects occurring during an MSLB. In this case, the sensitivity of the results to other sources of uncertainties becomes more apparent, e.g. to those related to the neutron data and/or the thermal-hydraulic boundary conditions. Considering that many other state-of-the-art advanced kinetics solvers have cross-section formalisms similar to that of CORETRAN, effects of the type currently investigated need to be taken into account while developing methodologies for assessing neutronics-related uncertainties in best-estimate transient analysis.

Magic-Sized Diamond Nanocrystals

The 2D structural transformation of a heavily boron-doped diamond surface has been revealed using scanning tunneling microscopy (STM). We found that at boron densities above the metal-insulator transition the diamond surface is comprised of spatially ordered magic-sized nanocrystals. The development of quantized electron gas inside these nanocrystals is directly confirmed by STM observation of standing electron waves. The experimental comparison of metallic and insulating diamond reveals the existence of the Fermi-sea-induced quantum selection rules for the self-assembly of nanostructures.