Small Density Differences Research Articles

Effect of powder properties, process parameters, and recoating speed on powder layer properties measured by in-situ laser profilometry and part properties in laser powder bed fusion

In laser-based powder bed fusion of metals (PBF-LB/M), the powder layer is the link between the powder properties and the resulting part quality. Powder layer quality is a key metric related to powder spreadability and ultimately part quality, yet it is still unclear how it can be quantified. This is due to the difficulty of studying powder layer properties during the process. This study investigates the influence of powder properties, process parameters, and recoating speed on the surface roughness of the powder layer and the part, as well as on the effective thickness of the powder layer and solidified layer, and the resulting relative part density. Utilizing in-situ laser profilometry, high-resolution topographical data of the powder layer and the part surface were acquired, with minimal interference to the PBF-LB/M process. Six AlSi10Mg powders with varying particle size distribution, morphology, and flowability were processed using a wide range of recoating speeds and scan speeds to create powder layers with a wide range of properties. The results reveal a strong correlation between energy input and the effective powder layer thickness where lower scan speed results in an increased effective powder layer thickness due to material losses. Additionally, faster recoating decreases the powder layer density, which is moderated by the median particle size where the effect is strongest for fine powders. The surface roughness of the powder layer and top part surface are influenced by the recoating speed, energy input, and particle size, and they are strongly linked to each other. This highlights the importance of considering realistic substrate surface roughnesses in both powder spreading experiments and simulations. Finally, layer properties affect the process stability, resulting in small differences in relative part density.

The roles of environment and interactions on the evolution of red and blue galaxies in the EAGLE simulation

We study the evolution of the red and blue galaxies from z=3 to z=0 using the EAGLE simulation. The galaxies in the blue cloud and the red sequence are separated at each redshift using a scheme based on Otsu's method. Our analysis shows that the two populations have small differences in the local density and the clustering strength until z=2, after which the red galaxies preferentially occupy the denser regions and exhibit a significantly stronger clustering than the blue galaxies. The significant disparities in cold gas mass and specific star formation rate (sSFR) observed before z=2 suggest that factors beyond environmental influences may also contribute to the observed dichotomy. Interacting galaxy pairs at a given separation exhibit a higher SFR at increasing redshifts, which may be linked to the rising gas fractions at higher redshift. As redshift decreases, the SFR decreases across all separations, suggesting a gradual depletion of the cold gas reservoir. At pair separations < 50 kpc, an anomalous increase in the SFR among paired galaxies in isolation around z ~ 2 suggests that environmental effects begin to dominate at this redshift, thereby increasing the rate of galaxy interactions and the occurrence of starburst galaxies. We observe a substantial decrease in the blue fraction in paired galaxies starting from z=1 to the present. However, the decrease in the blue fraction in paired galaxies with their second nearest neighbour at a distance greater than 500 kpc continues until z=0.5, after which the blue fraction begins to increase.

Analysis of application range of simplified models for field to thermo-field to thermionic emission processes from the cathode

Electron emission plays a dominant role in plasma–cathode interactions and is a key factor in many plasma phenomena and industrial applications. It is necessary to illustrate the various electron emission mechanisms and the corresponding applicable description models to evaluate their impacts on discharge properties. In this study, detailed expressions of the simplified formulas valid for field emission to thermo-field emission to thermionic emission typically used in the numerical simulation are proposed, and the corresponding application ranges are determined in the framework of the Murphy–Good theory, which is commonly regarded as the general model and to be accurate in the full range of conditions of the validity of the theory. Dimensionless parameterization was used to evaluate the emission current density of the Murphy–Good formula, and a deviation factor was defined to obtain the application ranges for different work functions (2.5‒5 eV), cathode temperatures (300‒6000 K), and emitted electric fields (105 to 1010 V·m−1). The deviation factor was shown to be a nonmonotonic function of the three parameters. A comparative study of particle number densities in atmospheric gas discharge with a tungsten cathode was performed based on the one-dimensional implicit particle-in-cell (PIC) with the Monte Carlo collision (MCC) method according to the aforementioned application ranges. It was found that small differences in emission current density can lead to variations in the distributions of particle number density due to changes in the collisional environment. This study provides a theoretical basis for selecting emission models for subsequent numerical simulations.

A review on free miscible buoyant jets

Buoyant jets have been a topic of interest due to their presence in various natural and industrial phenomena, as well as in our daily lives. Covering extensive literature, our focus in this review article is on free, miscible buoyant jets with small-density differences (under the Boussinesq approximation) within uniform and stagnant environments. We examine the latest research on Newtonian jet flows and expand our scope to their viscoplastic counterparts, reflecting a burgeoning trend in contemporary literature. Our analysis covers negatively buoyant, pure, and positively buoyant jets, via delving into their dynamics. This includes regime classifications and key jet characteristics, such as laminar length, penetration length, mixing behavior, velocity profiles, and turbulence parameters. In particular, the effects of the key governing dimensionless parameters, including the Reynolds number, the Froude number, the Bingham number, the viscosity ratio, and the injection angle, on these characteristics are explored.

Mixing of a cylindrical gravity current in a stratified ambient

Simulation results from direct numerical simulations (DNSs) of cylindrical gravity currents propagating into a linearly stratified ambient with stratification strengths ranging from 0 to 0.8, at a moderate Reynolds number of Re=3450 are presented. The simulations are based on the incompressible Navier–Stokes equations, assuming small density differences such that the Boussinesq approximation is valid. The density of the ambient fluid increases linearly from the top (ρ0) to the bottom (ρb) of the domain. A comparative analysis is conducted between the stratified cases and the unstratified case to investigate the impact of ambient stratification strength on the mixing behaviour of cylindrical gravity currents. The energy conversion processes are analysed using the mechanical framework proposed by Winters et al. (1995). The energy budget is formulated by considering gravitational available potential energy, and the evolution of potential energy due to reversible stirring and irreversible diapycnal mixing in the system. The findings reveal a decrease in available potential energy and kinetic energy with increasing stratification strength, indicating lower energy exchange for gravity currents propagating in the stratified environment. Instantaneous and cumulative mixing efficiency calculations during the slumping phase indicate that Kelvin–Helmholtz billow play an important role in stirring the heavy fluid and causing irreversible mixing with the ambient fluid.

MEMO: dataset and methods for robust multimodal retinal image registration with large or small vessel density differences

The measurement of retinal blood flow (RBF) in capillaries can provide a powerful biomarker for the early diagnosis and treatment of ocular diseases. However, no single modality can determine capillary flowrates with high precision. Combining erythrocyte-mediated angiography (EMA) with optical coherence tomography angiography (OCTA) has the potential to achieve this goal, as EMA can measure the absolute RBF of retinal microvasculature and OCTA can provide the structural images of capillaries. However, multimodal retinal image registration between these two modalities remains largely unexplored. To fill this gap, we establish MEMO, the first public multimodal EMA and OCTA retinal image dataset. A unique challenge in multimodal retinal image registration between these modalities is the relatively large difference in vessel density (VD). To address this challenge, we propose a segmentation-based deep-learning framework (VDD-Reg), which provides robust results despite differences in vessel density. VDD-Reg consists of a vessel segmentation module and a registration module. To train the vessel segmentation module, we further designed a two-stage semi-supervised learning framework (LVD-Seg) combining supervised and unsupervised losses. We demonstrate that VDD-Reg outperforms existing methods quantitatively and qualitatively for cases of both small VD differences (using the CF-FA dataset) and large VD differences (using our MEMO dataset). Moreover, VDD-Reg requires as few as three annotated vessel segmentation masks to maintain its accuracy, demonstrating its feasibility.

Bending creep behaviour of various polymer films analysed by surface strain measurement.

Understanding the temporal bending deformation of polymer films is key to designing mechanically durable flexible electronic devices. However, such creep behaviour under persistent bending remains unclear due to a lack of precise and accurate bending strain analysis methods. Herein, we quantitatively analysed the bending creep behaviour of various polymeric films using our developed strain measurement method that can precisely measure surface strain from optical diffraction. The surface strain measurement reveals that bending creep deformation differs depending on the polymer structure. The four-element Burgers model was employed to model the temporal strain increase on the bending surface successfully. By fitting the four-element model to the time course of the measured surface strain, we found that each polymer film has a different threshold surface strain for the appearance of bending creep deformation. Such disparity in the bending creep behaviour can be explained by the difference in strain energy density between the polymer films and their elastic model; polymer films with a small strain energy density difference show small bending creep deformation. The results obtained in this study contribute to the elucidation of the bending creep behaviour of polymer films and the development of flexible electronic devices operated under persistent bending.

Comparative study of WCSPH, EISPH and explicit incompressible-compressible SPH (EICSPH) for multi-phase flow with high density difference

This study presents three Smoothed Particle Hydrodynamics (SPH) methods capable of handling high-density differences in violent incompressible multiphase flows. The conventional Weakly Compressible SPH (WCSPH) is reformulated into a quasi-Lagrangian framework based on Arbitrary Lagrangian–Eulerian (ALE) context. The Explicit Incompressible SPH (EISPH) method is extended to handle multiphase flows and reformulated in the ALE framework. The Explicit Incompressible-Compressible SPH (EICSPH) method, which can handle both compressible and incompressible phases, is developed by combining these two approaches in a fully explicit algorithm. The proposed methods are validated and compared through four benchmark problems including Rayleigh-Taylor instability, hydrostatic, liquid sloshing and dam break problems. Firstly, the stability and accuracy of interface modeling of the three methods were verified through Rayleigh-Taylor instability with small density differences. In the hydrostatic problem with a large density difference, the results from all three methods exhibit good agreement in the pressure distribution. Particularly, EICSPH demonstrates the faster convergence when compared to WCSPH, with EISPH showing fastest convergence overall. In the transient sloshing problem, all three methods quantitatively converged to the experimental results and exhibited good agreement, although slight pressure noise was observed in WCSPH initially. Finally, in the dam break problem, all three methods successfully simulated sharp interfaces without non-physical voids. The temporal variations of pressure and height were well predicted by all three methods, with EISPH showing better conformity to the water-air interface morphology obtained from other incompressible numerical methods compared to WCSPH and EICSPH. Additionally, the impact of air speed of sound was examined in EICSPH, where while the difference in pressure variation at the probe was minimal, differences were observed in energy dissipation and the progression of the free surface due to the air cushion effect.

Research and Implementation of Denoising Algorithm for Brain MRIs via Morphological Component Analysis and Adaptive Threshold Estimation

The inevitable noise generated in the acquisition and transmission process of MRIs seriously affects the reliability and accuracy of medical research and diagnosis. The denoising effect for Rician noise, whose distribution is related to MR image signal, is not good enough. Furthermore, the brain has a complex texture structure and a small density difference between different parts, which leads to higher quality requirements for brain MR images. To upgrade the reliability and accuracy of brain MRIs application and analysis, we designed a new and dedicated denoising algorithm (named VST–MCAATE), based on their inherent characteristics. Comparative experiments were performed on the same simulated and real brain MR datasets. The peak signal-to-noise ratio (PSNR), and mean structural similarity index measure (MSSIM) were used as objective image quality evaluation. The one-way ANOVA was used to compare the effects of denoising between different approaches. p &lt; 0.01 was considered statistically significant. The experimental results show that the PSNR and MSSIM values of VST–MCAATE are significantly higher than state-of-the-art methods (p &lt; 0.01), and also that residual images have no anatomical structure. The proposed denoising method has advantages in improving the quality of brain MRIs, while effectively removing the noise with a wide range of unknown noise levels without damaging texture details, and has potential clinical promise.

Role of Interaction Range and Buoyancy on the Adhesion of Vesicles.

Vesicles on substrates play a fundamental role in many biological processes, ranging from neurotransmitter release at the synapse on small scales to the nutrient intake of trees by large vesicles. For these processes, the adsorption or desorption of vesicles to biological substrates is crucial. Consequently, it is important to understand the factors determining whether and for how long a vesicle adsorbs to a substrate and what shape it will adopt. Here, we systematically study the adsorption of a vesicle to planar substrates with short- and long-range interactions, with and without buoyancy. We assume an axially symmetric system throughout our simulations. Previous studies often considered a contact potential of zero range and neutral buoyancy. The interaction range alters the location and order of the adsorption transition and is particularly important for small vesicles, e.g., in the synapse. Whereas even small density differences between the inside and the outside of the vesicle give rise to strong buoyancy effects for large vesicles, e.g., giant unilamellar vesicles, as buoyancy effects scale with the fourth power of the vesicle size. We find that (i) an attractive membrane-substrate potential with nonzero spatial extension leads to a pinned state, where the vesicle benefits from the attractive membrane-substrate interaction without significant deformation. The adsorption transition is of first order and occurs when the substrate switches from repulsive to attractive. (ii) Buoyancy shifts the transversality condition, which relates the maximal curvature in the contact zone to the adhesion strength and bending rigidity, up/downward, depending on the direction of the buoyancy force. The magnitude of the shift is influenced by the range of the potential. For upward buoyancy, adsorbed vesicles are at most metastable. We determine the stability limit and the desorption mechanisms and compile the thermodynamic data into an adsorption diagram. Our findings reveal that buoyancy, as well as spatially extended interactions, are essential when quantitatively comparing experiments to theory.

Nutrition contributions of coral reef fisheries not enhanced by capture of small fish

Recent policy recommendations have highlighted the nutritional benefits of fisheries that capture small finfish species. Small fish, particularly those that feed in the pelagic zone, tend to be more nutrient dense than larger species, with increased concentrations of calcium, zinc, and omega-3 fatty acids. However, capturing fish below some recommended size limit (i.e., length at first maturity = Lmat) in coral reefs is frequently considered to be unsustainable and associated with reduced yields and losses of ecosystem functions. To evaluate the potential effects of fish body size, we analyzed nutrient concentrations of 424 demersal and pelagic finfish species reported from Western Indian Ocean artisanal fisheries. We found that length and food source are associated with only small differences in nutrient density in the artisanal catches of this region (≤7% of a child's daily requirement in most cases). We also analyzed 20 years of catch monitoring data from Kenya, where many of the common species have Lmat ∼20–25 cm, to test the potential benefits and tradeoffs of capturing small fishes. Small capture sizes were associated with low yields and sexually immature catches with a mean length of 15 cm resulting in 38% lower catch per unit effort, 37% lower nutrient yield, and a 22% lower maturity index compared to a mean body length of 30 cm. Catches of undersized fish were not associated with substantial increases or decreases in nutrient content relative to human nutritional requirements. Thus, coral reef artisanal fisheries should target moderate to large fishes (>20 cm) to maximize overall yield, nutrient yield, and sustainability.

Numerical investigation on a bimodal mixing system of solid–liquid mixture in an industrial mixing cooker

The powder mixing for foods is regarded to be unique because tons of products are manufactured through powder mixing processes in food industries; hence there is no room for doubt about the importance of a better understanding of the mixing mechanism. On the other hand, the powder mixing for foods has not been established so far. In this study, a series of simulations of the powder mixing in a liquid are performed using the discrete element method–computational fluid dynamics method. A monodispersed and bimodal mixing systems are computed to evaluate the effect of the particle density and the mixing shaft obliquity in a particular mixing cooker. The monodispersed system was computed purely for a purpose of reference to compare with the bimodal system. By performing a simulation on a monodispersed and bimodal mixing systems with very small density differences between solids as well as solid and liquid, this study provides significant evidence and novel insights into understanding the mixing mechanism of food products. In the monodispersed mixing system, the particles with the density close to water are revealed to be affected by the diffusive mixing which is due to buoyancy and the drag force. Furthermore, an increase in mixing shaft obliquity is shown to improve mixing performance in the monodispersed mixing system. In the bimodal mixing system the effect of the gravitational force is shown to be extremely important for the efficient powder mixing, even if the density difference is very small as 5%. The heavier particles initially positioned in the upper layer are drawn down into the layer of the lighter particles by gravitational force, facilitating mixing and enhancing efficiency. These are new findings which showed unique particle behaviour in mixing systems. This study provides new insights into the specific powder mixing mechanism in a liquid which are frequently encountered in the food industries.

Strong atom capture ability and pinning effect by doping an element with large electronegativity difference in Sb2Te3 phase change material

The ultimate goal of memory researchers and developers is to devise a universal memory to replace all memories in complex hierarchies of memory systems. Phase change memory is considered by the International Semiconductor Industry Association as the most likely emerging storage technology to achieve this ambitious goal due to its excellent performance. However, the relatively slow operation speed and poor cycle endurance are stumbling blocks for this purpose. Here, we propose a new doping strategy to construct stable bonding by introducing atoms with large electronegativity differences from the matrix Te atoms enhancing the capture ability and the pinning effect to Te atoms, which facilitates fast nucleation and suppresses migration of Te atoms to achieve significant performance improvement. Using ab initio molecular dynamics simulations, we explore and compare the atomic structures, chemical bonding, and dynamics properties of Hf doped Sb2Te3 (HfST), La doped Sb2Te3 (LaST), Rb doped Sb2Te3 (RbST) and Sr doped Sb2Te3 (SrST). In the XST (X refers to Hf, Rb, La and Sr atoms) systems, the increase of the more stable ring structure fluctuation and the stronger atom capture capability of rings enable more and faster formation of nuclei during crystallization. Higher Peierls-like distortions suggest better thermal stability. The motility of Te atoms is suppressed due to the pinning effect. High coordination of atoms and the concentration reduction of the lone-pair electron and void enable small density difference. Our results show that fast universal memory with good endurance can be achieved by the construction of strong bonding by doping atoms with large electronegativity differences, providing an important guidance for experiments and industries.

Near-Isotropic Local Attosecond Charge Transfer within the Anisotropic Puckered Layers of Black Phosphorus.

Black phosphorus possesses useful two-dimensional (2D) characteristics of van der Waals coupled materials but additionally features an in-plane anisotropic puckered layer structure that deviates from common 2D materials. Three distinct directions exist within the lattice of black phosphorus: the in-plane armchair and zigzag directions and the out-of-plane direction, with each distinct phosphorus 3p partial density of states. This structural anisotropy is imprinted onto various collective long-range properties, while the extent to which local electronic processes are governed by this directionality is unclear. At the P L1 edge, the directional selectivity of the core-hole clock method was used to probe the local charge transfer dynamics of electrons excited into the 3p-derived conduction band on an attosecond time scale. Here we show that the surprisingly small anisotropy of 3p electron transfer times reflects the similarly small differences in the 3p-derived unoccupied density of states caused by the underlying phosphorus bonding angles within the puckered layers.

Experimental Research into the Uniaxial Compressive Strength of Low-Density Reef Limestone Based on Image Recognition.

Low-density reef limestone is widely distributed in tropical oceans; exploring its mechanical properties is of significance to practices in marine foundation engineering. In this research, laboratory experiments on low-density reef limestones with two different types of porous structures were conducted using image recognition methods to study the special mechanical properties of low-reef limestone. S¯ was defined as the parameter quantifying the pore geometry, and the calculation method of S¯ was optimized based on image recognition data. Finally, the influencing factors of uniaxial compressive strength (UCS) of low-density reef limestone were analyzed, and a modified formula considering pore structure was proposed. The results indicate the following: Image recognition methods were used to determine feasibility and convenience of capturing 2D pore geometric information of specimens. The optimization method of S¯ is conducive to improving automatic image recognition accuracy. Low-density reef limestones with different porous structures show small difference in porosity and density, while they exhibit large differences in pore sizes and UCS. The UCS of low-density reef limestone is found to be jointly influenced by pore structure and density (it increases with the decrease in parameter S¯ and increase in density). The results may provide help for those investigating the mechanical properties of reef limestone and practices in marine foundation engineering.

Traveling internal waves in a two-layer shallow medium with variable bathymetry and current: Surface and near-bottom flows

In the linear approximation, we examine the one-dimensional problem of long internal wave propagation without reflection in a stationary two-layer flow with a free boundary in a channel of variable depth and width, in the limiting cases of surface and near-bottom currents. We use the shallow-water approximation and assume that liquids in the layers are ideal, immiscible, and having a small relative density difference, and that the flow is only in one of the layers. To filter fast processes (such as surface gravity waves), the flow velocity is assumed to have the same order of magnitude as the speed of internal waves. Both surface and near-bottom flows are divided into three classes, in accordance with conditions providing wave propagation without reflection. The properties of the flows in each of the classes are examined, and it is shown that the global reflectionless near-bottom flows do exist. The main results and conclusions are illustrated by a few particular solutions. A detailed comparison with the first part of the study concerning the flows with currents in both layers made it possible to clarify and refine some of the results obtained there, and to formulate questions for further investigation. The results obtained may be of interest for understanding of those natural processes in surface and near-bottom flows, which are significantly influenced by internal waves.

Traveling internal waves in a two-layer shallow medium with variable bathymetry and current

In the linear approximation, we examine one-dimensional problems of long internal wave propagation in a stationary flow of two-layer fluids with free boundary in a channel of variable depth and width. We use the shallow-water approximation and assume that liquids in the layers are ideal, immiscible, and have a small relative density difference inherent to natural currents. The conditions the flow must satisfy for wave propagation without reflection are found and analyzed. It is shown that there are three classes of such flows, and the characteristic properties of each of them are studied and compared with those found earlier in a similar problem for surface waves. A general analysis of the problem is illustrated by a few particular solutions. The results obtained can be of interest for understanding natural phenomena in which internal waves play a significant role.

Autumn stopover hotspots and multiscale habitat associations of migratory landbirds in the eastern United States

Halting the global decline of migratory birds requires a better understanding of migration ecology. Stopover sites are a crucial yet understudied aspect of bird conservation, mostly due to challenges associated with understanding broad-scale patterns of transient habitat use. Here, we use a national network of weather radar stations to identify stopover hotspots and assess multiscale habitat associations of migratory landbirds across the eastern United States during autumn migration. We mapped seasonal bird densities over 5 y (2015 to 2019) from 60 radar stations covering 63.2 million hectares. At a coarse scale, we found that landbirds migrate across a broad front with small differences in migrant density between radar domains. However, relatively more birds concentrate along the Mississippi River and Appalachian Mountains. At a finer scale, we identified radar pixels that consistently harbored high densities of migrants for all 5 y, which we classify as stopover hotspots. Hotspot probability increased with percent cover of all forest types and decreased with percent cover of pasture and cultivated crops. Moreover, we found strong concentrating effects of deciduous forest patches within deforested regions. We also found that the prairie biome in the Midwest (now mostly cropland) is likely a migration barrier, with large concentrations of migrants at the prairie-forest boundary after crossing the agricultural Midwest. Overall, the broad-front migration pattern highlights the importance of locally based conservation efforts to protect stopover habitats. Such efforts should target forests, especially deciduous forests in highly altered landscapes. These findings demonstrate the value of multiscale habitat assessments for the conservation of migratory landbirds.

Friday, June 17, 20222:30 PM - 3:30 PM PHSPP01 Presentation Time: 2:30 PM: Dosimetry for Beta Emitting Brachytherapy Sources

<h3>Purpose</h3> Dosimetry for photon-emitting brachytherapy sources is well established, to the point of commercial software for clinical treatment planning to account for material heterogeneities and patient-specific radiation scatter conditions. However, dosimetry standards for beta sources are simpler and have not been recently investigated. While the AAPM TG-149 and ICRU 72 Reports issue clinical guidance and reference data, a modern evaluation of beta-source dosimetry is missing. This investigation strives to identify the potential role for beta-emitting radionuclides as well as to examine theoretical sources for a deeper understanding of beta dosimetry. <h3>Materials and Methods</h3> Available beta-emitting brachytherapy sources include P-32, Sr/Y-90, and Ru/Rh-106 with Y-90 commonly used for selective internal radiation therapy (SIRT) of cancers in the liver. Energy spectra of these sources were taken from ICRU 72 with dose simulations performed using the MCNP v6.2 Monte Carlo radiation transport code. From ICRU 72, the maximum energies for P-32, Sr/Y-90, Y-90, and Ru/Rh-106 were 1.71, 2.28, 2.28, and 3.54 MeV, respectively. Monoenergetic sources of 0.5, 1, 1.5, 2, 3, and 4 MeV were also simulated to extend the energy range and scrutinize dose-distribution dependence on electron energy. Since 2D dose distribution are highly dependent on the design of a source, point sources were simulated to evaluate the depth-dose distribution as a function of radius, <i>r</i>, as well radial dose functions that removed the inverse-square effect. Dose was sampled in 15 cm diameter spheres comprised of water, tissue (ICRU 44 skeletal muscle), or bone (ICRU 44 cortical bone). To mass density effect was quantified in water for 0.5, 1, and 2 g/cm³ with 1 and 2 g/cm³ for tissue and bone, respectively. Observed dose differences were then ascribed to medium differences and not confounded by small differences in mass density. Dose was sampled with radial increments from 0.001 to 0.02 cm for direct comparisons to ICRU 72 results and to evaluate the effect of volume averaging. While only electron sources were considered, dose contributions from bremsstrahlung photons were quantified through comparing results of electron- and electron:photon-transport. Energy deposition was calculated using the MCNP *F8 pulse-height tally, then dividing by the voxel mass to obtain absorbed dose per electron history. With 10⁷ source electrons, statistical uncertainties (<i>k</i>=1) were <0.1% for <i>r</i><0.7 CSDA. When studying bremsstrahlung, 10⁸ source electrons produced 4% statistical uncertainties for <i>r</i>>1.2 CSDA. <h3>Results</h3> In the high dose-gradient region for radionuclide-based beta emitters, the calculated dose distributions in water were within 20% of those from the ICRU 72 Report, which used binned monoenergetic sources to approximate the continuous beta spectra. The radionuclide-based sources exhibited radial dose functions in water that diminished much more steeply than Pd-103, similar to a <0.01 MeV photon source. Sr/Y-90 had a bimodal distribution. Radial dose functions for radionuclide-based sources were inversely proportional to changes in mass density, differing from the scaling-factor models by Cross (1967) and the ICRU 56 Report over the large range of media and densities examined herein. Further, radial dose functions for monoenergetic sources demonstrated a Bragg peak with dose enhancement at <i>r</i>∼0.7 CSDA for all studied energies and media. A sampling of results for radionuclides and monoenergetic sources in water is given in the figure. <h3>Conclusions</h3> Beta dosimetry is largely governed by the mass density with medium composition being a much lesser effect, unlike for low-energy photons where the photoelectric effect is highly sensitive to <i>Z</i>_eff. Unlike for high-energy photon dosimetry, beta dosimetry is negligibly impacted by radiation scatter conditions due to their short range. While Ru/Rh-106 and Sr/Y-90 are proven for shallow plaque brachytherapy and Y-90 for SIRT, beta-emitting radionuclides do not have adequate range for interstitial implants like prostate brachytherapy since dose uniformity is much worse than with HDR Ir-192 or LDR Pd-103 brachytherapy.

Buoyancy segregation suppresses viscous fingering in horizontal displacements in a porous layer

We consider the axisymmetric displacement of an ambient fluid by a second input fluid of lower density and lower viscosity in a horizontal porous layer. If the two fluids have been segregated vertically by buoyancy, then the flow becomes self-similar with the input fluid preferentially flowing near the upper boundary. We show that this axisymmetric self-similar flow is stable to angular-dependent perturbations for any viscosity ratio. The Saffman–Taylor instability is suppressed due to the buoyancy segregation of the fluids. The radial extent of the segregated flow is inversely proportional to the viscosity ratio. This horizontal extension of the intrusion eliminates the discontinuity in the pressure gradient between the fluids associated with the viscosity contrast. Hence at late times, viscous fingering is shut down even for arbitrarily small density differences. The stability is confirmed through numerical integration of a coupled problem for the interface shape and the pressure gradient, and through complementary asymptotic analysis, which predicts the decay rate for each mode. The results are extended to anisotropic and vertically heterogeneous layers. The interface may have relatively steep shock-like regions, but the flow is always stable when the fluids have been segregated by buoyancy, as in a uniform layer.