Radiation Transport Model Research Articles

On sampling of scattering phase functions

Abstract Monte Carlo radiative transfer, which has been demonstrated as a successful algorithm for modelling radiation transport through the astrophysical medium, relies on sampling of scattering phase functions. We review several classic sampling algorithms such as the tabulated method and the accept–reject method for sampling the scattering phase function. The tabulated method uses a piecewise constant approximation for the true scattering phase function; we improve its sampling performance on a small scattering angle by using piecewise linear and piecewise log-linear approximations. It has previously been believed that certain complicated analytic phase functions such as the Fournier–Forand phase function cannot be simulated without approximations. We show that the tabulated method combined with the accept–reject method can be applied to sample such complicated scattering phase functions accurately. Furthermore, we introduce the Gibbs sampling method for sampling complicated approximate analytic phase functions. In addition, we propose a new modified Henyey–Greenstein phase function with exponential decay terms for modelling realistic dust scattering. Based on Monte Carlo simulations of radiative transfer through a plane-parallel medium, we also demonstrate that the result simulated with the new phase function can provide a good fit to the result simulated with the realistic dust phase function.

Radiation transport and scaling of optical depth in Nd:YAG laser-produced microdroplet-tin plasma

Experimental scaling relations of the optical depth are presented for the emission spectra of a tin-droplet-based, 1-μm-laser-produced plasma source of extreme-ultraviolet (EUV) light. The observed changes in the complex spectral emission of the plasma over a wide range of droplet diameters (16–65 μm) and laser pulse durations (5–25 ns) are accurately captured in a scaling relation featuring the optical depth of the plasma as a single, pertinent parameter. The scans were performed at a constant laser intensity of 1.4 × 1011 W/cm2, which maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for industrial EUV lithography. Using a one-dimensional radiation transport model, the relative optical depth of the plasma is found to linearly increase with the droplet size with a slope that increases with the laser pulse duration. For small droplets and short laser pulses, the fraction of light emitted in the 2% bandwidth around 13.5 nm relative to the total spectral energy is shown to reach high values of more than 14%, which may enable conversion efficiencies of Nd:YAG laser light into—industrially—useful EUV radiation rivaling those of current state-of-the-art CO2-laser-driven sources.

Possis: predicting spectra, light curves, and polarization for multidimensional models of supernovae and kilonovae

ABSTRACT We present possis, a time-dependent three-dimensional Monte Carlo code for modelling radiation transport in supernovae and kilonovae. The code incorporates wavelength- and time-dependent opacities, and predicts viewing-angle dependent spectra, light curves, and polarization for both idealized and hydrodynamical explosion models. We apply the code to a kilonova model with two distinct ejecta components, one including lanthanide elements with relatively high opacities and the other devoid of lanthanides and characterized by lower opacities. We find that a model with total ejecta mass $M_\mathrm{ej}=0.04\, \mathrm{M}_\odot$ and half-opening angle of the lanthanide-rich component Φ = 30° provides a good match to GW 170817/AT 2017gfo for orientations near the polar axis (i.e. for a system viewed close to face-on). We then show how crucial is the use of self-consistent multidimensional models in place of combining one-dimensional models to infer important parameters, such as the ejecta masses. We finally explore the impact of Mej and Φ on the synthetic observables and highlight how the relatively fast computation times of possis make it well-suited to perform parameter-space studies and extract key properties of supernovae and kilonovae. Spectra calculated with possis in this and future studies will be made publicly available.

Microphysical Characteristics of Precipitation during Pre-monsoon, Monsoon, and Post-monsoon Periods over the South China Sea

Raindrop size distribution (RSD) characteristics over the South China Sea (SCS) are examined with onboard Parsivel disdrometer measurements collected during marine surveys from 2012 to 2016. The observed rainfall is divided into pre-monsoon, monsoon, and post-monsoon periods based on the different large-scale circumstances. In addition to disdrometer data, sounding observation, FY-2E satellite, SPRINTARS (Spectral Radiation-Transport Model for Aerosol Species), and NCEP reanalysis datasets are used to illustrate the dynamical and microphysical characteristics associated with the rainfall in different periods. Significant variations have been observed in respect of raindrops among the three periods. Intercomparison reveals that small drops (D < 1 mm) are prevalent during pre-monsoon precipitation, whereas medium drops (1–3 mm) are predominant in monsoon precipitation. Overall, the post-monsoon precipitation is characterized by the least concentration of raindrops among the three periods. But, several large raindrops could also occur due to severe convective precipitation events in this period. Classification of the precipitation into stratiform and convective regimes shows that the lg(Nw) value of convective rainfall is the largest (smallest) in the pre-monsoon (post-monsoon) period, whereas the Dm value is the smallest (largest) in the pre-monsoon (post-monsoon) period. An inversion relationship between the coefficient A and the exponential b of the Z—R relationships for precipitation during the three periods is found. Empirical relations between Dm and the radar reflectivity factors at Ku and Ka bands are also derived to improve the rainfall retrieval algorithms over the SCS. Furthermore, the possible causative mechanisms for the significant RSD variability in different periods are also discussed with respect to warm and cold rain processes, raindrop evaporation, convective activities, and other meteorological factors.

Methodological approach to development of dosimetric models of the human skeleton for beta-emitting radionuclides

Objective of the study : to develop a skeleton model for assessing red bone marrow dose from osteotropic beta-emitting radionuclides. This article describes the modeling methodology which takes into account the individual variability of the macro- and microstructure of bone tissue. Materials and methods: it is proposed to model bone sites with active hematopoiesis by dividing them into small segments described by simple geometric shapes. Spongiosa, which fills the segments, is modeled as an isotropic three-dimensional grid (carcass) of rod-like trabeculae that “run through” the bone marrow. In the process of randomization, multiple carcass deformations are simulated by changing the positions of the grid nodes and the thickness of the rods. Model grid parameters are selected in accordance with the parameters of spongiosa microstructures taken from the published papers. Stochastic modeling of radiation transport in heterogeneous environments simulating distribution of bone tissue and marrow in each of the segments is performed by Monte Carlo method. The model output for the lumbar vertebra is given as an example. The generated vertebral model allowed us to obtain the dosimetric characteristics of bone marrow irradiation, which are comparable to the results obtained with ICRP model developed based on the data of micro-images of bone structures. For the first time ever confidence intervals of dosimetric characteristics associated with individual variability of bone structure were evaluated. The developed methodology for the calculation of doses absorbed in the bone marrow from osteotropic radionuclides does not require additional studies of autopsy material. The obtained results will be used to calculate individual doses in a cohort of Techa riverside residents who were exposed due to Techa River contamination as a result of liquid radioactive waste discharges by the Mayak Production Association.

Surrogate-Based Robust Design for a Non-Smooth Radiation Source Detection Problem

In this paper, we develop and numerically illustrate a robust sensor network design to optimally detect a radiation source in an urban environment. This problem exhibits several challenges: penalty functionals are non-smooth due to the presence of buildings, radiation transport models are often computationally expensive, sensor locations are not limited to a discrete number of points, and source intensity and location responses, based on a fixed number of sensors, are not unique. We consider a radiation source located in a prototypical 250 m × 180 m urban setting. To address the non-smooth properties of the model and computationally expensive simulation codes, we employ a verified surrogate model based on radial basis functions. Using this surrogate, we formulate and solve a robust design problem that is optimal in an average sense for detecting source location and intensity with minimized uncertainty.

New Constraints on the Thermal Conductivity of the Upper Mantle From Numerical Models of Radiation Transport

AbstractTo address uncertainties in the values and mathematical form of the radiative thermal conductivity krad in the mantle, we developed new models for the transport, scattering, and absorption of thermal radiation in semitransparent multiphase polycrystalline assemblages. We show that the Rosseland diffusion equation correctly describes the diffusion of thermal radiation and infer the form of the effective spectral coefficients through numerical experimentation. We show that the scattering coefficient depends on the grain size and on interphase contact statistics in complicated ways, but that simplifications can be employed in practice. The effective opacity of a composite random material is a harmonically weighted mixture in the limit of infinitely large grain size and an arithmetically weighted mixture in the limit of infinitesimal grain size. Using existing absorption spectra for major upper mantle minerals, we estimate krad as a function of temperature, grain size, and petrology. In mantle assemblages, the scattering effect is important for small grain sizes (&lt;1 mm), but the grain size effect on the effective opacity of a multiphase medium is important for grain sizes up to 10 cm. We calculate that upper mantle krad is about 2–3.5 W·m−1·K−1 for a representative mean grain size range of 0.01 to 1 cm. This translates to a total thermal conductivity of 5.5–7 W·m−1·K−1. Application of our model to the cooling of oceanic lithosphere shows that krad increases net cooling by about 25%.

Radiation Exposure of Workers and Volunteers in Shelters and Community Reception Centers in the Aftermath of a Nuclear Detonation.

After a nuclear detonation, workers and volunteers providing first aid, decontamination, and population monitoring in public shelters and community reception centers will potentially be exposed to radiation from people they are assisting who may be contaminated with radioactive fallout. A state-of-the-art computer-aided design program and radiation transport modeling software were used to estimate external radiation dose to workers in three different exposure scenarios: performing radiation surveys/decontamination, first aid, and triage duties. Calculated dose rates were highest for workers performing radiation surveys due to the relative proximity to the contaminated individual. Estimated cumulative doses were nontrivial but below the occupational dose limit established for normal operations by the Occupational Safety and Health Administration.

GEANT4 models of HPGe detectors for radioassay

Radiation transport models of two high purity germanium detectors, GeII and GeIII, located at the University of Alabama have been created in GEANT4 \cite{geant4}. These detectors have been used extensively for radioassay measurements of materials used in various low background experiments. The two models have been validated against actual data under several scenarios typically seen in radioassay measurements. The systematic uncertainties of the models for GeII and GeIII are estimated to be $\sim$12\% and $\sim$9\% respectively.

Microscale Thermal Energy Transfer Over a Combined System of Thin Films: Analytical Approach

An analytical approach for the solution of the equation for phonon transport is presented for the combination thin films system. The transient phonon radiative transport model is considered and the combination of the silicon–diamond–silicon films is accommodated in the analysis. The multi-film system is thermally disturbed from the edges through introducing temperature difference across the combined films. Equivalent equilibrium temperature is considered quantifying the distribution of the phonon intensity in the films. Equivalent equilibrium temperature obtained from the analytical approach is compared with that predicted from the numerical solution. It is found that numerical predictions of equivalent equilibrium temperature agree well with those obtained from the analytical approach. The boundary scattering of phonons at the film edges causes equivalent equilibrium temperature jump at film edges, which becomes apparent in the early heating periods. Phonon scattering in the combined films causes sharp decay of temperature in the films, which is more pronounced in the silicon film than that of the diamond.

Verification of the Elekta Monaco TPS Monte Carlo in modelling radiation transmission through metals in a water equivalent phantom.

Many studies have performed dosimetric studies using various metal implants however these are difficult to translate to other implants of a different geometry or material (Rijken and Colyer, J Appl Clin Med Phys 18:5:301-306, 2017; Ade and du Plessis, J Appl Clin Med Phys 18:5:162-173, 2017; Prabhakar et al. Rep Pract Oncol Radiother 18:209-213, 2013; Ng et al. Rep Pract Oncol Radiother 20:273-277, 2015; Reft et al. Med Phys 30:1162-1182, 2003; Sasaki et al., Nihon Hoshasen Gijutsu Gakkai Zasshi 72(9):735-745, 2016). In this study, the ability of the Monaco Monte Carlo algorithm (Elekta AB, Stockholm, Sweden) to model radiation transport through different types of metals was evaluated. Investigation of the capabilities and limitations of the algorithm is required for the potential use of Monaco for planning radiotherapy treatments when avoidance of metal implants is clinically undesirable. A MapCHECK 2 diode array (Sun Nuclear Corp, Melbourne, USA) and a PTW 30013 Farmer chamber was used to measure the dose at depth, downstream of 1cm × 5cm × 5cm metal blocks of three known compositions; stainless steel, aluminium and MCP96. The setup was imaged using a CT scanner and imported into the Monaco TPS where the beam arrangement was replicated. The density of the metals was overridden using the known electron density of each (IMPAC Medical Systems Inc, Monaco dose calculation technical reference. IMPAC Medical Systems, Sunnydale, CA, 2013). The differences between the dose measured using the ion chamber and calculated using Monaco downstream of the 1cm metal blocks were respectively: - 1.2%, - 2.2% and 9.5% when irradiated using a 6 MV beam, and - 0.9%, - 1.3% and 14%, when irradiated using a 15 MV beam. This was then repeated using 2cm and 3cm of each metal type giving similar results for aluminium and stainless steel and increased discrepancy for MCP96. Discrepancies between treatment planning software and measurements at depth have been shown to give uncertainties between 5 and 23% in previous studies (Rijken and Colyer, J Appl Clin Med Phys 18:5:301-306, 2017; Ade and du Plessis, J Appl Clin Med Phys 18:5:162-173, 2017; Prabhakar et al. Rep Pract Oncol Radiother 18:209-213, 2013; Ng et al. Rep Pract Oncol Radiother 20:273-277, 2015; Reft et al. Med Phys 30:1162-1182, 2003; Sasaki et al., Nihon Hoshasen Gijutsu Gakkai Zasshi 72(9):735-745, 2016). This study uses basic shapes providing results that remove the uncertainties in geometry and can therefore be applied to any shape. This will help determine whether errors in dose calculations are due to the TPS particle transport algorithms or due to other effects, such as inaccurate contouring or incorrect densities. Thus giving the planner an additional degree of freedom in their planning and decision making process.

Hourly Aerosol Assimilation of Himawari‐8 AOT Using the Four‐Dimensional Local Ensemble Transform Kalman Filter

AbstractThe next‐generation geostationary satellite Himawari‐8 has a much higher observation frequency of the aerosol field than polar‐orbiting satellites. Aerosol analyses with a geostationary satellite can advance our understanding of the rapid spatiotemporal evolution of aerosols, which is especially critical for studies of air pollution and its mechanisms. We present a one‐monthlong hourly aerosol analysis using an aerosol data assimilation based on the local ensemble Kalman filter (LETKF), Himawari‐8‐retrieved hourly aerosol optical thicknesses (AOTs), and a global model named Non‐hydrostatic Icosahedral Atmospheric Model coupled with an aerosol model named Spectral Radiation Transport Model for Aerosol Species (NICAM‐SPRINTARS). To assimilate asynchronous observations and avoid frequent switching between the assimilation and ensemble aerosol forecasts, the LETKF is also extended to the four‐dimensional LETKF (4D‐LETKF). The hourly aerosol analyses are evaluated with both the assimilated Himawari‐8 AOTs and independent Moderate Resolution Imaging Spectroradiometer (MODIS)‐ and AErosol RObotic NETwork (AERONET)‐retrieved AOTs. All evaluations show that the assimilations positively affect the model performances and produce simulated AOTs that are closer to the observations. The analyses correctly reduce the significantly positive biases and root‐mean‐square errors of the control experiment, especially over East China and Australia. Our results also show that hourly aerosol analyses with more frequent Himawari‐8 observations are superior to those using the polar satellite MODIS observations. The performances among the LETKF and 4D‐LETKF experiments are generally not so different, but the computational load of the 4D‐LETKF is much lighter than that of the LETKF.

A source biasing and variance reduction technique for Monte Carlo radiation transport modeling of emission tomography problems

In the original article, Equation 4 was incorrectly published. The correct equation is provided in this correction

Model of the thyroid gland irradiation in the radiobiological experiment analysis

Radiation accidents at nuclear installations are usually accompanied by releases of volatile and biologically dangerous iodine radionuclides into the environment, which can lead to contamination of large areas and cause irradiation of the thyroid gland (TG) in the population and mammals. The reference representatives of biota in radioactive contamination of agricultural ecological systems can be agricultural animals. The basis for the selection of farm animals as reference species is the availability of data of constant veterinary control of animal health indicators and characteristics of their changes due to radiation exposure. To date, the regularities of radiation pathology of the thyroid gland in farm animals with the arrival of radioactive isotopes of iodine have been studied. At the same time, the computational base and software for quantitative analysis of dynamics of formation of radiobiological effects are not sufficiently developed. Using numerical models, namely: a) a compartment model of the 131I metabolism and (b) precise radiation transport model for the thyroid gland the radiobiological characteristics for calves’ thyroid irradiation is derived for the experimental conditions. As a result of experimental and calculated data comparison, critical dose resulting in rapid (within a day) radiation destruction of the parenchymal thyroid tissue was estimated.

Testing requirements for active interrogation systems

The role of active-interrogation systems for nuclear security is to detect the presence of special nuclear material inside an object by observing the radiation emitted by that object when it has been exposed to known sources of external radiation. Because of the cost, complexity, and the need to avoid irradiating occupants, active-interrogation systems are intended for cargo applications where shielding can prevent detection by passive radiation detection systems. To ensure that active-interrogation systems for detection of special nuclear material are designed and tested to a consistent level of performance, technical standards are needed for evaluating such systems. This paper addresses the testing standards needed for active-interrogation systems to detect high atomic number materials, fissionable materials, and specific special nuclear materials. Because the use of special nuclear material for a testing standard is not practical, this work focuses on the determination of materials that could be used as surrogates in that they provide a similar response as targets of special nuclear materials. The results of this paper, determined through analytic calculations and radiation transport modeling, are based upon scenarios constructed and applied to specific active-interrogation modalities.

Entropy analysis for thermally disturbed thin films

Entropy generation rate during phonon transport across consecutively placed two thin films is examined. The films are thermal disturbed via setting the temperature differential at the edges. The Boltzmann transport equation is introduced to model the micro/nano scale heat transfer across the films. Thermal resistance is incorporated at the edges and at the interface of the films to model the boundary scattering. Thermodynamic irreversibility in association with the entropy generation rate is formulated in the films while adopting the phonon intensity distribution. The thermal conductivity predictions are validated adopting the previous data. It is demonstrated that the thermal conductivity data predicted and obtained from the experiments are in good agreement. The phonon scattering at the film edges and at the interface results in temperature jump in these regions. As the film thickness reduces, the entropy generation rate becomes large, which is related to the thermodynamic irreversibility caused by temperature jump. The entropy generation rate predicted through the Fourier heating model remains significantly larger than that obtained from the phonon radiative transport model, which is more pronounced for the small film thicknesses.

Investigation of Dose Rates Exterior to an Above-Ground Waste Storage Facility Using Radiation Transport Models

The dose rate profile at different heights above the ground and as a function of distance from the north, west, and south walls of an above-ground waste storage facility was analyzed using the Monte Carlo n-Particle Transport eXtended (MCNPX) radiation transport code. The waste storage facility houses 9,996 waste barrels of conditioned waste. The facility has concrete shielding added to the building walls on the north, west, and east sides, with no such additional shielding towards the roof or the south side wall; instead, the distance from the first row of barrels to the wall is extended to allow for maneuverability of a crane on the south side. The dose rate is computed as a function of distance using MCNPX and assuming a homogeneous Co distribution in each waste barrel. Different dose regions are identified and analyzed based on graphical features and best-fit functions. The dose rates were expected to be largest at the wall of the facility and subsequently decrease continuously with distance from the repository; however, our analysis indicates a peak in dose rate observed for all heights on the north and west sides of the facility. This peak is likely due to scattering in the shielding material and atmosphere, and possibly could be ascribed to skyshine. The difference between the dose rate at 1 m outside the wall and the peak dose rate is significant, and indicates that the dose rate measured close to the wall may not always be conservative for extended sources, such as an above-ground waste storage facility.

Impacts of meteorological nudging on the global dust cycle simulated by NICAM coupled with an aerosol model

In this study, we present simulations of the global dust cycle for present day conditions using a new dust-atmosphere model based on the Non-hydrostatic Icosahedral Atmospheric Model (NICAM) coupled with the Spectral Radiation Transport Model for Aerosol Species (SPRINTARS). We focus on evaluations of the dust simulation with respect to emissions, depositions, surface concentrations, aerosol optical depths (AODs), and the dust-aerosol direct radiative effects (DREs). The sensitivities of the dust simulation to the meteorological fields are also investigated through with and without meteorological nudging. NICAM without meteorology nudging tends to systemically overestimate the 10 m wind speeds by approximately 30%–40%, whereas the daily magnitudes and variations in the 10 m wind speeds are both significantly improved with meteorological nudging, especially over the Sahara Desert. The estimated annual global mean dust emission flux, dust AOD, and dust-aerosol shortwave DRE at the top of the atmosphere with meteorological nudging are 1463 Tg yr−1, 0.033, and −1.3 Wm-2, respectively. Due to the approximately 30%–40% overestimations of the 10 m wind speeds over the two major desert regions, the estimated annual global mean dust emission flux, AOD, and DRE without meteorological nudging are significantly greater than those with meteorological nudging. The overestimations of 10 m wind speeds and the associated dust emissions are mainly caused by the positive biases of wind speeds especially from surface to approximately 2 km and slightly affected by the temperature fields. The monthly variations in the dust depositions over the Atlantic and the surface dust concentrations over the Pacific are all better simulated with meteorological nudging. Compared to both the AERONET (Aerosol Robotics Network)- and MODIS (Moderate-Resolution Imaging Spectroradiometer)- retrieved AODs, the simulated daily AOD variations are significantly improved with meteorological nudging, especially over the dust-aerosol dominated regions. The global and annual mean dust lifetime and size distribution, which are two critical factors for estimating dust radiative effects, are quite similar between the dynamic and nudged NICAMs. We therefore can use the dynamic model to understand climate-dust interactions in a global and annual scale. Furthermore, we can improve the model performances for some applications in regional and seasonal scales by meteorological nudging which probably cannot be achieved by just tuning the dust emission.

Organ doses in pediatric patients undergoing cardiac-centered fluoroscopically guided interventions: Comparison of three methods for computational phantom alignment.

To assess various computational phantom alignment techniques within Monte Carlo radiation transport models of pediatric fluoroscopically guided cardiac interventional studies. Logfiles, including all procedure radiation and machine data, were extracted from a Toshiba infinix-I unit in the University of Florida Pediatric Catheterization Laboratory for a cohort of 10 patients. Two different alignment methods were then tested against a ground truth standard based upon identification of a unique anatomic reference point within images co-registered to specific irradiation events within each procedure. The first alignment method required measurement of the distance from the edge of the exam table to the top of the patient's head (table alignment method). The second alignment method fixed the anatomic reference point to be the geometric center of the heart muscle, as all 10 studies were cardiac in nature. Monte Carlo radiation transport simulations were performed for each patient and intervention using morphometry-matched hybrid computational phantoms for the reference and two tested alignment methods. For each combination, absorbed doses were computed for 28 organs and root mean square organ doses were assessed and compared across the alignment methods. The percent error in root mean square organ dose ranged from -57% to +41% for the table alignment method, and from -27% to +22% for the heart geometric centroid alignment method. Absorbed doses to specific organs, such as the heart and lungs, demonstrated higher accuracy in the heart geometric centroid alignment method, with average percent errors of 10% and 1.4%, respectively, compared to average percent errors of -32% and 24%, respectively, using the table alignment method. Of the two phantom alignment methods investigated in this study, the use of an anatomical reference point - in this case the geometric centroid of the heart - provided a reliable method for radiation transport simulations of organ dose in pediatric interventional cardiac studies. This alignment method provides the added benefit of requiring no physician input, making retrospective calculations possible. Moving forward, additional anatomical reference methods can be tested to assess the reliability of anatomical reference points beyond cardiac centered procedures.

Practical considerations for modeling streamer discharges in air with radiation transport

Two basic challenges limiting the simulation capabilities of the streamer discharge community are the efficient resolution of Poisson’s equation and the proper treatment of photoionization. This paper addresses both of these challenges, beginning with a graphics processing unit executed multigrid (MG) algorithm to efficiently solve Poisson’s equation on a massively parallel platform. When utilized in a 3D particle-in-cell (PIC) model with radiation transport, the MG solver is demonstrated to reduce the required simulation time by approximately a factor of three over a conventional Jacobi scheme. Next, a fully theoretical photoionization model, based on the basic properties of N2 and O2 molecules is developed as an alternative to widely utilized semi-empirical models. Following a review of N2 emission properties, a total of eight transitions from only three excited states are reported as a base set of transitions for a practical physics-based photoionization model. A 3D PIC simulation of streamer formation is demonstrated with two dominant transitions included in the radiation transport model.