Monte Carlo Radiation Transport Research Articles

Radiation pressure in super star cluster formation

The physics of star formation at its extreme, in the nuclei of the densest and the most massive star clusters in the universe - potential massive black hole nurseries - has for decades eluded scrutiny. Spectroscopy of these systems has been scarce, whereas theoretical arguments suggest that radiation pressure on dust grains somehow inhibits star formation. Here, we harness an accelerated Monte Carlo radiation transport scheme to report a radiation hydrodynamical simulation of super star cluster formation in turbulent clouds. We find that radiation pressure reduces the global star formation efficiency by 30-35%, and the star formation rate by 15-50%, both relative to a radiation-free control run. Overall, radiation pressure does not terminate the gas supply for star formation and the final stellar mass of the most massive cluster is $\sim 1.3\times10^6\,M_\odot$. The limited impact as compared to in idealized theoretical models is attributed to a radiation-matter anti-correlation in the supersonically turbulent, gravitationally collapsing medium. In isolated regions outside massive clusters, where the gas distribution is less disturbed, radiation pressure is more effective in limiting star formation. The resulting stellar density at the cluster core is $\ge 10^{8}\,M_{\odot}\,\textrm{pc}^{-3}$, with stellar velocity dispersion $\gtrsim 70\,\text{km}\,\text{s}^{-1}$. We conclude that the super star cluster nucleus is propitious to the formation of very massive stars via dynamical core collapse and stellar merging. We speculate that the very massive star may avoid the claimed catastrophic mass loss by continuing to accrete dense gas condensing from a gravitationally-confined ionized phase.

Comparison of normal tissue dose calculation methods for epidemiological studies of radiotherapy patients

Radiation dosimetry is an essential input for epidemiological studies of radiotherapy patients aimed at quantifying the dose–response relationship of late-term morbidity and mortality. Individualised organ dose must be estimated for all tissues of interest located in-field, near-field, or out-of-field. Whereas conventional measurement approaches are limited to points in water or anthropomorphic phantoms, computational approaches using patient images or human phantoms offer greater flexibility and can provide more detailed three-dimensional dose information. In the current study, we systematically compared four different dose calculation algorithms so that dosimetrists and epidemiologists can better understand the advantages and limitations of the various approaches at their disposal. The four dose calculations algorithms considered were as follows: the (1) Analytical Anisotropic Algorithm (AAA) and (2) Acuros XB algorithm (Acuros XB), as implemented in the Eclipse treatment planning system (TPS); (3) a Monte Carlo radiation transport code, EGSnrc; and (4) an accelerated Monte Carlo code, the x-ray Voxel Monte Carlo (XVMC). The four algorithms were compared in terms of their accuracy and appropriateness in the context of dose reconstruction for epidemiological investigations. Accuracy in peripheral dose was evaluated first by benchmarking the calculated dose profiles against measurements in a homogeneous water phantom. Additional simulations in a heterogeneous cylinder phantom evaluated the performance of the algorithms in the presence of tissue heterogeneity. In general, we found that the algorithms contained within the commercial TPS (AAA and Acuros XB) were fast and accurate in-field or near-field, but not acceptable out-of-field. Therefore, the TPS is best suited for epidemiological studies involving large cohorts and where the organs of interest are located in-field or partially in-field. The EGSnrc and XVMC codes showed excellent agreement with measurements both in-field and out-of-field. The EGSnrc code was the most accurate dosimetry approach, but was too slow to be used for large-scale epidemiological cohorts. The XVMC code showed similar accuracy to EGSnrc, but was significantly faster, and thus epidemiological applications seem feasible, especially when the organs of interest reside far away from the field edge.

FreeCAD based modeling study on MCNPX for accelerator driven system

MCNPX is a general purpose Monte Carlo radiation transport code designed to track many particle types over broad ranges of energies and it has the potential to deal with Accelerator Driven System (ADS) problems. The neutronics design and analysis of ADS using MCNPX code is significantly complex mainly on constructing the three dimensional geometry model, especially when there is an additional spallation target coupled with the subcritical reactor constructed by abundant nested repeated structures in several levels. This modeling process has long been recognized as a time consuming, tedious and error-prone task, which is hard to master for novice users. Therefore, it is imperative to build a code system that can translate CAD models of ADS to the native language of MCNPX code. In this context of demand, a code system named CAD-PSMC (FreeCAD based parsing script for MCNPX code) has been developed to solve the ADS modeling conversion problems. In the framework of this code, hierarchical tree-based basic geometry classes and Boolean&Affine operations have been established with a mapping relationship to MCNPX code. Additionally, ray-casting technology and Markov chain based iteration method have been proposed to solve the problem of spline surfaces in complex geometries. Finally, the applicability and accuracy of CAD-PSMC code have been demonstrated by comparing with various reference models and numerical calculation results.

RECORDS: improved Reporting of montE CarlO RaDiation transport Studies

Studies involving Monte Carlo simulations are common in both diagnostic and therapy medical phy- sics research, as well as other fields of basic and applied science. As with all experimental studies , the conditions and parameters used for Monte Carlo simulations impact their scope, validity, limita- tions, and generalizability. Unfortunately, many published peer-reviewed articles involving Mont e Carlo simulations do not provide the level of detail needed for the reader to be able to properly assess the quality of the simulations. The American Association of Physicists in Medicine Task Group #268 developed guidelines to improve reporting of Monte Carlo studies in medical physics research. By following these guidelines, manuscripts submitted for peer-review will include a level of relevant detail that will increase the transparency, the ability to reproduce results, and the overall scientific value of these studies. The guidelines include a checklist of the items that should be included in the Methods, Results, and Discussion sections of manuscripts submitted for peer-review. These guideli- nes do not attempt to replace the journal reviewer, but rather to be a tool during the writing and review process. Given the varied nature of Monte Carlo studies, it is up to the authors and the review- ers to use this checklist appropriately, being conscious of how the different items apply to each partic- ular scenario. It is envisioned that this list will be useful both for authors and for reviewers, to help ensure the adequate description of Monte Carlo studies in the medical physics literatur.

Neutron-neutron angular correlations in spontaneous fission of Cf252 and Pu240

Background: Angular anisotropy has been observed between prompt neutrons emitted during the fission process. Such an anisotropy arises because the emitted neutrons are boosted along the direction of the parent fragment.Purpose: To measure the neutron-neutron angular correlations from the spontaneous fission of $^{252}\mathrm{Cf}$ and $^{240}\mathrm{Pu}$ oxide samples using a liquid scintillator array capable of pulse-shape discrimination. To compare these correlations to simulations combining the Monte Carlo radiation transport code MCNPX with the fission event generator FREYA.Method: Two different analysis methods were used to study the neutron-neutron correlations with varying energy thresholds. The first is based on setting a light output threshold while the second imposes a time-of-flight cutoff. The second method has the advantage of being truly detector independent.Results: The neutron-neutron correlation modeled by FREYA depends strongly on the sharing of the excitation energy between the two fragments. The measured asymmetry enabled us to adjust the FREYA parameter $x$ in $^{240}\mathrm{Pu}$, which controls the energy partition between the fragments and is so far inaccessible in other measurements. The $^{240}\mathrm{Pu}$ data in this analysis was the first available to quantify the energy partition for this isotope. The agreement between data and simulation is overall very good for $^{252}\mathrm{Cf}(\mathrm{sf})$ and $^{240}\mathrm{Pu}(\mathrm{sf})$.Conclusions: The asymmetry in the measured neutron-neutron angular distributions can be predicted by FREYA. The shape of the correlation function depends on how the excitation energy is partitioned between the two fission fragments. Experimental data suggest that the lighter fragment is disproportionately excited.

Radiation shielding calculations for a 3D ITER benchmark model using TRIPOLI-4® Monte Carlo code

The International Thermonuclear Experimental Reactor (ITER) is currently under construction in France. From the radiation shielding and radiation protection points of view, an intense and large neutron source with energy around 14.1 MeV will be generated from the D-T plasma zone during the ITER operation and diverse gamma-ray sources from neutron activation of the reactor structure materials and coolant should be considered for the reactor operation and maintenance. To decrease the radiation impacts caused by these neutron and gamma sources, iterative designs and nuclear analyses of ITER components are currently performed with three dimensional Monte Carlo radiation transport calculations. Due to the important dimensions of ITER, the thick tokamak blanket modules, and the diagnostic and functional port openings, variance reduction techniques are essential in these Monte Carlo neutron transport calculations. To verify the reactor components design models and to check the radiation transport calculation results, advanced graphic features of the calculation tool are also necessary. With the growing interest in using the TRIPOLI-4® Monte Carlo radiation transport code for ITER applications, the aim of this paper is to study the feasibility to use variance reduction features of TRIPOLI-4 code on a 3D ITER benchmark model which is a 40° toroidal segment including 5796 vol cells. The calculation results reported in this paper include the axial and radial profiles of the inboard TF coil heating and the neutron flux attenuation through the equatorial port plugs and shield. The performance of the TRIPOLI-4 graphic tool under its parallel computing mode was also evaluated.

Evaluation of the UF/NCI hybrid computational phantoms for use in organ dosimetry of pediatric patients undergoing fluoroscopically guided cardiac procedures**This work was supported by Advanced Laboratory for Radiation Dosimetry Studies, University of Florida, American Association of University Women, National Cancer Institute Grant 1F31 CA159464.

Epidemiologic data demonstrate that pediatric patients face a higher relative risk of radiation induced cancers than their adult counterparts at equivalent exposures. Infants and children with congenital heart defects are a critical patient population exposed to ionizing radiation during life-saving procedures. These patients will likely incur numerous procedures throughout their lifespan, each time increasing their cumulative radiation absorbed dose. As continued improvements in long-term prognosis of congenital heart defect patients is achieved, a better understanding of organ radiation dose following treatment becomes increasingly vital. Dosimetry of these patients can be accomplished using Monte Carlo radiation transport simulations, coupled with modern anatomical patient models. The aim of this study was to evaluate the performance of the University of Florida/National Cancer Institute (UF/NCI) pediatric hybrid computational phantom library for organ dose assessment of patients that have undergone fluoroscopically guided cardiac catheterizations. In this study, two types of simulations were modeled. A dose assessment was performed on 29 patient-specific voxel phantoms (taken as representing the patient’s true anatomy), height/weight-matched hybrid library phantoms, and age-matched reference phantoms. Two exposure studies were conducted for each phantom type. First, a parametric study was constructed by the attending pediatric interventional cardiologist at the University of Florida to model the range of parameters seen clinically. Second, four clinical cardiac procedures were simulated based upon internal logfiles captured by a Toshiba Infinix-i Cardiac Bi-Plane fluoroscopic unit. Performance of the phantom library was quantified by computing both the percent difference in individual organ doses, as well as the organ dose root mean square values for overall phantom assessment between the matched phantoms (UF/NCI library or reference) and the patient-specific phantoms. The UF/NCI hybrid phantoms performed at percent differences of between 15% and 30% for the parametric set of irradiation events. Among internal logfile reconstructed procedures, the UF/NCI hybrid phantoms performed with RMS organ dose values between 7% and 29%. Percent improvement in organ dosimetry via the use of hybrid library phantoms over the reference phantoms ranged from 6.6% to 93%. The use of a hybrid phantom library, Monte Carlo radiation transport methods, and clinical information on irradiation events provide a means for tracking organ dose in these radiosensitive patients undergoing fluoroscopically guided cardiac procedures.

Electromagnetic Chirps from Neutron Star-Black Hole Mergers.

We calculate the electromagnetic signal of a gamma-ray flare coming from the surface of a neutron star shortly before merger with a black hole companion. Using a new version of the Monte Carlo radiation transport code Pandurata that incorporates dynamic spacetimes, we integrate photon geodesics from the neutron star surface until they reach a distant observer or are captured by the black hole. The gamma-ray light curve is modulated by a number of relativistic effects, including Doppler beaming and gravitational lensing. Because the photons originate from the inspiraling neutron star, the light curve closely resembles the corresponding gravitational waveform: a chirp signal characterized by a steadily increasing frequency and amplitude. We propose to search for these electromagnetic chirps using matched filtering algorithms similar to those used in LIGO data analysis.

An angular biasing method using arbitrary convex polyhedra for Monte Carlo radiation transport calculations

This paper presents a new method for performing angular biasing in Monte Carlo radiation transport codes using arbitrary convex polyhedra to define regions of interest toward which to project particles (DXTRAN regions). The method is derived and is implemented using axis-aligned right parallelepipeds (AARPPs) and arbitrary convex polyhedra. Attention is paid to possible numerical complications and areas for future refinement. A series of test problems are executed with void, purely absorbing, purely scattering, and 50% absorbing/50% scattering materials. For all test problems tally results using AARPP and polyhedral DXTRAN regions agree with analog and/or spherical DXTRAN results within statistical uncertainties. In cases with significant scattering the figure of merit (FOM) using AARPP or polyhedral DXTRAN regions is lower than with spherical regions despite the ability to closely fit the tally region. This is because spherical DXTRAN processing is computationally less expensive than AARPP or polyhedral DXTRAN processing. Thus, it is recommended that the speed of spherical regions be considered versus the ability to closely fit the tally region with an AARPP or arbitrary polyhedral region. It is also recommended that short calculations be made prior to final calculations to compare the FOM for the various DXTRAN geometries because of the influence of the scattering behavior.

RECORDS: improved Reporting of montE CarlO RaDiation transport Studies: Report of the AAPM Research Committee Task Group 268.

Studies involving Monte Carlo simulations are common in both diagnostic and therapy medical physics research, as well as other fields of basic and applied science. As with all experimental studies, the conditions and parameters used for Monte Carlo simulations impact their scope, validity, limitations, and generalizability. Unfortunately, many published peer-reviewed articles involving Monte Carlo simulations do not provide the level of detail needed for the reader to be able to properly assess the quality of the simulations. The American Association of Physicists in Medicine Task Group #268 developed guidelines to improve reporting of Monte Carlo studies in medical physics research. By following these guidelines, manuscripts submitted for peer-review will include a level of relevant detail that will increase the transparency, the ability to reproduce results, and the overall scientific value of these studies. The guidelines include a checklist of the items that should be included in the Methods, Results, and Discussion sections of manuscripts submitted for peer-review. These guidelines do not attempt to replace the journal reviewer, but rather to be a tool during the writing and review process. Given the varied nature of Monte Carlo studies, it is up to the authors and the reviewers to use this checklist appropriately, being conscious of how the different items apply to each particular scenario. It is envisioned that this list will be useful both for authors and for reviewers, to help ensure the adequate description of Monte Carlo studies in the medical physics literature.

40. Voxel-based multi-model fitting method for modelling time activity curves in SPECT images

Introduction Estimating the biodistribution and the pharmacokinetics from time- sequence SPECT images on a per-voxel basis is useful for studying activity non-uniformity or computing absorbed dose distributions by convolution of voxel kernels or Monte-Carlo radiation transport. Current approaches are either region- based, thus assuming uniform activity within the region, or voxel-based but using the same fitting model for all voxels. Methods We propose a voxel-based multi-model fitting method (VoMM) that estimates a fitting function for each voxel by automatically selecting the most appropriate model among a predetermined set with Akaike criteria. This approach can be used to compute the time integrated activity (TIA) for all voxels in the image. To control fitting optimization that may fail due to excessive image noise, an approximated version based on trapezoid integration, named restricted method, is also studied. From this comparison, the number of failed fittings within images was estimated and analyzed. Numerical experiments were used to quantify uncertainties and feasibility was demonstrated with real patient data. Results Regarding numerical experiments, root mean square errors of TIA obtained with VoMM were similar to those obtained with bi-exponential fitting functions, and were lower ( 10%) than with single model approaches that consider the same fitting function for all voxels. Failure rates were lower with VoMM and restricted approach than with single-model methods. On real clinical data, VoMM was able to fit 90% of the voxels and led to less failed fits than single- model approaches. On regions of interest (ROI) analysis, the difference between ROI-based and voxel-based TIA estimations was low, less than 4%. However, the computation of the mean residence time exhibited larger differences, up to 25%. Conclusions The proposed voxel-based multi-model fitting method, VoMM, is 35 feasible on patient data. VoMM leads to similar organ-based TIA estimations as conventional ROI-based method. However, for pharmacokinetics analysis, studies of spatial heterogeneity or voxel-based absorbed dose assessment, VoMM could be used preferentially as it prevents model overfitting.

Phantom validation of quantitative Y-90 PET/CT-based dosimetry in liver radioembolization

BackgroundPET/CT has recently been shown to be a viable alternative to traditional post-infusion imaging methods providing good quality images of 90Y-laden microspheres after selective internal radiation therapy (SIRT). In the present paper, first we assessed the quantitative accuracy of 90Y-PET using an anthropomorphic phantom provided with lungs, liver, spine, and a cylindrical homemade lesion located into the hepatic compartment. Then, we explored the accuracy of different computational approaches on dose calculation, including (I) direct Monte Carlo radiation transport using Raydose, (II) Kernel convolution using Philips Stratos, (III) local deposition algorithm, (IV) Monte Carlo technique (MCNP) considering a uniform activity distribution, and (V) MIRD (Medical Internal Radiation Dose) analytical approach. Finally, calculated absorbed doses were compared with those obtained performing measurements with LiF:Mg,Cu,P TLD chips in a liquid environment.ResultsOur results indicate that despite 90Y-PET being likely to provide high-resolution images, the 90Y low branch ratio, along with other image-degrading factors, may produce non-uniform activity maps, even in the presence of uniform activity. A systematic underestimation of the recovered activity, both for the tumor insert and for the liver background, was found. This is particularly true if no partial volume correction is applied through recovery coefficients. All dose algorithms performed well, the worst case scenario providing an agreement between absorbed dose evaluations within 20%. Average absorbed doses determined with the local deposition method are in excellent agreement with those obtained using the MIRD and the kernel-convolution dose calculation approach.Finally, absorbed dose assessed with MC codes are in good agreement with those obtained using TLD in liquid solution, thus confirming the soundness of both calculation approaches. This is especially true for Raydose, which provided an absorbed dose value within 3% of the measured dose, well within the stated uncertainties.ConclusionsPatient-specific dosimetry is possible even in a scenario with low true coincidences and high random fraction, as in 90Y–PET imaging, granted that accurate absolute PET calibration is performed and acquisition times are sufficiently long. Despite Monte Carlo calculations seeming to outperform all dose estimation algorithms, our data provide a strong argument for encouraging the use of the local deposition algorithm for routine 90Y dosimetry based on PET/CT imaging, due to its simplicity of implementation.

Surface applicator of a miniature X-ray tube for superficial electronic brachytherapy of skin cancer.

We designed and fabricated a surface applicator of a novel carbon nanotube (CNT)-based miniature X-ray tube for the use in superficial electronic brachytherapy of skin cancer. To investigate the effectiveness of the surface applicator, the performance of the applicator was numerically and experimentally analyzed. The surface applicator consists of a graphite flattening filter and an X-ray shield. A Monte Carlo radiation transport code, MCNP6, was used to optimize the geometries of both the flattening filter and the shield so that X-rays are generated uniformly over the desired region. The performance of the graphite filter was compared with that of conventional aluminum (Al) filters of different geometries using the numerical simulations. After fabricating a surface applicator, the X-ray spatial distribution was measured to evaluate the performance of the applicator. The graphite filter shows better spatial dose uniformity and less dose distortion than Al filters. Moreover, graphite allows easy fabrication of the flattening filter due to its low X-ray attenuation property, which is particularly important for low-energy electronic brachytherapy. The applicator also shows that no further X-ray shielding is required for the application because unwanted X-rays are completely protected. As a result, highly uniform X-ray dose distribution was achieved from the miniature X-ray tube mounted with the surface applicators. The measured values of both flatness and symmetry were less than 5% and the measured penumbra values were less than 1 mm. All these values satisfy the currently accepted tolerance criteria for radiation therapy. The surface applicator exhibits sufficient performance capability for their application in electronic brachytherapy of skin cancers.

Monte Carlo simulation of radiation transport and dose deposition from locally released gold nanoparticles labeled with 111In, 177Lu or 90Y incorporated into tissue implantable depots

Permanent seed implantation (PSI) brachytherapy is a highly conformal form of radiation therapy but is challenged with dose inhomogeneity due to its utilization of low energy radiation sources. Gold nanoparticles (AuNP) conjugated with electron emitting radionuclides have recently been developed as a novel form of brachytherapy and can aid in homogenizing dose through physical distribution of radiolabeled AuNP when injected intratumorally (IT) in suspension. However, the distribution is unpredictable and precise placement of many injections would be difficult. Previously, we reported the design of a nanoparticle depot (NPD) that can be implanted using PSI techniques and which facilitates controlled release of AuNP. We report here the 3D dose distribution resulting from a NPD incorporating AuNP labeled with electron emitters (90Y, 177Lu, 111In) of different energies using Monte Carlo based voxel level dosimetry.The MCNP5 Monte Carlo radiation transport code was used to assess differences in dose distribution from simulated NPD and conventional brachytherapy sources, positioned in breast tissue simulating material. We further compare these dose distributions in mice bearing subcutaneous human breast cancer xenografts implanted with 177Lu-AuNP NPD, or injected IT with 177Lu-AuNP in suspension. The radioactivity distributions were derived from registered SPECT/CT images and time-dependent dose was estimated.Results demonstrated that the dose distribution from NPD reduced the maximum dose 3-fold when compared to conventional seeds. For simulated NPD, as well as NPD implanted in vivo, 90Y delivered the most homogeneous dose distribution. The tumor radioactivity in mice IT injected with 177Lu-AuNP redistributed while radioactivity in the NPD remained confined to the implant site. The dose distribution from radiolabeled AuNP NPD were predictable and concentric in contrast to IT injected radiolabeled AuNP, which provided irregular and temporally variant dose distributions.The use of NPD may serve as an intermediate between PSI and radiation delivered by radiolabeled AuNP by providing a controlled method to improve delivery of prescribed doses as well as homogenize dose from low penetrating electron sources.

Voxel-based multimodel fitting method for modeling time activity curves in SPECT images.

Estimating the biodistribution and the pharmacokinetics from time-sequence SPECT images on a per-voxel basis is useful for studying activity nonuniformity or computing absorbed dose distributions by convolution of voxel kernels or Monte-Carlo radiation transport. Current approaches are either region-based, thus assuming uniform activity within the region, or voxel-based but using the same fitting model for all voxels. We propose a voxel-based multimodel fitting method (VoMM) that estimates a fitting function for each voxel by automatically selecting the most appropriate model among a predetermined set with Akaike criteria. This approach can be used to compute the time integrated activity (TIA) for all voxels in the image. To control fitting optimization that may fail due to excessive image noise, an approximated version based on trapezoid integration, named restricted method, is also studied. From this comparison, the number of failed fittings within images was estimated and analyzed. Numerical experiments were used to quantify uncertainties and feasibility was demonstrated with real patient data. Regarding numerical experiments, root mean square errors of TIA obtained with VoMM were similar to those obtained with bi-exponential fitting functions, and were lower (<5% vs. >10%) than with single model approaches that consider the same fitting function for all voxels. Failure rates were lower with VoMM and restricted approaches than with single-model methods. On real clinical data, VoMM was able to fit 90% of the voxels and led to less failed fits than single-model approaches. On regions of interest (ROI) analysis, the difference between ROI-based and voxel-based TIA estimations was low, less than 4%. However, the computation of the mean residence time exhibited larger differences, up to 25%. The proposed voxel-based multimodel fitting method, VoMM, is feasible on patient data. VoMM leads organ-based TIA estimations similar to conventional ROI-based method. However, for pharmacokinetics analysis, studies of spatial heterogeneity or voxel-based absorbed dose assessment, VoMM could be used preferentially as it prevents model overfitting.

Physical validation of a Monte Carlo-based, phantom-derived approach to computed tomography organ dosimetry under tube current modulation.

To physically validate the accuracy of a Monte Carlo-based, phantom-derived methodology for computed tomography (CT) dosimetry that utilizes organ doses from precomputed axial scans and that accounts for tube current modulation (TCM). The output of a Toshiba Aquilion ONE CT scanner was modeled, based on physical measurement, in the Monte Carlo radiation transport code MCNPX (v2.70). CT examinations were taken of two anthropomorphic phantoms representing pediatric and adult patients (15-yr-old female and adult male) at various energies, in which physical organ dose measurements were made using optically stimulated luminescence dosimeters (OSLDs). These exams (chest-abdomen-pelvis) were modeled using organ dose data obtained from the computationally equivalent phantom of each anthropomorphic phantom. TCM was accounted for by weighting all organ dose contributions by both the relative attenuation of the phantom and the image-derived mA value (from the DICOM header) at the same z-extent (cranial-caudal direction) of the axial dose data. The root mean squares of percent difference in organ dose when comparing the physical organ dose measurements to the computational estimates were 21.2, 12.1, and 15.1% for the uniform (no attenuation weighting), weighted (computationally derived), and image-based methodologies, respectively. Overall, these data suggest that the Monte Carlo-based dosimetry presented in this work is viable for CT dosimetry. Additionally, for CT exams with TCM, local attenuation weighting of organ dose contributions from precomputed axial dosimetry libraries increases organ dose accuracy.

Dose assessment following an overexposure of a worker at a Swiss nuclear power plant

Aim. The aim of this work was to assess the doses received by a diver exposed to a radiation source during maintenance work in the fuel transfer pool at a Swiss nuclear power plant, and to define whether the statutory limit was breached or not. Method. Onsite measurements were carried out and different scenarios were simulated using the MicroShield Software and the MCNPX Monte Carlo radiation transport code to estimate the activity of the irradiating object as well as the doses to the limbs and the effective dose delivered to the operator. Results. The activity of the object was estimated to 1.8 TBq. From the various dose estimations, a conservative value of 7.5 Sv was proposed for the equivalent dose to the skin on the hands and an effective dose of 28 mSv. Conclusion. The use of different experimental and calculation methods allowed us to accurately estimate the activity of the object and the dose delivered to the diver, useful information for making a decision on the most appropriate scheme of follow up for the patient.

In-line (α,n) source sampling methodology for monte carlo radiation transport simulations

Abstract A new in-line method for sampling neutrons emitted in (α,n) reactions based on alpha particle source information has been developed for continuous-energy Monte Carlo simulations. The new method uses a continuous-slowing-down model coupled with (α,n) cross section data to precompute the expected neutron yield over the alpha particle lifetime. This eliminates the complexity and computational cost associated with explicit charged particle transport. When combined with an integrated alpha particle decay source sampling capability, the proposed method provides an efficient and accurate method for sampling (α,n) neutrons based solely on nuclide inventories in the problem, with no additional user input required. Results from several example calculations show that the proposed method reproduces the (α,n) neutron yields and energy spectra from reference experiments and calculations.

Particle tracking acceleration via signed distance fields in direct-accelerated geometry Monte Carlo

Abstract Computer-aided design (CAD)-based Monte Carlo radiation transport is of value to the nuclear engineering community for its ability to conduct transport on high-fidelity models of nuclear systems, but it is more computationally expensive than native geometry representations. This work describes the adaptation of a rendering data structure, the signed distance field, as a geometric query tool for accelerating CAD-based transport in the direct-accelerated geometry Monte Carlo toolkit. Demonstrations of its effectiveness are shown for several problems. The beginnings of a predictive model for the data structure's utilization based on various problem parameters is also introduced.

On the effect of Lyman α trapping during the initial collapse of massive black hole seeds

One viable seeding mechanism for supermassive black holes is the direct gaseous collapse route in pre-galactic dark matter halos, producing objects on the order of $10^4 - 10^6$ solar masses. These events occur when the gas is prevented from cooling below $10^4$ K that requires a metal-free and relatively H$_2$-free medium. The initial collapse cools through atomic hydrogen transitions, but the gas becomes optically thick to the cooling radiation at high densities. We explore the effects ofLyman-$\alpha$ trapping in such a collapsing system with a suite of Monte Carlo radiation transport calculations in uniform density and isotropic cases that are based from a cosmological simulation. Our method includes both non-coherent scattering and two-photon line cooling. We find that Lyman-$\alpha$ radiation is marginally trapped in the parsec-scale gravitationally unstable central cloud, allowing the temperature to increase to 50,000 K at a number density of $3 \times 10^4$ cm$^{-3}$ and increasing the Jeans mass by a factor of five. The effective equation of state changes from isothermal at low densities to have an adiabatic index of 4/3 around the temperature maximum and then slowly retreats back to isothermal at higher densities. Our results suggest that Lyman-$\alpha$ trapping delays the initial collapse by raising the Jeans mass. Afterward the high density core cools back to $10^4$ K that is surrounded by a warm envelope whose inward pressure may alter the fragmentation scales at high densities.