Photon Transport Research Articles

Modeling and Monte Carlo simulation on light transmission, absorption, and photoelectric conversion in core-shell nanoparticles for photocatalysis

Up-conversion core-shell nanostructures have been reported to be an effective way to enhance photocatalytic performance. However, the processes of photon transport and conversion utilization between core-shell structures have not been clarified, which limits the design and selection of shell materials for optimal photocatalytic activity. In this work, the model of light transmission, absorption, and photoelectric conversion in core-shell nanoparticles are established. By integrating the Monte Carlo method with fast stochastic algorithms, a new algorithm is designed to comprehensively simulate the main processes of photocatalysis in core-shell nanoparticles. The simulation results indicate that among the traditional semiconductor materials, CdS demonstrates superior photocatalytic efficiency as the shell material with an optimal thickness. Additionally, the incorporation of a SiO2 layer between the core and CdS results in a further increase in photocatalytic efficiency, reaching up to 77.6%.

Observation of Topological Transmission in Terahertz Domino Waveguide Array

In this paper, we propose on-chip terahertz domino waveguide array structures which can enable the topological effect and enhance the robustness of terahertz transmission. Terahertz topological effect (include topologically-protected edge states and binding states) is excited by constructing kink and antikink structures in designed domino waveguide arrays. Furthermore, the robustness of topological waveguide arrays is successfully demonstrated. The topological transmission effects have been experimentally demonstrated using scanning near-field terahertz microscopy. Our study provides a new approach to study the topological photonic transport of waveguide arrays in the terahertz regime and has potential application in terahertz integrated circuit.

Effect of the various tritium breeding materials on the tritium breeding ratio in ARC fusion reactor

This study deals with TBR (Tritium Breeding Ratio) performance of the four different types of molten salt (FLiBe, FLiNaK, FLiNaBe, FLiNaRb) which has four different Li-6 enrichment ratios (0%, 30%, 60%, 90%) Moreover, two different vacuum vessel materials (SiC composite and V4Cr4Ti alloy) have been used in ARC reactor geometry. In models with TBR ≥1.05, it has been desired to examine the sustainability of the tritium production of the reactor by removing the neutron multiplication layer from the design without using the toxic Beryllium element. The neutronic performance studies have been examined in OpenMC, which is an open-source Monte Carlo neutron and photon transport code. The TBR values of the different models obtained and the change of these values according to Li-6 enrichment were examined. Obtaining result showed that the FLiBe molten salt and V4Cr4Ti alloy vacuum vessel wall material could be the best choice for the ARC reactor.

Investigation of the pulsed neutron source method measuring the fission decay properties of a subcritical assembly with low-Z reflectors

The pulsed neutron source (PNS) method measuring the fission decay properties of a subcritical system with reflectors was analyzed numerically. Using the Monte Carlo code JMCT, a spherical subcritical assembly with a low atomic number (low-Z) reflector was modeled, and the neutron and photon transport process initiated by an instantaneous neutron source was simulated. The time dependence of the leakage photon count rate and the neutron populations in the fissile region and the reflector was obtained. It is indicated that the neutron population in the assembly is dominated by that in the reflector after a relatively short period, the length of which is mainly dependent on the reflector's thickness. That is to say, the neutron population in the assembly is more affected by the reflector than by the fissile region. The leakage photon count rate is approximately proportional to the neutron population in the fissile region during the specific time window of hundreds of nanoseconds after the source pulse. The fission decay properties of the fissile region can therefore be approximately derived from the leakage photon count rate based on the PNS method. The reason for the existence of the above time window was analyzed, and the effective time range of leakage photon count rate was discussed.

Kinetic modelling of rarefied gas flows with radiation

Two kinetic models are proposed for high-temperature rarefied (or non-equilibrium) gas flows with internal degrees of freedom and radiation. One of the models uses the Boltzmann collision operator to model the translational motion of gas molecules, which has the ability to capture the influence of intermolecular potentials, while the other adopts the relaxation time approximations, which has higher computational efficiency. In our kinetic model equations, not only the transport coefficients such as the shear/bulk viscosity and thermal conductivity but also their underlying relaxation processes are recovered. The non-equilibrium dynamics of gas flow and radiation are tightly coupled, where the transport properties of gas molecules and photons are correlatively dependent. The proposed kinetic models are validated by the direct simulation Monte Carlo method in several non-radiative rarefied gas flows (e.g. the normal shock wave, Fourier flow, Couette flow and the creep flow driven by the Maxwell demon), and the experimental data of planar heat transfer and normal shock waves in nitrogen. Then, the rarefied gas flows with strong radiation are studied based on the kinetic models, not only in the above one-dimensional gas flows, but also in the two-dimensional radiative hypersonic flow passing a cylinder. The characteristics of heat transfer in the tightly coupled fields of gas and radiation are systematically investigated, particularly the influence of the non-equilibrium photon transport and their interactions with gas molecules are revealed. It is found that the radiation makes a profound contribution to the total heat transfer in radiative hypersonic flow at an intermediate photon Knudsen number.

Rapid prototyping Lab-on-Chip devices for the future: A numerical optimisation of bulk optical parameters in microfluidic systems

Nuclear reactor process control is typically monitored for pure β-emitting radionuclides via manual sampling followed by laboratory analysis, leading to delays in data availability and response times. The development of an in situ microfluidic Lab on Chip (LoC) system with integrated detection capable of measuring pure β-emitting radionuclides presents a promising solution, enabling a reduction in occupational exposure and cost of monitoring whilst providing improved temporal resolution through near real-time data acquisition. However, testing prototypes with radioactive sources is time-consuming, requires specialist facilities/equipment, generates contaminated waste, and cannot rapidly evaluate a wide range of designs or configurations. Despite this, modelling multiple design parameters and testing their impact on detection with non-radioactive substitutes has yet to be adopted as best practice. The measurement of pure β emitters in aqueous media relies on the efficient transport of photons generated by the Cherenkov effect or liquid scintillators to the detector. Here we explore the role of numerical modelling to assess the impact of optical cell geometry and design on photon transmission and detection through the microfluidic system, facilitating improved designs to realise better efficiency of integrated detectors and overall platform design. Our results demonstrate that theoretical modelling and an experimental evaluation using non-radiogenic chemiluminescence are viable for system testing design parameters and their impact on photon transport. These approaches enable reduced material consumption and requirement for specialist facilities for handling radioactive materials during the prototyping process. This method establishes proof of concept and the first step towards numerical modelling approaches for the design optimisation of microfluidic LoC systems with integrated detectors for the measurement of pure β emitting radionuclides via scintillation-based detection.

Decay Dose Shielding Analysis with Hybrid Unstructured Mesh/Constructive Solid Geometry Monte Carlo Calculation and ADVANTG Acceleration

The target segments of the Oak Ridge National Laboratory Second Target Station (STS) neutron production facility become highly activated due to spallation reactions or nuclei transmutation by primary protons and emitted neutrons. Once the target segments are removed from their location within the core vessel, decay dose rates must be accurately quantified to determine the shielding configurations of remote-handling tools and transport casks and to aid in planning maintenance activities. For this analysis, we utilized a hybrid unstructured mesh (UM)/constructive solid geometry approach for calculating spallation products and neutron fluxes, activation calculations using the AARE package that includes the CINDER2008 activation code to calculate the decay photon source at different cooling times, and the ADVANTG code to accelerate the final decay photon transport calculation. Both Type 316 stainless steel (SS-316) and lead were investigated as candidates for shielding materials. The decay photon transport calculation through the thick SS-316 or lead shields exhibited between 25 and 30 orders-of-magnitude attenuations in the radial direction, depending on the shield. Such a difficult shielding calculation required advanced variance reduction. ADVANTG has some missing features, which limits its usability in spallation neutron source applications. It does not support volumetric sources created for MCNP6.2 UM capability. An approximate source was created for this problem. Not only was this approximate source needed for running the ADVANTG calculation to generate the weight windows, but also it was essential to develop source biasing (SB) parameters that were crucial for dramatically accelerating the decay photon transport in this problem. With this approximate source, the analysis was completed in a very reasonable computational time, and the design of the STS remote-handling equipment was finalized. This paper compares the efficiency of Monte Carlo simulations with different weight window and SB parameters calculated using different approximate ADVANTG calculations.

Shutdown Dose Rate with Cartesian Mesh for High-Energy Nuclear Systems

In accelerator-driven systems, charged particles and high-energy neutrons can contribute to the production of nuclides that can persist long after the system has been shut down. These nuclides release photons that contribute to the biological dose. It is essential to quantify the biological dose as a function of time after shutdown to ensure safe working conditions for laborers during maintenance procedures. The shutdown dose rate (SDR) can be calculated with the Rigorous Two-Step (R2S) method, which includes a neutron and photon transport coupled with an activation calculation. For accelerator-driven systems, calculating SDR presents challenges related to the neutron cross-sectional data available for high-energy neutrons. A tally was implemented to collect isotope production data directly in a Monte Carlo N-Particle (MCNP) calculation. The output of this RNUCS tally is then used directly in an activation calculation, bypassing the need to use cross-section data with the neutron flux to obtain the isotope production and destruction data. A mesh-based RNUCS-R2S workflow has been developed based on this tally to calculate SDR in accelerator-driven systems. This workflow operates directly on computer-aided design geometry and supports using a meshed photon source. This workflow has been verified against a cell-based RNUCS-R2S workflow. A test problem with the essential characteristics of an accelerator-driven system was created to use in this analysis. The SDR results are within 40% of the cell-based RNUCS-R2S results. The workflow was also validated with the spallation neutron source system. Most detectors’ SDR results are within 50%, with a few detectors having a significantly lower SDR result than the experimental value.

A review of the Geant4 simulation platform for applications involving optical-based sensing and dosimetry

Geant4 is a versatile simulation platform for numerous applications involving transport of ionizing radiation, including ionizing photons. Moreover, the physics needed for tracking optical photons is also embedded in the Geant4 simulation toolkit. From its introduction two decades ago there have been a variety of reports concerning the use of the Geant4 code in optical photons transport applications. Further to this, in particular for the interests herein, characterizations of radioluminescence radiation detectors have also been conducted. Noting the rapid development of such detectors, reliant on light collection and detection, there has been an implicit need for optimization of the different components forming the respective optical devices. Compared to ionizing radiation transport modeling less attention has been paid to this capability among the radiation physics community and Geant4 users. This review first describes in brief terms the physics built into Geant4 to provide for the transport of optical photons. This is followed by a retrospective on reported applications, discussion of the strengths and deficiencies of optical transport analyses by Geant4 and contemplation of possible future applications.

Current Status of TRIPOLI-4® Monte Carlo Radiation Transport Code on Adult and Pediatric Computational Phantoms for Radiation Dosimetry Study

TRIPOLI-4® is a general-purpose Monte Carlo radiation transport code developed by the Service d’Études des Réacteurs et de Mathématiques Appliquées at CEA-Saclay. It uses continuous-energy nuclear data to simulate neutron, photon, electron, and positron transport in fields like radiation shielding, reactor physics, and nuclear criticality safety. To study radiation protection dosimetry in human tissues and organs, male and female adult computational phantoms from the Medical Internal Radiation Dose–Oak Ridge National Laboratory phantoms family and the International Commission on Radiological Protection (ICRP) publication 110 were recently modeled and calculated using the geometry options of TRIPOLI-4. To easily use the ICRP 110 voxel-based phantoms in different exposure scenarios, a newly developed phantom option is available in TRIPOLI-4 and its display tool T4G. This new phantom option is helpful for modeling one or more phantoms and for improving calculation performance in real irradiation environments. The 2020 published pediatric computational reference phantoms are accessible from ICRP publication 143. Male and female pediatric phantoms are also verified with the new T4G tool and TRIPOLI-4 code. This paper reports on recent works using TRIPOLI-4 on adult and pediatric computational phantoms. The modeling methods of stylized and voxel-based phantoms, the graphic displays of modeled phantoms with T4G, and the verification procedures for single-phantom and two-phantom application cases are presented. Validation for external and internal dosimetry calculations has been performed. Calculation results on organ dose S values for nuclear medicine applications are presented for single female and single male voxel phantoms using 131I and 177Lu radiation sources. Effective dose calculations for two-phantom cases using 99mTc and 18F sources are compared with traditional H*(10) calculations from nuclear medicine patient to patient caregiver.

Response Matrix/Discrete Ordinates Solution of the 1D Fokker-Planck Equation

The Fokker-Planck equation (FPE) is one of the quintessential equations of particle transport theory. Representing small angle scattering characteristics of electron and photon transport by differential scattering indeed is a mathematical/numerical challenge. Here, we address the challenge with the method of response matrix applied to the Sn approximation to arrive at a nearly six-place-precision benchmark. Our approach aligns with the response matrix solution of the radiative transfer equation for anisotropic scattering published previously. We conclude with the comparison of the response matrix benchmark to a classical finite difference approximation.

Influence of NH4Cl additive in a VO2+/VO2+ - AQDS/AQDS2− solar redox flow battery

Solar redox flow batteries are a relatively new type of redox flow battery technology that uses solar energy to directly store chemical energy. Here we present a solar redox flow battery that uses a MoS2@TiO2 thin film with a Nafion protection layer supported on FTO glass substrate as photoanode, employing VO2+/VO2+ and AQDS/AQDS2− as redox active species. When the solar radiation strikes the photoelectrode, the photogenerated holes oxidize VO2+ to VO2+, while the photogenerated electrons reduce AQDS to AQDS2− at the counter electrode. The oxidized form of V5+ and reduced form of AQDS2− thus retain the chemical energy and can release the stored charged via the reverse electrochemical reaction. The addition of NH4Cl to the electrolyte was found to have a positive impact on the electrochemical performance of the redox flow cell. This effect was more evident for the VOSO4 electrolyte, leading to an enhancement of the voltaic and energy efficiencies of more than 17.5%. The results suggest that NH4Cl promotes both mass transport of the vanadium redox species and charge transfer of the AQDS in the electrolyte. The solar-to-output energy conversion efficiency (SOEE) of the solar redox flow battery using 1.6 g L−1 NH4Cl in both anolyte and catholyte reached 9.73%, and an energy density of 87.45% after 10 consecutive one-hour photocharging cycles. Additionally, the use of Nafion to protect the MoS2@TiO2 photoanode from photocorrosion was explored. The Nafion layer ensured an increased stability of MoS2@TiO2 against the strong acidic environment while maintaining effective light response, which translated into enhanced photon and mass transport. An energy storage capacity of ∼60 mAh L−1 after 1-hour photocharging was observed.

Observation of Unidirectional Bulk Modes and Robust Edge Modes in Triangular Photonic Crystals

AbstractThe valley contrasting physics in photonic systems gives rise to many interesting phenomena such as photonic valley Hall effect and robust valley transport. However, most reported valley photonic crystals are demonstrated in the honeycomb lattice. Here, unidirectional bulk modes and robust edge modes are numerically and experimentally shown in triangular photonic crystals. By switching the phase vortex of an input chiral source, dynamic unidirectional propagation of bulk modes at different valleys is observed. Unidirectional edge modes are also demonstrated on the domain wall between two distinct triangular photonic crystals. In addition, robust edge modes around sharp corners are experimentally confirmed. This work paves the way for the realization of dynamic unidirectional photonic transport and robust valley transport in photonic crystals.

Negative differential thermal conductance by photonic transport in electronic circuits

The negative differential thermal conductance (NDTC) provides the key mechanism for realizing thermal transistors. This exotic effect has been the object of an extensive theoretical investigation, but the implementation is still limited to a few specific physical systems. Here, we consider a simple circuit of two electrodes exchanging heat through electromagnetic radiation. We theoretically demonstrate that the existence of an optimal condition for power transmission, well known as impedance matching in electronics, provides a natural framework for engineering NDTC: the heat flux is reduced when the temperature increase is associated to an abrupt change of the electrode's impedance. As a case study, we numerically analyze a hybrid structure based on thin-film technology, in which the increased resistance is due to a superconductor-resistive phase transition. For typical metallic superconductors operating below $1\phantom{\rule{4pt}{0ex}}\mathrm{K}$, NDTC reflects in a temperature drop of the order of a few mK by increasing the power supplied to the system. Our numerical work draws new routes for implementing a thermal transistor in nanoscale circuits.

A generative adversarial network to speed up optical Monte Carlo simulations

Detailed simulation of optical photon transport and detection in radiation detectors is often used for crystal-based gamma detector optimization. However, the time and memory burden associated with the track-wise approach to particle transport and detection in commonly used Monte Carlo codes makes optical simulation prohibitive at a system level, where hundreds to thousands of scintillators must be modeled. Consequently, current large system simulations do not include detailed detector models to analyze the potential performance gain with new radiation detector technologies. Generative adversarial networks (GANs) are explored as a tool to speed up the optical simulation of crystal-based detectors. These networks learn training datasets made of high-dimensional data distributions. Once trained, the resulting model can produce distributions belonging to the training data probability distribution. In this work, we present the proof of concept of using a GAN to enable high-fidelity optical simulations of nuclear medicine systems, mitigating their computational complexity. The architecture of the first network version and high-fidelity training dataset is discussed. The latter is generated through accurate optical simulation with GATE/Geant4, and contains the position, direction, and energy distributions of the optical photons emitted by 511 keV gamma rays in bismuth germanate and detected on the photodetector face. We compare the GAN and simulation-generated distributions in terms of similarity using the Jensen–Shannon distance. Excellent agreement was found with similarity values higher than 93.5% for all distributions. Moreover, the GAN speeded the optical photon distribution generation by up to two orders of magnitude. These very promising results have the potential to drastically change the use of nuclear imaging system optical simulations by enabling high-fidelity system-level simulations in reasonable computation times. The ultimate is to integrate the GAN within GATE/Geant4 since numerous applications (large detectors, bright scintillators, Cerenkov-based timing positron emission tomography) can benefit from these improvements.

Accelerated Prediction of Photon Transport in Nanoparticle Media Using Machine Learning Trained With Monte Carlo Simulations

Abstract Monte Carlo simulations for photon transport are commonly used to predict the spectral response, including reflectance, absorptance, and transmittance in nanoparticle laden media, while the computational cost could be high. In this study, we demonstrate a general purpose fully connected neural network approach, trained with Monte Carlo simulations, to accurately predict the spectral response while dramatically accelerating the computational speed. Monte Carlo simulations are first used to generate a training set with a wide range of optical properties covering dielectrics, semiconductors, and metals. Each input is normalized, with the scattering and absorption coefficients normalized on a logarithmic scale to accelerate the training process and reduce error. A deep neural network with ReLU activation is trained on this dataset with the optical properties and medium thickness as the inputs, and diffuse reflectance, absorptance, and transmittance as the outputs. The neural network is validated on a validation set with randomized optical properties, as well as nanoparticle medium examples including barium sulfate, aluminum, and silicon. The error in the spectral response predictions is within 1% which is sufficient for many applications, while the speedup is 1–3 orders of magnitude. This machine learning accelerated approach can allow for high throughput screening, optimization, or real-time monitoring of nanoparticle media's spectral response.

Verification and Analysis of the Problem-Dependent Multigroup Macroscopic Cross-Sections for Shielding Calculations

Multigroup constants are the foundation of neutron and photon transport problems, and the accuracy of multigroup cross-sections has a significant impact on shielding calculation. Challenges have arisen in generating accurate multigroup macroscopic cross-sections for some problems using the widely used cross-section processing code TRANSX 2.15. We developed the multigroup neutron-photon macroscopic cross-section processing module in the shielding transport code ARES. The module is capable of handling the neutron-photon coupled master libraries in MATXS format and providing the problem-dependent multigroup macroscopic cross-sections tailored to each specific shielding physics problem. The self-designed problems with a single nuclide, as well as the iron and OKTAVIAN experiments, are used to verify and analyze the accuracy of neutron and photon macroscopic cross-sections. Results indicate that the macroscopic cross-sections, neutron flux and neutron-photon leakage spectrum obtained by the MGMXS module are consistent with corresponding reference values. As for the JANUS Phase I fixed source shielding benchmark, the relative differences in the reaction rate between the calculation results and experimental data are within 20%. The module provides better problem-dependent multigroup macroscopic cross-sections for neutron and photon shielding calculations that satisfy the accuracy requirements of engineering applications.

Sensitivity Analysis in Coupled Monte Carlo Radiation Transport Simulations

Sensitivity analysis methods have found extensive use in nuclear criticality safety applications for understanding the impact of uncertain nuclear data on eigenvalue estimates. Significant uncertainty exists in nuclear data and nuclear physics models for photon and electron transport applications, and the goal of this work is to explore whether recently developed adjoint-based, first-order generalized perturbation theory reaction rate sensitivity methods can be extended to coupled Monte Carlo radiation transport simulations. This paper presents a rigorous theoretical derivation for this extended sensitivity analysis method, which is then implemented in a one-dimensional test Monte Carlo code. The adjoint-based sensitivity coefficients are found to agree well with reference direct perturbation and deterministic SENSMG sensitivity coefficients for a simple test problem.

Optoelectronic Properties of In0.87Ga0.13As0.25P0.75(001)β2(2×4) Surface: A First-Principles Study.

InGaAsP photocathode surface affects the absorption, transport and escape of photons, and has a great influence on quantum efficiency. In order to study InGaAsP photocathode surface, the electronic structure, work function, formation energy, Mulliken population and optical properties of In0.87Ga0.13As0.25P0.75(001)β2(2×4) reconstruction surface were calculated from first principles. Results show that stabilized the In0.87Ga0.13As0.25P0.75(001)β2(2×4) surface is conducive to the escape of low-energy photoelectrons. The narrow bandgap and emerging energy levels of the reconstruction surface make the electron transition easier. Under the action of the dipole moment, the electrons transfer from inner layers to the surface during the surface formation process. By contrast to the bulk, the surface absorption coefficient and reflectivity considerably decrease, and the high-reflection range becomes narrower as the falling edge redshifts. On the contrary, the surface transmissivity increases, which is conducive for the photons passing through the surface into the bulk to excite more photoelectrons. Meanwhile, the higher absorption coefficient of surface in low-energy side is favorable for long-wave absorption. The dielectric function peaks of the surface move toward the low-energy side and peak values decrease.

Photonic heat transport from weak to strong coupling

Superconducting circuits provide a favorable platform for quantum thermodynamic experiments. An important component for such experiments is a heat valve, i.e., a device which allows one to control the heat power flowing through the system. Here we theoretically study the heat valve based on a superconducting quantum interference device (SQUID) coupled to two heat baths via two resonators. The heat current in such a system can be tuned by magnetic flux. We investigate how the heat current modulation depends on the coupling strength $g$ between the SQUID and the resonators. In the weak coupling regime the heat current modulation grows as ${g}^{2}$, but, surprisingly, at the intermediate coupling it can be strongly suppressed. This effect is linked to the resonant nature of the heat transport at weak coupling, where the heat current dependence on the magnetic flux is a periodic set of narrow peaks. At the intermediate coupling the peaks become broader and overlap, thus reducing the heat modulation. At very strong coupling the heat modulation grows again and finally saturates at a constant value.