Radiation Transport Effects Research Articles

Simulations of radiatively cooled magnetic reconnection driven by pulsed power

Magnetic reconnection is an important process in astrophysical environments, as it reconfigures magnetic field topology and converts magnetic energy into thermal and kinetic energy. In extreme astrophysical systems, such as black hole coronae and pulsar magnetospheres, radiative cooling modifies the energy partition by radiating away internal energy, which can lead to the radiative collapse of the reconnection layer. In this paper, we perform two- and three-dimensional simulations to model the MARZ (Magnetic Reconnection on Z) experiments, which are designed to access cooling rates in the laboratory necessary to investigate reconnection in a previously unexplored radiatively cooled regime. These simulations are performed in GORGON, an Eulerian two-temperature resistive magnetohydrodynamic code, which models the experimental geometry comprising two exploding wire arrays driven by 20 MA of current on the Z machine (Sandia National Laboratories). Radiative losses are implemented using non-local thermodynamic equilibrium tables computed using the atomic code Spk, and we probe the effects of radiation transport by implementing both a local radiation loss model and $P_{1/3}$ multi-group radiation transport. The load produces highly collisional, super-Alfvénic (Alfvén Mach number $M_A \approx 1.5$ ), supersonic (Sonic Mach number $M_S \approx 4-5$ ) strongly driven plasma flows which generate an elongated reconnection layer (Aspect Ratio $L/\delta \approx 100$ , Lundquist number $S_L \approx 400$ ). The reconnection layer undergoes radiative collapse when the radiative losses exceed the rates of ohmic and compressional heating (cooling rate/hydrodynamic transit rate = $\tau _{\text {cool}}^{-1}/\tau _{H}^{-1}\approx 100$ ); this generates a cold strongly compressed current sheet, leading to an accelerated reconnection rate, consistent with theoretical predictions. Finally, the current sheet is also unstable to the plasmoid instability, but the magnetic islands are extinguished by strong radiative cooling before ejection from the layer.

Migration of low-mass planets in inviscid discs: the effect of radiation transport on the dynamical corotation torque

ABSTRACT Low-mass planets migrate in the type-I regime. In the inviscid limit, the contrast between the vortensity trapped inside the planet’s corotating region and the background disc vortensity leads to a dynamical corotation torque, which is thought to slow down inward migration. We investigate the effect of radiative cooling on low-mass planet migration using inviscid 2D hydrodynamical simulations. We find that cooling induces a baroclinic forcing on material U-turning near the planet, resulting in vortensity growth in the corotating region, which in turn weakens the dynamical corotation torque and leads to 2–3× faster inward migration. This mechanism is most efficient when cooling acts on a time-scale similar to the U-turn time of material inside the corotating region, but is none the less relevant for a substantial radial range in a typical disc (R ∼ 5–50 au). As the planet migrates inwards, the contrast between the vortensity inside and outside the corotating region increases and partially regulates the effect of baroclinic forcing. As a secondary effect, we show that radiative damping can further weaken the vortensity barrier created by the planet’s spiral shocks, supporting inward migration. Finally, we highlight that a self-consistent treatment of radiative diffusion as opposed to local cooling is critical in order to avoid overestimating the vortensity growth and the resulting migration rate.

A novel calorimeter for synchrotron produced monochromatic x-ray beams.

Electron synchrotrons produce x-ray beams with dose rates orders of magnitude greater than conventional x-ray tubes and with beam sizes on the order of a few millimeters. These characteristics put severe challenges on current dosimeters to accurately realize absorbed dose or air kerma. This work seeks to investigate the suitability of a novel aluminum-based calorimeter to determine absorbed dose to water with an uncertainty significantly smaller than currently possible with conventional detectors. A lower uncertainty in the determination of absolute dose rate would impact both therapeutic applications of synchrotron-produced x-ray beams and research investigations. A vacuum-based calorimeter prototype with an aluminum core was built, matching the beam profile of the 140keV monochromatic x-ray beam, produced by the Canadian Light Source Biomedical Imaging and Therapy beamline. The choice of material and overall calorimeter design was optimized using FEM thermal modeling software while Monte Carlo radiation transport simulations were used to model the impact of interactions of the radiation beam with the detector components. Corrections for both the thermal conduction and radiation transport effects were of the order of 3% and the simplicity of the geometry, combined with the monochromatic nature of the incident x-ray beam, meant that the uncertainty in each correction was ≤0.5%. The calorimeter performance was found to be repeatable over multiple irradiations of 1Gy at the±0.6% level, and no systematic dependence on environmental effects or total dose was observed. The combined standard uncertainty in the determination of absorbed dose to aluminum was estimated to be 0.8%, indicating that absorbed dose to water, the ultimate quantity of interest, could be determined with an uncertainty on the order of 1%. This value is an improvement over current techniques used for synchrotron dosimetry and comparable with the state-of-the art for conventional kV x-ray dosimetry.

Sensitivity of synchrotron radiation to the superthermal electron population in mildly relativistic plasma

Synchrotron radiation has markedly different behavior in ∼ 10 keV and in ∼ 100 keV plasma. We show that high-energy electrons that occupy the tail of velocity distribution function have disproportionate impact on power loss of ∼ 100 keV plasma. If electrons with energy more than cutoff energy are redistributed while keeping the Maxwellian distribution function below cutoff energy intact, both emission and absorption of synchrotron radiation act to decrease the lost power. These novel radiation transport effects in non-equilibrium plasma suggest large utility in the deconfinement of high-energy electrons to reduce synchrotron radiation in applications where the radiation is deleterious.

X-ray imaging and radiation transport effects on cylindrical implosions.

Magnetization of inertial confinement implosions is a promising means of improving their performance, owing to the potential reduction of energy losses within the target and mitigation of hydrodynamic instabilities. In particular, cylindrical implosions are useful for studying the influence of a magnetic field, thanks to their axial symmetry. Here, we present experimental results from cylindrical implosions on the OMEGA-60 laser using a 40-beam, 14.5 kJ, 1.5ns drive and an initial seed magnetic field of B0 = 30T along the axes of the targets, compared with reference results without an imposed B-field. Implosions were characterized using time-resolved x-ray imaging from two orthogonal lines of sight. We found that the data agree well with magnetohydrodynamic simulations, once radiation transport within the imploding plasma is considered. We show that for a correct interpretation of the data in these types of experiments, explicit radiation transport must be taken into account.

Effects of Radial Thermal Conduction and Radiation Transport During Fuel Pellet Implosion in Heavy-Ion Inertial Fusion

We investigated the effects of radial thermal conduction and radiation transport from a fuel pre-heating for during the implosion process. We proposed a target structure using a Pb pusher to prevent the pre-heating phenomenon. We compared the electron heat and radiation fluxes, optical thickness, radiation temperature of the fuel, and fuel compression ratio for the Al and Pb pushers. For the Pb pusher, the compression ratio of the fuel increased when pre-heating was prevented. The results indicated that a pusher with a high-Z and dense material could minimize pre-heating and achieve a high fuel compression ratio. This is because such a material can maintain higher opacity during the implosion process.

Modeling of deuterium and carbon radiation transport in MAST-U tokamak advanced divertors

Modest effects of deuterium and carbon radiation opacity in the Super-X and snowflake divertor plasmas are predicted for MAST Upgrade tokamak with core plasma input power 2.5–5 MW and plasma current 1 MA. The radiation transport modeling is based on the SOLPS-EIRENE and UEDGE code divertor plasma predictions. Two radiation transport models are used: one is based on a full radiation transport equation implemented in the radiation transport and collisional-radiative code CRETIN (without feedback on the background plasma), and another is an internal self-consistent UEDGE model with ionization, recombination, and heating rates corrected for Ly α line trapping based on the escape probability model implemented in CRETIN. In MAST-U, the Super-X and snowflake divertor plasmas are predicted to reach detached regimes at lower upstream densities than the standard divertor, and the conclusion still holds with radiation transport effects included. At neutral densities m−3, modest Ly α deuterium line trapping with optical depths 10–15 is predicted in the Super-X divertor. Divertor plasmas are optically thin to other Lyman and Balmer lines, as well as to strong C III and C IV lines that are responsible for most of divertor radiated power. Insignificant changes (within a few percent) to divertor deuterium ionization and recombination rates are found. Radiation fluxes on outer divertor target are modified within a factor of 2–3 when the radiation transport is accounted for, and a similar variation is found due to the line shape models that define the absorption and emission line profiles in the radiation transport modeling. The predicted Lyman and Balmer spectral intensities are significantly modified due to radiation trapping. A measurement of divertor radiation transport effects is discussed using the Ly β /Ba α line ratio. In the snowflake divertor configuration, divertor plasmas are found to be optically thin to Lyman series lines within a large range of parameter variations that include upstream density, divertor transport coefficients, and magnetic configurations. Modest radiation transport effects are only found in a few cases with strongest divertor transport and magnetic configurations closest to the ideal snowflake configuration, however, the plasma background models that were used are yet to be validated with an experiment.

Effects of Haze Radiation and Eddy Heat Transport on the Thermal Structure of Pluto’s Lower Atmosphere

Abstract The temperature profile of Pluto’s atmosphere has generally been assumed in a radiative–conductive equilibrium. Recent studies further highlighted the importance of radiative heating and cooling effects by haze particles. In this study, we update results from Zhang et al. by taking into account the icy haze composition proposed by Lavvas et al., and find that radiation of such an icy haze could still dominate the energy balance in the middle and upper atmosphere and explain the cold temperature observed by New Horizons. However, additional considerations are needed to explain the rapid decrease in temperature toward the icy surface at altitudes <25 km. We propose that vertical eddy heat transport might help maintain radiative–diffusive equilibrium in the lower atmosphere. In this scenario, our radiative–conductive–diffusive model (including both gas and haze) would match observations if the eddy diffusivity is on the order of 103 cm2 s−1. Alternatively, if eddy heat transport is not effective on Pluto, in order to match observations, haze albedo must increase rapidly with decreasing altitude and approach unity near the surface. This is a plausible result of additional ice condensation and/or cloud formation. In this scenario, haze radiation might still dominate over gas radiation and heat conduction to maintain radiative equilibrium. Better constraints on haze albedo at ultraviolet and visible wavelengths would be a key to distinguish these two scenarios. Future mid-infrared observations from the James Webb Space Telescope could also constrain the thermal emission and haze properties in Pluto’s lower atmosphere.

Long-term evolution of a merger-remnant neutron star in general relativistic magnetohydrodynamics: Effect of magnetic winding

Long-term ideal and resistive magnetohydrodynamics (MHD) simulations in full general relativity are performed for a massive neutron star formed as a remnant of binary neutron star mergers. Neutrino radiation transport effects are taken into account as in our previous papers. The simulation is performed in axial symmetry and without considering dynamo effects as a first step. In the ideal MHD, the differential rotation of the remnant neutron star amplifies the magnetic-field strength by the winding in the presence of a seed poloidal field until the electromagnetic energy reaches $\sim 10\%$ of the rotational kinetic energy, $E_{\rm kin}$, of the neutron star. The timescale until the maximum electromagnetic energy is reached depends on the initial magnetic-field strength and it is $\sim 1$ s for the case that the initial maximum magnetic-field strength is $\sim 10^{15}$ G. After a significant amplification of the magnetic-field strength by the winding, the magnetic braking enforces the initially differentially rotating state approximately to a rigidly rotating state. In the presence of the resistivity, the amplification is continued only for the resistive timescale, and if the maximum electromagnetic energy reached is smaller than $\sim 3\%$ of $E_{\rm kin}$, the initial differential rotation state is approximately preserved. In the present context, the post-merger mass ejection is induced primarily by the neutrino irradiation/heating and the magnetic winding effect plays only a minor role for the mass ejection.

Thermal and efficiency droop in InGaN/GaN light-emitting diodes: decoupling multiphysics effects using temperature-dependent RF measurements

Multiphysics processes such as recombination dynamics in the active region, carrier injection and transport, and internal heating may contribute to thermal and efficiency droop in InGaN/GaN light-emitting diodes (LEDs). However, an unambiguous methodology and characterization technique to decouple these processes under electrical injection and determine their individual roles in droop phenomena is lacking. In this work, we investigate thermal and efficiency droop in electrically injected single-quantum-well InGaN/GaN LEDs by decoupling the inherent radiative efficiency, injection efficiency, carrier transport, and thermal effects using a comprehensive rate equation approach and a temperature-dependent pulsed-RF measurement technique. Determination of the inherent recombination rates in the quantum well confirms efficiency droop at high current densities is caused by a combination of strong non-radiative recombination (with temperature dependence consistent with indirect Auger) and saturation of the radiative rate. The overall reduction of efficiency at elevated temperatures (thermal droop) results from carriers shifting from the radiative process to the non-radiative processes. The rate equation approach and temperature-dependent pulsed-RF measurement technique unambiguously gives access to the true recombination dynamics in the QW and is a useful methodology to study efficiency issues in III-nitride LEDs.

Enhanced GEANT4 Monte Carlo simulations of the space radiation effects on the International Space Station and Apollo missions using high-performance computing environment

A significant challenge to current and future manned and/or unmanned space missions is due to deep space radiation. An improvement in more realistic (more accurate) simulation models in predicting the effects of radiation within the spacecraft is required, especially to better predict dose to astronauts, energy deposition within sensitive electronics, and effectiveness of radiation shielding for long-term space missions. The International Space Station provides an invaluable resource for long-term measurements of the radiation environment in Low Earth Orbit (LEO); however, the only manned missions with dosimetry data available beyond LEO are the Apollo missions. Thus the physiological effects and dosimetry for deep space missions are not well understood in planning extended missions. GEANT4, a Monte Carlo method, represents a powerful physics simulation tool to assess the effects of radiation transport through spacecraft. The newest version of GEANT4 supports multithreading and Message Passing Interface (MPI) allowing for much faster distributive processing of simulations, using a high-performance computing environment. This paper introduces a new simulation model and application using GEANT4 that greatly reduces computational time to hours instead of weeks without any post simulation processing based on high-performance computing. This paper also introduces a new set of GEANT4 computational detectors for calculating dose distribution, besides the historically used International Commission of Radiation Units simulation spheres. The computational detectors include a thermoluminescent detector, tissue equivalent proportional counter, and human phantom, along with additional new scorers to calculate dose equivalence based on the International Commission of Radiation Protection standards. This study presents GEANT4 simulations of the dose deposition for the International Space Station and the Apollo 11 and 14 missions, which replicate well the dose measurements during these missions. The simulations of both Apollo missions show consistent doses from galactic cosmic rays and radiation belts with a small variation in dose distribution across the Apollo capsule. The greatest contributor to radiation dose for both Apollo missions in the simulations came from galactic cosmic rays. Simulations of historical solar particle events during an Apollo missions show a solar particle event would not be fatal and below mission limits. These GEANT4 models also provides the values of the dose deposition and dose equivalent for various organs within a human phantom in the International Space Station and Apollo command module, which are developed for the first time using this GEANT4 based application.

Influence of Resonance Radiation Transport on Chemical Equilibrium in an Argon Arc

Deviations from chemical equilibrium in argon arc plasma are analysed by means of collisional-radiative model. Corresponding comprehensive kinetic scheme has been developed and applied form study of free-burning arc at the conditions typical for welding applications. While the natural lifetime have been used for radiation emitted from highly excited argon states, the resonance radiation was described taking into account the radiation transport effects. Resulting spatial distributions of excited argon atoms are compared for the cases of LTE and two-temperature plasma using different approaches for the description of the resonance radiation transport.

Photo-thermochemical decomposition of carbon-dioxide in a direct solar receiver-reactor

The synergistic combination of the spectral and intensity features of solar energy can potentially lead to chemical synthesis processes under milder temperature conditions than those in solar thermochemical processes. A direct solar receiver-reactor is built and used for the catalytic photo-thermochemical processing of carbon dioxide. The reactor is designed to be representative of field application solar reactors by depicting fluid flow, heat transfer, mass transport, and radiation transport effects, often suppressed in reactors aimed to fundamental chemical kinetics studies. Two different types of catalytic monoliths, tubular quartz and zirconia foam, are tested with coatings of 1 and 2 wt% Cu-doped TiO2. The zirconia foam monolith is optically thick and has more than four times higher coating mass compared to the tubular quartz monolith; nevertheless, the latter, while being optically thin, leads to four times higher CO production, with the 1 wt% coated monolith producing the highest CO yield. Moreover, the experimental results show that the CO production rate increases almost exponentially with increasing incident solar radiation flux. A computational fluid dynamics (CFD) model, describing fluid flow, radiation transport, and heterogeneous photo- and thermo-chemistry, is used to analyze the CO2 decomposition process in the 1 wt% tubular quartz reactor. The modeling results provide assessment of the photochemical and thermochemical effects, and suggest the synergistic enhancement due to the photolytic mechanism on the thermochemical process. Solar photo-thermochemical processing of CO2 using an optically and catalytically tuned catalytic monolith can lead to higher decomposition yields than photochemical processes, potentially comparable to those of solar thermochemical approaches while operating at lower temperatures.

Effect of Radiation Transport on Minimal Sparkplug Ignition Energy of Nanosized Coal-Dust Suspension

In this paper we consider a numerical investigation on sparkplug ignition of nanosized coal-dust suspension in the air. The aim of the research is to determine the effect of radiation transport on minimal sparkplug ignition energy of a coal-dust suspension. The paper provides the data on the sparkplug ignition energy dependence on the suspension emissivity factor and coal-dust content.

Effects of density gradients and fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade

Electron cyclotron emission imaging (ECEI) provides measurements of electron temperature (Te) and its fluctuations (δTe). However, when measuring at the plasma edge, in the steep gradient region, radiation transport effects must be taken into account. It is shown that due to these effects, the scrape-off layer region is not accessible to the ECEI measurements in steady state conditions and that the signal is dominated by the shine-through emission. Transient effects, such as filaments, can change the radiation transport locally, but cannot be distinguished from the shine-through. Local density measurements are essential for the correct interpretation of the electron cyclotron emission, since the density fluctuations influence the temperature measurements at the plasma edge. As an example, a low frequency 8 kHz mode, which causes 10%–15% fluctuations in the signal level of the ECEI, is analysed. The same mode has been measured with the lithium beam emission spectroscopy density diagnostic, and is very well correlated in time with high frequency magnetic fluctuations. With radiation transport modelling of the electron cyclotron radiation in the ECEI geometry, it is shown that the density contributes significantly to the radiation temperature (Trad) and the experimental observations have shown the amplitude modulation in both density and temperature measurements. The poloidal velocity of the low frequency mode measured by the ECEI is 3 km s–1. The calculated velocity of the high frequency mode measured with the magnetic pick-up coils is about 25 km s–1. Velocities are compared with the E × B background flow velocity and possible explanations for the origin of the low frequency mode are discussed.

Abstract ID: 45 Development and analysis of the track-LET, dose-LET and RBE calculations with a therapeutical proton and ion beams using Geant4 Monte Carlo code

A reliable prediction of the spatial Linear Energy Transfer(LET) distribution in a biological tissues is a crucial point for the estimation of the radiobiological parameters on which are based the current treatment planning [1] . Nowadays, the accuracy and approach for the LET calculation can significantly affect the reliability of the calculated Relative Biological Effectiveness(RBE) [2] . The LET is not a physical quantity that can be easily measured, and therefore, values are usually estimated by calculating stopping power based on the Bethe-Bloch equation. Its definition, as reported in ICRU Report No.85, is related to a non-stochastic quantity that describes the average energy transfer from electronic interactions per unit length travelled by charged primary particles. The Monte Carlo(MC) technique is the most accurate method to account for complex radiation transport effects and energy losses in a medium.However, as a computation method, the accuracy and precision of the MC calculation results strongly depend on the physics interaction cross sections applied as well as on the simulation algorithms used and the transport parameters chosen [3] . The main aim of this study was to develop a completely open source tool based on Geant4 code for the calculation of the LET-track and LET-dose distributions of therapeutic proton and ion beams.Furthermore, the main parameters that can influence the LET calculation in a typical MC simulation based on a voxelized equivalent tissue phantom, were identified and investigated. Particular attention was given to the energy loss process due to the electronic interactions such as ionization and excitation of the primary incident particles and to the production cut and step dependence.Moreover, the secondary nuclei generation was considered and opportunely weighted to the computational total-LET-dose calculation.A module to couple the MC calculation to LEM radiobiological model has been studied and a series of calculations were performed to illustrate the impact of different LET values on the RBE-weighted dose.

Benchmark Experiment for Photoionized Plasma Emission from Accretion-Powered X-Ray Sources.

The interpretation of x-ray spectra emerging from x-ray binaries and active galactic nuclei accreted plasmas relies on complex physical models for radiation generation and transport in photoionized plasmas. These models have not been sufficiently experimentally validated. We have developed a highly reproducible benchmark experiment to study spectrum formation from a photoionized silicon plasma in a regime comparable to astrophysical plasmas. Ionization predictions are higher than inferred from measured absorption spectra. Self-emission measured at adjustable column densities tests radiation transport effects, demonstrating that the resonant Auger destruction assumption used to interpret black hole accretion spectra is inaccurate.

Interaction of short pulse collimated irradiation with inhomogeneity: An accurate model

The accurate formulation of radiative source is established with the transient effect of radiation transport for evaluating the correct thermal damage in the proximity of laser source. The accurate formulation of radiative source consists of two parts: the conventional absorption term and the propagation term. The inclusion of the propagation term significantly deviates the energy distribution from earlier steady state radiative source especially in the ultrashort time scale where this term has significance. In the present work, this effect has been investigated in the presence of absorbing, emitting, and anisotropically scattering medium within a two-dimensional rectangular domain subjected to collimated irradiation at one of its boundaries. A small inhomogeneity is taken at the center of the top wall which is irradiated with a laser source. The effects of optical parameters such as absorption coefficient, scattering coefficient, and scattering albedo on the temperature distribution in the medium are analyzed. It is found that the modified radiation energy flux gives an early signal of crossing the thermal damage threshold temperature surrounding the inhomogeneity. Inclusion of propagation term in radiation energy source helps to precisely access the extent of thermal damage surrounding the inhomogeneity which is not traceable as per the existing models.

The 2014 southeast Brazil austral summer drought: regional scale mechanisms and teleconnections

The southeast region of Brazil experienced in austral summer 2014 a major drought event leading to a number of impacts in water availability for human consumption, agricultural irrigation and hydropower production. This study aims to perform a diagnostic analysis of the observed climate conditions during this event, including an inspection of the occurred precipitation anomalies in the context of previous years, and an investigation of possible relationships with sea surface temperatures and atmospheric circulation patterns. The sea surface temperature analysis revealed that the southwestern South Atlantic Ocean region near the coast of southeast Brazil showed strong negative association with precipitation over southeast Brazil, indicating that increased sea temperatures in this ocean region are consistent with reduced precipitation as observed in summer 2014. The circulation analysis revealed prevailing anti-cyclonic anomalies at lower levels (850 hPa) with northerly anomalies to the west of southeast Brazil, channeling moisture from the Amazon towards Paraguay, northern Argentina and southern Brazil, and drier than normal air from the South Atlantic Ocean towards the southeast region of Brazil. This circulation pattern was found to be part of a large-scale teleconnection wave train linked with the subsidence branch of the Walker circulation in the tropical east Pacific, which in turn was generated by an anomalous tropical heat source in north/northeastern Australia. A regional Hadley circulation with an ascending branch to the south of the subsidence branch of the Walker circulation in the tropical east Pacific was identified as an important component connecting the tropical and extratropical circulation. The ascending branch of this Hadley circulation in the south Pacific coincided with an identified Rossby wave source region, which contributed to establishing the extratropical component of the large-scale wave train connecting the south Pacific and the Atlantic region surrounding southeast Brazil. This connection between the Pacific and the Atlantic was confirmed with Rossby ray tracing analyses. The local circulation response was associated to downward air motion (subsidence) over Southeast Brazil, contributing to the expressive negative precipitation anomalies observed during summer 2014, and leading to a major drought event in the historical context. The analysis of atmospheric and oceanic patterns of this event helped defining a schematic framework leading to the observed drought conditions in southeast Brazil, including the involved teleconnections, blocking high pressure, radiative and humidity transport effects.

Radiation Transport in Z-Pinches

Z-pinches, formed by passing large currents through wires or gas puffs, are the most powerful and energetic laboratory sources of X-rays. As higher current generators become available, Z-pinches incorporate increasingly larger masses of plasma and are nearly always optically thick to some or most of the photons that they produce. Understanding and modeling the transport of this radiation is an important element in attaining an overall picture of the physics of Z-pinches and in optimizing their properties for applications. This paper focuses on the aspects of radiation transport that are most relevant to the Z-pinch environment. The topics covered include: theory of radiation transport and its strengths and limitations, sources of opacity in Z-pinches, requirements for local thermodynamic equilibrium in the presence of opacity, transition to the blackbody limit, and Z-pinch experiments that demonstrate the effects of radiation transport.