High Energy Electron Transport Research Articles

Mitochondria, oxidative stress and PARP‐1 network: a new target for neuroprotective effects of tetracyclines?

Mitochondria, oxidative stress and PARP‐1 network: a new target for neuroprotective effects of tetracyclines?

High energy electron transport in dense plasma in fast ignition scenario

We investigate the high energy electron transport in dense plasma for different laser intensity by using 3D PIC (Three-Dimensional Particle-In-Cell) simulation. We have observed dependence of electron energy flux on incident laser intensity. The electron energy fluxes normalized by the incident laser flux, I/I0, become ≈ 0.2 for the normalized laser electric field of a = 3, ≈ 0.1 for a = 1 and no visible flux for a = 0.2 in dense plasma region, where a = eE/meω0c. For the case of a = 3, we investigated space and time resolved velocity distribution functions and induced longitudinal electric field. We show that the high energy electrons have beam-like velocity distribution with relativistic thermal spread near the energy flux front. We also discuss the electromagnetic instabilities associated with the high energy electron beam taking the relativistic thermal spread of the beam into account.

Observation of electron transport dynamics in high intensity laser interactions using multi-energy monochromatic x-ray imaging

We describe recent measurements in which a novel imaging technique was used to investigate the transport of high energy electrons produced by the interaction of a femtosecond laser pulse with a three-layer target at an intensity of 5 × 1019 W cm−2. The imaging system was configured to work in a single-photon detection regime to identify the energy of the x-ray photons and to discriminate among Kα photons generated in each target layer. Electrons emerging from the rear side after propagation through all the target layers were also detected using a custom developed detector. The results on fast electron propagation are combined with the information obtained from electron diagnostics and are modelled using analytical and numerical codes to obtain a detailed description of electron propagation dynamics.

Regrowth of diluted magnetic semiconductor GaMnAs on InGaP (001) surfaces to realize freestanding micromechanical structures

Low temperature molecular beam epitaxy regrowths of Ga1−xMnxAs (x≈0.04) diluted magnetic semiconductors on GaAs∕In1−yGayP∕GaAs(001) and In1−yGayP∕GaAs(001) (y≈0.51) heterostructures prepared by metal-organic chemical vapor deposition are described. The resulting Ga1−xMnxAs properties are comparable to epitaxial films grown directly on GaAs (001) substrates from in situ reflection high-energy electron diffraction, x-ray diffraction, magnetometry, and transport measurements with magnetic ordering temperature of as-grown films to range between ∼50 and ∼60K. Postgrowth low temperature annealing enhances both magnetic and transport properties. Perfect etch selectivity between Ga1−xMnxAs∕GaAs and In1−yGayP is utilized to realize suspended Ga1−xMnxAs∕GaAs doubly clamped beam micromechanical freestanding structures.

High energy electron transport in solids

With the addition of recent PW shots, the propagation of short-pulse laser generated electron beams have been studied using laser pulse energies from 30 J to 300 J, generating currents up to ∼15 MA in solid Al:Cu targets. This is ∼5% of the current that will be required in an ignition pulse. To this level, the current appears to simply scale with laser power, the propagation spread not change at all. The resistance of the aluminum does not seem to play a role in the propagation characteristics, though it might in setting the current starting parameters. We do find that at the highest currents parts of these targets reach temperatures high enough to modify the Cu-K, emission spectrum rendering our Bragg imaging mirrors ineffective; spectrometers will be needed to collect data at these higher temperatures.

Theory of unipolar ballistic and quasiballistic transit-time oscillators for a terahertz range

We are the first to consider DC characteristics and a linear admittance of a very short undoped transit space (T-space) in the case of a ballistic transport of electrons injected into this space from a heavily doped cathode contact. We have considered both a classic case of a unipolar space-charge-limited injection current and a case of a unipolar tunnel injection through a rectangular or triangular heterostructure barrier. In both cases, the effective injection occurs in not very strong electric fields (⩽(1−3)×10 5 V/ cm) . To reach a high-energy ballistic electron transport, we have assumed highly positioned noncentral electron valleys over the bottom of a central Γ-valley and have selected the appropriate semiconductors for the T-space. For all the considered examples, we have obtained frequency windows of a negative conductance in a deep terahertz range. This means that such unipolar devices are perspective as terahertz oscillators. Numerical estimates are completed for the InP-T-space with the length of 70 nm .

SOLAR MICROWAVE BURSTS AND ELECTRON KINETICS

Solar flares present a number of radiative characteristics indicative of kinetic processes of high energy particles. Proper understanding of the kinetic processes, however, relies on how well we can separate the acceleration from transport characteristics. In this paper, we discuss microwave and hard X-ray bursts as a powerful tool in investigating the acceleration and transport of high energy electrons. After a brief review of the studies devoted to the kinetic process of solar flare particles, we cast them into a simple formulation which allows us to handle the injection, trap, and precipitation of flare electrons self-consistently. The formulation is then taken as a basis for interpreting and analyzing a set of impulsive and gradual bursts occurred on 2001 April 6 observed with the Owens Valley Solar Array, and HXT/WBS onboard Yohkoh satellite. We quantify the acceleration, trap, and precipitation processes during each burst in terms of relevant time scales, and also determine ambient density and magnetic field. Our result suggests that it should be the acceleration property, in particular, electron pitch angle distribution, rather than the trap condition, that is mainly responsible for the distinctive properties of the impulsive and gradual flares.

High-energy ballistic transport in hetero- and nano-structures

Ballistic electron emission microscopy (BEEM) is a three terminal extension of scanning tunneling microscopy and yields topographic and spectroscopic information on high-energy electron transport in semiconductors at nm-resolution. In BEEM on GaAs–AlGaAs double barrier resonant tunneling diodes (DBRTDs) ballistic electrons which tunnel through a resonant state inside the DBRTD result in a characteristic linear behavior in the BEEM spectrum. On DBRTDs nanostructured into narrow quantum wires, however, this tunneling is quenched for electron energies below the AlGaAs barrier heights. This quenching of the ballistic current can be explained in terms of a transfer Hamiltonian formalism applied to tunneling processes between electron systems of different dimensionality. We measured BEEM spectra on InAs self-assembled quantum dots (SAQDs) for positions on the dots and for “off-dot” regions on the so-called InAs wetting layer. From these data, we determined the local InAs–GaAs band offsets on the dots and on the wetting layer and investigated the temperature dependence of the InAs–GaAs barrier height.

Electron Transport during the 1999 August 20 Flare Inferred from Microwave and Hard X‐Ray Observations

We discuss injection and transport of high-energy electrons during a GOES X-ray class M9.8 flare observed in microwaves with the Owens Valley Solar Array (OVSA) and in hard X-rays (HXRs) with the hard X-ray telescope (HXT) on board Yohkoh. Observed at 1 s timescales or better in both wavelength regimes, the event shows (1) a large difference in scale between the microwave source and the HXR source; (2) an unusually hard HXR spectrum (maximum spectral index ~-1.6), followed by rapid spectral softening; and (3) a microwave light curve containing both impulsive peaks (3 s rise time) simultaneous with those of the HXRs and a long, extended tail with a uniform decay rate (2.3 minutes). We analyze the observations within the framework of the electron trap-and-precipitation model, allowing a time-dependent injection energy spectrum. Assuming thick-target bremsstrahlung for the HXRs, we infer the electron injection function in the form Q(E, t) ~ (E/E0)-δ(t), where the timescale for δ(t) to change by unity is ~7 s. This injection function can account for the characteristics of the impulsive part of the microwave burst by considering the bulk of the electrons to be directly precipitating without trapping. The same injection function also accounts for the gradual part of the microwave emission by convolving the injection function with a kernel representing the trapping process, which at late times gives N(E, t) ~ e-νt(E/E0)-b. We require b ~ 1.4 and ν ~ 6 × 10-3β s-1, where β is the electron speed divided by the speed of light. Therefore, the derived form of the precipitation rate ν itself indicates strong pitch-angle diffusion, but the slow decay of the microwave radiation requires a small loss cone (~4°) and a low ambient density in the coronal trap. Also, the numbers of electrons needed to account for the two components of the microwave emission differ by an order of magnitude. We estimate that the ≥100 keV number of the directly precipitating electrons is ~1033, while the trapped population requires ~1032 electrons. This leads us to a model of two interacting loops, the larger of which serves as an efficient trap while the smaller provides the impulsive source. These characteristics are consistent with the spatially resolved observations.

Quantum theory of high-energy electron transport in the surface region

Quantum theory of high-energy electron transport in the surface region

Pyruvate and hydroxycitrate/carnitine may synergize to promote reverse electron transport in hepatocyte mitochondria, effectively ‘uncoupling’ the oxidation of fatty acids

In a recent pilot study, joint administration of pyruvate, hydroxycitrate (HCA), and carnitine to obese subjects was associated with a remarkable rate of body-fat loss and thermogenesis, strongly suggestive of uncoupled fatty-acid oxidation. Hepatocytes possess an uncoupling mechanism – reverse electron transport – that enables fasting ketogenesis to proceed independent of respiratory control. Electrons entering the respiratory chain at the coenzyme Q (CoQ) level via FAD-dependent acyl coA dehydrogenase, can be driven ‘up’ the chain by the electrochemical proton gradient to reduce NAD+; if these electrons are then shuttled to the cytoplasm, returning to the respiratory chain at the CoQ level, the net result is heat generation at the expense of the proton gradient, enabling the uncoupled flow of electrons to oxygen. Pyruvate's bariatric utility may stem from its ability to catalyze the rapid transport of high-energy electrons from mitochondria to the cytoplasm, thus stimulating electron shuttle mechanisms. By enabling rapid mitochondrial uptake of fatty acids and thus disinhibiting hepatocyte ketogenesis, HCA/carnitine should initiate reverse electron transport: concurrent amplification of electron shuttle mechanisms by pyruvate can be expected to accelerate this reverse electron transport, thereby decreasing the electrochemical proton gradient. As a result, hepatocytes may be able to convert fatty acids to CO2and heat with little net generation of ATP. These considerations suggest that it may be feasible to render hepatocytes functionally equivalent to activated brown fat, such that stored fat can be selectively oxidized in the absence of caloric restriction. Other measures which enhance the efficiency of hepatocyte electron shuttle mechanisms may increase the efficacy of this strategy.

Monte Carlo simulation of high-field electron transport in GaAs using an analytical band-structure model

We present a study of high energy electron transport in GaAs using an analytical model of the band structure. This model is based on piecewise polynomial approximation of the dispersion relation in different regions of the Brillouin zone. It accounts for the first two conduction bands and reproduces all important features of the full band structure. We have used this model to set up a Monte Carlo simulation of electron transport accounting for impact ionization. It has been shown that this “extended valley” model yields essentially the same results as the most rigorous full band Monte Carlo calculations. We have found a large influence of high energy anisotropy on electron transport. Another important result is that most impact ionization events are due to electrons in the second conduction band.

MMC-a high-performance Monte Carlo code for electron beam treatment planning

The macro Monte Carlo (MMC) method has been developed to improve the speed of traditional Monte Carlo (MC) high-energy electron transport calculations without loss in accuracy. The MMC algorithm uses results derived from conventional MC simulations of electron transport through macroscopic spheres of various radii and consisting of a variety of media. Based on these results, electrons are transported in macroscopic steps through the absorber. The absorber geometry is represented by a three-dimensional (3D) density matrix, typically derived from computer tomographic (CT) data. Energy lost by the electrons along their paths through the absorber is scored in a 3D dose matrix. Transport of secondary electrons and bremsstrahlung photons is taken into account. Major modifications of the original implementation of the MMC algorithm have resulted in an improved version of the code, resolving earlier problems with electron transport across interfaces of different materials, and running at a substantially higher speed. Furthermore, the code has been integrated into a clinical 3D treatment planning system. MMC results are in good agreement with results from conventional MC codes and are obtained with a speed gain of about one order of magnitude for clinically relevant irradiation situations. Calculation times to obtain a relative statistical accuracy of 2% per dose grid voxel for small electron field sizes are short enough to be routinely useful in radiotherapy clinics on present day affordable workstation computers. Considering speed, accuracy and memory requirements, MMC is a promising alternative to currently available electron dose planning algorithms.

A comparison of numerical solutions of the Boltzmann transport equation for high-energy electron transport silicon

In this work we have undertaken a comparison of several previously reported computer codes which solve the semiclassical Boltzmann equation for electron transport in silicon. Most of the codes are based on the Monte Carlo particle technique, and have been used here to calculate a relatively simple set of transport characteristics, such as the average electron energy. The results have been contributed by researchers from Japan, Europe, and the United States, and the results were subsequently collected by an independent observer. Although the computed data vary widely, depending on the models and input parameters which are used, they provide for the first time a quantitative (though not comprehensive) comparison of Boltzmann Equation solutions. >

Monte Carlo simulation of the transport of fast electrons and positrons in solids

We give a detailed description of a Monte Carlo simulation method for the transport of high energy (1–50 keV) positrons and electrons in solids using either the Penn dielectric loss function or the Lindhard dielectric function to model inelastic scattering and elastic scattering cross sections obtained from a partial wave expansion. Results for positron implantation and backscattering for Be, Al, Cu, Ag and Au are presented. Simulation results and experiments for positrons are found to be in very good agreement, which confirms the accuracy of the Penn model for describing inelastic scattering of positrons in solids. In the case of electrons agreement between simulated and experimental backscattering probabilities is reasonable but there are significant discrepancies which can be explained by the neglect of the indistinguishability of electrons in the Penn model.

Cross sections for elastic scattering of fast electrons and positrons by atoms

In order to provide reliable cross section data needed for Monte Carlo simulation of high-energy electron and positron transport, a simple procedure to evaluate cross sections for elastic scattering of these particles by neutral atoms is described. Our approach starts from the exact Mott cross section for scattering by a bare point nucleus. Screening effects are accounted for by means of the Born approximation. A correction, for which we give a simple analytical expression, is introduced to compensate for the inaccuracies of the Born approximation at low energies. This correction has been determined from an extensive partial wave calculation involving 20 elements within the periodic system and electrons and positrons with kinetic energies from 1 keV to 10 MeV. Finite nuclear size effects are introduced by means of the Born approximation and a simple nuclear charge distribution. The resulting differential cross sections are adequate for describing multiple elastic scattering of electrons and positrons with kinetic energies above ∼ZkeV, where Z is the atomic number of the target atom.

Carrier transport analysis with Monte Carlo simulation including new simplified band structure

A new analytical band structure model for high energy carrier transport is presented, in which not only X valley (the lowest set of minima of conduction band) but also L or Gamma valleys are taken into account. All the physical parameters (effective mass, nonparabolicity and the location of each valley) are adjusted to fit to the calculated local empirical pseudopotential band structure. The main features of the density of states, which influence the scattering rate, are well reproduced with the new band model. It was found that the Monte Carlo simulation using the high energy valley model makes it possible to accurately predict high energy electron transport.

A macro Monte Carlo method for electron beam dose calculations

The macro Monte Carlo (MMC) method is a fast Monte Carlo (MC) algorithm for high energy electron transport in an absorbing medium. Incident electrons are transported in large-scale macroscopic steps through the absorber. Electron parameters after each step are calculated from probability distributions. Transport of secondary electrons and bremsstrahlung photons is taken into account. For electron beam dose calculations, the energy, angular and spatial distributions of primary electrons have to be provided. The absorber geometry is represented by a 3D density matrix and the energy lost by the electrons along their paths is scored in a 3D dose matrix of user-definable resolution. Measurements in the beam of a Philips SL75-20 linear accelerator incident on homogeneous and simple inhomogeneous phantoms are in good agreement with MMC simulations.

Surface structure of the photosystem II complex

The Photosystem II (PS-II) complex is organized around a photosynthetic reaction center (RC) embedded in the photosynthetic membrane. PS-II traps the energy of sunlight and uses it drive highenergy electron transport across the photosynthetic membrane. PS-II is closely associated with a group of proteins known as the oxygen-evolving complex (OEC), which are bound to the inner surface of the photosynthetic membrane. This complex splits water to yield electrons that are passed to the RC, releasing molecular oxygen. We have used freeze-etch electron microscopy to study 2-dimensional crystals of the PS-II complex obtained from a photosynthetic mutant of barley (viridiszb63) kindly provided by Dr. David Simpson of the Carlsberg Institute of Copenhagen (Simpson & von Wettstein, 1980). The photosynthetic membranes of these mutant plants lack photosystem I, and consequently contain extensive crystalline membrane regions enriched in PS-II. These plants are an excellent source of PS-II sheetlike crystals, obtainable without the use of detergents or chemical modification: Figure 1, prepared by quick-freezing, deep-etching, and rotary shadowing, illustrates the appearance of these sheetlike crystals.

An improved impact-ionization model for high-energy electron transport in Si with Monte Carlo simulation

A new model for impact ionization in Si is presented, which goes beyond the limitations of the Keldysh formula and is based on a more realistic scheme developed starting from a first-order perturbation theory. This scattering mechanism is modeled by an extended band structure which includes many bands for electrons and one band for holes in a finite Brillouin zone. Some processes have been identified to bring the dominant contribution to the scattering probability, in the present approach, for electron energies ranging up to 3 eV. Expressions for the differential and integrated scattering probabilities have been obtained which are consistent with the band model and can be included in a Monte Carlo simulation of the electron gas. Results for transport quantities are shown for a bulk material in presence of homogeneous and static electric fields under physical conditions where impact ionization influences the carrier dynamics. A comparison with theoretical and experimental data from the literature is also given.