Path Length Of Electrons Research Articles

Development of an electron track-structure mode for arbitrary semiconductor materials in PHITS

Abstract Optimizing the design of semiconductor detector requires an electron track-structure code that can trace an incident electron motion with energies down to a few eV and simulate the production process of excited electrons in semiconductors. We therefore developed an Electron Track Structure mode applicable to ARbitrary Targets (ETSART) and implemented it into Particle and Heavy Ion Transport code System (PHITS). ETSART can simulate the electrons’ motion in arbitrary semiconductor materials using their bandgap energies. The accuracy of ETSART was validated by comparing the calculated electron path lengths in semiconductor materials with the corresponding data in the ICRU Report 37. Using ETSART, we also computed the mean energy required to produce an electron–hole pair (epsilon value) in various semiconductors and found that the epsilon value varies nonlinearly with the bandgap energy. Thus, ETSART can be useful for initial and mechanistic evaluations of electron–hole pair formation in new materials.

Revealing the plasma confinement behavior of an axial ring cusp hybrid discharge in a miniature ion thruster using PIC/MCC simulation

Micro direct current (DC) ion thrusters have broad application prospects as propulsion systems for micro spacecrafts due to their advantages of high discharge reliability and efficiency. Experiments in the literature show that the plasma discharge under the axial ring cusp hybrid (ARCH) magnetic field has higher discharge efficiency in the discharge chamber of micro DC ion thruster. In this paper, a 2d-3v axisymmetric particle-in-cell/Monte Carlo collision numerical model is developed for the ARCH discharge. This model takes the thermal electron emission including the Schottky effect, various collision processes including the Bohm-type anomalous conductivity and the uneven background gas density distribution in the cathode-anode gap into account. The spatial distributions of plasma characteristics are presented and the advantages of ARCH discharge compared with traditional 3-ring discharge in the discharge chamber of the micro DC ion thruster are analyzed. The longer electron path length and the change of ionization region improve the discharge efficiency in small-scale discharges. Two primary methods for the discharge confinement in the miniature ion thrusters, that is, the magneto-static ‘cusp confinement’ through magnetic cusps and the electrostatic ‘sheath refection confinement’ through the backplate with the lower potential are investigated. The sensitivity of macroscopic current characteristics and microscopic plasma characteristics in the ARCH discharge to the magnetic field strength and backplate biased potential are explored. It is found that there is an optimal magnetic field to maximize the utilization of propellant and minimize the discharge loss. The electrostatic ‘sheath refection confinement’ is conducive to the reduction of discharge loss, however, it is also accompanied by the decline of propellant utilization. The above results provide further support for the design optimization of the micro DC ion thruster discharge chamber.

Energy Spectrum of Solar Energetic Electron Events over 25 Years

We investigate the peak flux energy spectrum of 458 solar energetic electron (SEE) events with a clear velocity dispersion detected at energies from ≤4.2 to ≥108 keV by Wind/3DP from 1994 December through 2019 December, utilizing a pan-spectrum fitting method. According to the fitted spectral parameters, these 458 events are self-consistently classified into five spectral types: 304 downward double-power-law (DDPL) events, 32 upward double-power-law (UDPL) events, 23 single-power-law (SPL) events, 44 Ellison–Ramaty (ER) events, and 55 logarithmic–parabola (LP) events. The DDPL events can be further divided into two types: 231 events and 73 events, since their break energy E B exhibits a double-peak distribution separated by a dip at ∼20 keV. The () events show a power-law spectral index of () at energies below () keV and an index of () at energies above. The UDPL events have a spectral index of at energies below keV and an index of at energies above. The SPL events exhibit a spectral index of . The ER events show a spectral index of at energies below keV. The LP events are characterized by a spectral slope of () at 2.8 keV (108 keV). The six spectral types also behave differently in the relationship between spectral parameters and in solar cycle variations. The spectral shape of most SEE events appears to be unrelated to the estimated electron path lengths. These results suggest that the formation of SEE events can involve complex processes/sources.

Emission of fused silica and KBr samples in the UV and visible spectral ranges under irradiation with 2.7 MeV electrons

Fused silica and KBr samples were irradiated with a 2.7 MeV electron beam. The emission of fused silica and KBr samples in the UV and visible regions was studied under various experimental conditions. Numerical simulation of optical emission was carried out using the GEANT4 computer platform. Simulations of energy spectra and angular distributions of the beam electrons were performed for different target thicknesses. The results reveal the effect of scattering of the beam electrons on the angular distribution of Cherenkov radiation in fused silica samples with thicknesses exceeding the electron path length.

Weibull-Fréchet random path length model for avalanche gain and noise in photodiodes

A four-parameter Weibull-Fréchet (WF) distribution function has been introduced in the random path length (RPL) model for nonlocal modelling of soft-threshold ionisation in semiconductors. The WF function has been demonstrated to be capable of reproducing the realistic probability distribution function (PDF) of electron and hole ionisation path lengths extracted from full band Monte Carlo (FBMC) transport simulations of bulk GaAs. The electron-initiated multiplication in GaAs avalanche photodiodes (APDs) calculated by the WF-RPL model is in excellent agreement with the results from FBMC. The predicted excess noise factor is closer to that of FBMC as compared to the hard threshold RPL model. The advantage of this improved RPL model as a tool for predicting the PDF of electron and hole ionisation path lengths in AlAs0.56Sb0.44 from the experimentally measured avalanche gain and noise has been analysed. The electron ionisation path length PDF of AlAs0.56Sb0.44 has a unique feature of two decay rates with a narrow full width at half maximum and a long tail. The extremely low hole ionisation coefficient in AlAs0.56Sb0.44 is found with a PDF of ionisation path length spanning over 50 µm at an electric field of 600 kVcm−1, supporting the very low hole feedback ionisation in AlAs0.56Sb0.44 APDs. The combination of the detailed and peaked electron’s ionisation path length PDF and of the broad hole’s ionisation path length PDF is responsible for the extremely low avalanche noise in the 1.55 µm thick AlAs0.56Sb0.44 APDs.

Electrons on a straight path: A novel ionisation vacuum gauge suitable as reference standard

The consortium of the European project 16NRM05 designed a novel ionisation vacuum gauge in which the electrons take a straight path from the emitting cathode through the ionisation space into a Faraday cup. Compared to existing ionisation vacuum gauges, this has the advantage that the electron path length is well defined. It is independent of the point and angle of emission and is not affected by space charge around the collector. In addition, the electrons do not hit the anode where they can be reflected, generate secondary electrons or cause desorption of neutrals or ions. This design was chosen in order to develop a more stable ionisation vacuum gauge suitable as reference standard in the range of 10−6 Pa to 10−2 Pa for calibration purposes of other vacuum gauges and quadrupole mass spectrometers. Prototype gauges were produced by two different manufacturers and showed predictable sensitivities with a very small spread (<1.5%), very good short-term repeatability (<0.05%) and reproducibility (<1%), even after changing the emission cathode and drop-down tests. These characteristics make the gauge also attractive for industrial applications, because a gauge exchange does not require calibration or re-adjustment of a process.

Numerical simulation of the performance of the ASDEX pressure gauge

The normalised output Ii/(Ie-Ii) and sensitivity of the ASDEX pressure gauge are numerically simulated in this paper. The effects of the electrode spacing, width of grid slit and electrode voltage on the sensitivity of the ASDEX gauge are analyzed respectively, and the orthogonal analysis method is applied to optimize the structural and electrical parameters of the ASDEX pressure gauge. According to the simulation result, the sensitivity of the optimized gauge is increased to 0.01138Pa−1 without magnetic field. The motion trajectories of emitted electrons and sensitivity are simulated when the magnetic induction intensity is in the range of 0~1 T. The studies have demonstrated that the magnetic field inhibited the motion of emission electrons in the direction perpendicular to the base plate, increasing the probability of the electrons reaching the effective ionization volume and extending the electron path length due to the helical trajectories. When the magnetic induction intensity increased from 0 to 1 T, the sensitivity of the gauge increases from 0.01138 Pa-1 to 0.149 Pa-1.

Diffracted Transitive Radiation as a Means for Indicating the Divergence of an Ultrarelativistic Electron Beam

Diffracted transition radiation (DTR) emitted by a beam of relativistic electrons traversing a thin single-crystal plate in the Laue scattering geometry is considered. An expression has been obtained describing the angular DTR density when the electron path length in a target is far less than the extinction length of x-rays in the crystal. It is shown that in this case, the DTR process has a clearly pronounced kinematic character. Numerical calculations of the DTR photon yield in the direction of Bragg’s scattering performed for different solid angles of recording show that it is significantly affected by the electron beam divergence. We have concluded that the DTR photon yield measured for a given solid angle can be used for indicating the electron beam divergence. Results of model calculations of the electron beam divergence parameters for a given yield of DTR photons traversing the slit collimator have shown that the formula proposed in this work can be successfully used as a basis for the development of methods for measuring the divergence of ultrahigh-energy relativistic electron beams based on the angular DTR distribution.

An Alternative Interpretation of Impulsive SEP Events Occurring on 1999 January 9–10

Abstract We have examined two impulsive solar energetic particle events that occurred on 1999 January 9–10 (earlier event A and later event B). Both events showed earlier velocity dispersion and later intensity dropout of ions. In particular, in event A, the dropout repeated five times. Through the onset time analysis of solar particles, we find that only at higher energies (&gt;0.2 MeV nucleon−1 for heavy ions and &gt;2.8 keV for electrons) can the analysis provide a consistent path length of ions and electrons. The path length in event A is larger than that in event B. In contrast, at lower energies, the analysis fails to predict the distribution of first arrival of solar particles. The divergence between observation and prediction would increase if the interplanetary scattering of ions were taken into account. We then focus on the lower-energy region, where a negative correlation of ion intensities with plasma β is displayed. We have found that the repeated dropout of ions can be caused by the magnetic reconnection acceleration in the solar wind. In addition, we have discovered an isolated proton dropout event in which a sharply anisotropic pitch-angle distribution of low-energy electrons is also seen. Our observation is consistent with the prediction of Tautz et al. that a minimum power spectral density component parallel to the magnetic field can reduce the magnetic mirroring effect, preventing electron scattering through 90°.

On pathlength and energy straggling of megavoltage electrons slowing down

Purposeto elucidate the effects of multiple scattering and energy-loss straggling on electron beams slowing down in materials. MethodsEGSnrc Monte Carlo simulations are done using a purpose-written user-code. ResultsPlots are presented of the primary electron’s energy as a function of pathlength for 20 MeV electrons incident on water and tantalum as are plots of the overall distribution of pathlengths as the 20 MeV electrons slow down under various Monte Carlo scenarios in water and tantalum. The distributions range from 1 % to 135 % of the CSDA range in water and from 1 % to 186 % in tantalum. The effects of energy-loss straggling on energy spectra at depth and electron fluence at depth are also presented. ConclusionsThe role of energy-loss straggling and multiple scattering are shown to play a significant role in the range straggling which determines the dose fall-off region in electron beam dose vs depth curves and a significant role in the energy distributions as a function of depth.

Optical Absorption of a Composite Based on Bilayer Metal–Dielectric Spherical Nanoparticles

Optical properties of core–shell (metal) nanoparticles and composites based on them were investigated in the classical approximation. Frequency dependences of the real and imaginary parts of the dielectric permittivity of bilayer particles and absorption coefficients of composites based on them were calculated. Small-scale oscillations of the real and imaginary parts of the dielectric permittivity and absorption coefficient were found in the low-frequency spectral region. The behavior of the dielectric permittivity of bilayer nanoparticles was analyzed for the limiting cases of thin and thick shells. The influence of the effective mean free pathlength of electrons on the optical characteristics of the bilayer nanoparticles was considered. Two maxima of the composite absorption coefficient were demonstrated to be due to hybridization of polar modes.

Mesoscale Chemomechanical Interplay of the LiNi0.8Co0.15Al0.05O2 Cathode in Solid-State Polymer Batteries

Presently, the challenges faced by batteries are twofold. Firstly, they are expected to possess high energy and power density for powering zero emission electric vehicles. Secondly, they are also expected to be inexpensive and safe for deployment in electric vehicles and consumer applications. A potential candidate ideally suited for this case are solid-state polymer lithium-ion batteries coupled with layered nickel-rich oxides such as LiNi0.8Co0.15Al0.05O2 (NCA) or LiNi1−x−yCoxMnyO2 (NCM). Replacing the non-aqueous, liquid electrolytic solution that is comprised of flammable components, with a solid-state electrolyte such as an inexpensive polymer is highly advantageous. However, layered nickel-rich oxides such as NCA are not only known for their high reversible capacity, but also their inherent structural and thermal instability at charged state, which leads to several degradation mechanisms including intergranular cracking. While this phenomenon has been studied extensively for lithium-ion batteries consisting of a non-aqueous, liquid electrolyte, it has not been studied for solid polymer batteries. Secondary NCA particles harvested from cycled NCA-poly(ethylene oxide) batteries were subjected to a combination of two- and three-dimensional full-field transmission X-ray microscopy (2D/3D-FF-TXM) and high resolution broad ion beam- and focuses ion beam milling-scanning electron microscopy (BIB-/FIB-SEM). Doing so, the complex chemomechanical interplay within the hierarchically structured lithium-ion battery was investigated to reveal morphological defects and chemical heterogeneity in the secondary particles. Intergranular cracking was found to be extremely apparent after only a few cycles. It was found, that the absence of liquid electrolyte severely affects transport in fractured particles and may also hastens and accelerates fracture due to lower uniformity in current density. Intergranular cracks significantly increase diffusion path lengths of both charge carriers, electrons and lithium ions. While in liquid lithium-ion batteries, electrolyte filling the cracks should maintain lithium ion transport pathways but not electron transport pathways, in solid-state batteries, intergranular cracking compromises not only the electronic pathways, but also the ionic pathways. Eventually, intergranular cracking in polymer batteries seems to lead to isolated and inactivated primary grains within the core of secondary particles. Figure 1

Characterizing Lithiation/Delithiation of Thick Electrodes without Binders and Conductive Additives Using Neutron Imaging

The advantages of thick electrodes in improving energy density at the cell level has been reported in many lithium-ion battery papers. Thick electrodes increase energy density by reducing the fraction of the cell dedicated to inactive components, in particular current collectors and separators. However, thick electrodes increase electron and ion transport path lengths through the electrode which increases cell resistance and can result in limited current densities of battery operation. In this presentation the fabrication and characterization of cells with thick electrodes will be described. These electrodes are produced via hydraulic pressing and mild thermal treatment of active material powders as opposed to conventional casting of composite slurries. These electrodes thus have no binders or conductive additives, which would be expected to increase the diffusion of ions through the electrolyte-impregnated void regions within the electrode. However, these all active material electrodes can now have multiple processes that might limit their rate of charge/discharge, including transport of ions through the electrode microstructure, transport of ions through the solid active material particles, and transport of electrons through the all active material electrode matrix to the current collector. To provide insights into the processes that are limiting the current density in the high energy electrodes, we will describe the use of neutron imaging. Neutron imaging is highly sensitive to the lithium concentration within the battery cell, and thus regions undergoing lithiation/delithiation can be interrogated in operando during charge/discharge of the cell. The lithiation/delithiation profiles provide insights into what process limits the cell and opportunities to improve the performance of these thick electrodes fabricated with less common processing routes.

Synergic effect of adsorbed gas and charging on surface flashover

Flashover is a crucial issue in both high-voltage engineering and surface physics. It not only challenges the existing theories about its dynamic evolution, but also inhibits the clean energy revolution by limiting the accessible voltage rating of power equipment. It is of significance to elucidate the microscopic process along the interface to improve the flashover performance. In the present study, the synergic effect of adsorbed gas and surface charging is investigated, which reveals a long ignored factor for determining the flashover voltage. Depending on the relative amount of adsorbed gas, the flashover voltage varies, which exhibit different behavior from the bulk breakdown of the same gas. The amount of N2 gas adsorbed on epoxy resin (EP) surface is much larger than that on Al2O3 ceramic surface, corresponding to the observed higher flashover voltage on EP. It is proposed that the adsorbed gas molecules not only modify the local surface charging state via their interaction with the trapped charges, but also capture free electrons due to the distortion of their electronic distribution. Both effects suppress the free path length of electrons in the gas-solid interface. This work explores another possibility to improve the surface flashover performance.

Control of 1-dimensionally structured tungsten oxide thin films by precursor feed rate modulation in flame vapor deposition

Flame vapor deposition (FVD) process is the fast and effective method to prepare high quality 1-D nanostructured tungsten oxide thin film, which has several advantages in photoelectrochemical (PEC) water splitting such as more sunlight absorption, straight electron path and short diffusion length of electron hole. The precursor concentration in FVD process is the most important process variable to determine the morphology of prepared nanostructures. In this study, the precursor concentration in FVD process was controlled precisely and flexibly by adjusting the tungsten feed rate with the wire feeding device which we developed. Several interesting nanostructures were prepared in FVD process by modulating the precursor concentration with time and the reasonable growth mechanisms for developments of those nanostructures are also proposed. The narrower and longer 1-D tungsten oxide nanostructure could be prepared by controlling the precursor feed rate and deposition time in FVD process, which is desirable for efficient PEC water splitting.

In-situ analysis of ion-induced physicochemical change of Si surface by secondary electron yield detection

We report the variation of secondary electron yield with ion fluence during 10 keV N+1 and Ar+1 ions bombardment on a Si (1 0 0) surface at normal and oblique incidence. The initial high secondary electron yield from Si surface is observed due to the native oxide layer on Si for both nitrogen and argon ion bombardment. The secondary electron yield is expected to decrease and stabilize for continue ion bombardment, which is observed for the argon ion irradiation. However, a significant variation of the electron yield with continuing nitrogen ion bombardment is observed. Ex situ measurement of ion bombarded surfaces by Atomic Force Microscope shows that the variation of secondary electron yield is correlated with the surface topography development due to ion bombardment. Also, the reaction of the projectile with target atom alters the secondary electron emission in case of N+ ion bombardment. The mean free path length of the secondary electron is estimated using the Parilis-Kishinevsky theory and the variation is presented. Real-time physicochemical changes of Si surface due to low energy ion bombardment have been probed in-situ by monitoring the secondary electron emission yield measurements.

The impact of mass density variations on an electron Monte Carlo algorithm for radiotherapy dose calculations.

Background and PurposeA key step in electron Monte Carlo dose calculation requires converting Computed Tomography (CT) numbers from a tomographic acquisition to a mass density. This study investigates the dosimetric consequences of perturbations applied to a calibration table between CT number and mass density.Materials and MethodsA literature search was performed to define lower and upper bounds for physically reasonable perturbations to a reference CT number to mass density calibration table. Electron beam dose was calculated for ten patients using these variations and the results were compared to clinical plans originally derived with a reference calibration table. Dose differences both globally and in the Planning Target Volume (PTV) were assessed using dose- and volume-based metrics and 3- dimensional gamma analysis for each patient.ResultsSmall but statistically significant differences were observed between perturbations and reference data for certain metrics including volume of the 50% prescription isodose. Upper and lower variations in CT number to mass density calibration yielded mean values of V50% that were 4.4% larger and 2.1% smaller than reference values respectively. Gamma analysis using 3%/3mm criteria indicated >99% passing rate for the PTV for all patients. Global gamma analysis for some patients showed larger discrepancies possibly due to large electron path lengths through inhomogeneities.ConclusionsIn most patients, physically reasonable perturbations in CT number to mass density curves will not induce clinically significant impact on calculated target dose distributions. Strong dependence of electron transport on voxel material may produce dose speckle throughout the volume. Care should be taken in evaluating critical structures at depths beyond the target volume in highly heterogeneous regions.

Ionization cross sections of the Au L subshells by electron impact from the L3 threshold to 100 keV

We measured the cross sections for Au Lα, Lβ, Lγ, Lℓ and Lη x-ray production by the impact of electrons with energies from the L3 threshold to 100 keV using a thin Au film whose mass thickness was determined by Rutherford Backscattering Spectrometry. The x-ray spectra were acquired with a Si drift detector, which allowed to separate the components of the Lγ multiplet lines. The measured Lα, Lβ, , L, , , and Lη x-ray production cross sections were then employed to derive Au L1, L2 and L3 subshell ionization cross sections with relative uncertainties of 8%, 7% and 7%, respectively; these figures include the uncertainties in the atomic relaxation parameters. The correction for the increase in electron path length inside the Au film was estimated by means of Monte Carlo simulations. The experimental ionization cross sections are about 10% above the state-of-the-art distorted-wave calculations.

Chemical Interface Damping Depends on Electrons Reaching the Surface.

Metallic nanoparticles show extraordinary strong light absorption near their plasmon resonance, orders of magnitude larger compared to nonmetallic nanoparticles. This "antenna" effect has recently been exploited to transfer electrons into empty states of an attached material, for example to create electric currents in photovoltaic devices or to induce chemical reactions. It is generally assumed that plasmons decay into hot electrons, which then transfer to the attached material. Ultrafast electron-electron scattering reduces the lifetime of hot electrons drastically in metals and therefore strongly limits the efficiency of plasmon induced hot electron transfer. However, recent work has revived the concept of plasmons decaying directly into an interfacial charge transfer state, thus avoiding the intermediate creation of hot electrons. This direct decay mechanism has mostly been neglected, and has been termed chemical interface damping (CID). CID manifests itself as an additional damping contribution to the homogeneous plasmon line width. In this study, we investigate the size dependence of CID by following the plasmon line width of gold nanorods during the adsorption process of thiols on the gold surface with single particle spectroscopy. We show that CID scales inversely with the effective path length of electrons, i.e., the average distance of electrons to the surface. Moreover, we compare the contribution of CID to other competing plasmon decay channels and predict that CID becomes the dominating plasmon energy decay mechanism for very small gold nanorods.

3D-simulation of ionisation gauges and comparison with measurements

3D-simulations using the Vector Fields OPERA 3D software have been carried out on the CERN-type modulated Bayard-Alpert gauge. The program allows to simulate the ion creation inside the gauge and takes into account space charge effects. Parameters such as sensitivity, ion and electron path lengths inside and outside the ionisation grid, location of ion creation, collection efficiency, and potential distribution were studied as a function of emission current. This investigation resulted in a deeper understanding about the behaviour of the gauge, in particular about the effect of space charge. The achieved results were compared with experimental measurements; the results are satisfactory and encourage further studies.