Interaction Of Secondary Electrons Research Articles

Selected Area Manipulation of MoS2 via Focused Electron Beam-Induced Etching for Nanoscale Device Editing.

We demonstrate direct-write patterning of single and multilayer MoS2 via a focused electron beam-induced etching (FEBIE) process mediated with the XeF2 precursor. MoS2 etching is performed at various currents, areal doses, on different substrates, and characterized using scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopies. Scanning transmission electron microscopy reveals a sub-40 nm etching resolution and the progression of point defects and lateral etching of the consequent unsaturated bonds. The results confirm that the electron beam-induced etching process is minimally invasive to the underlying material in comparison to ion beam techniques, which damage the subsurface material. Single-layer MoS2 field-effect transistors are fabricated, and device characteristics are compared for channels that are edited via the selected area etching process. The source-drain current at constant gate and source-drain voltage scale linearly with the edited channel width. Moreover, the mobility of the narrowest channel width decreases, suggesting that backscattered and secondary electrons collaterally affect the periphery of the removed area. Focused electron beam doses on single-layer transistors below the etching threshold were also explored as a means to modify/thin the channel layer. The FEBIE exposures showed demonstrative effects via the transistor transfer characteristics, photoluminescence spectroscopy, and Raman spectroscopy. While strategies to minimize backscattered and secondary electron interactions outside of the scanned regions require further investigation, here, we show that FEBIE is a viable approach for selective nanoscale editing of MoS2 devices.

Secondary Electrons in Gold Nanoparticle Clusters and Their Role in Therapeutic Ratio: The Outcome of a Monte Carlo Simulation Study.

Gold nanoparticles (GNPs) are used in proton therapy radio-sensitizers to help increase the dose of radiation to targeted tumors by the emission of secondary electrons. Thus, this study aimed to investigate the link between secondary electron yields produced from a nanoshell of GNPs and dose absorption according to the distance from the center of the nanoparticles by using a Monte Carlo model. Microscopic evaluation was performed by modeling the interactions of secondary electrons in a phase-space file (PSF), where the number of emitted electrons was calculated within a spherical GNP of 15 nm along with the absorbed dose near it. Then, the Geant4-DNA physics list was used to facilitate the tracking of low-energy electrons down to an energy below 50 eV in water. The results show a remarkable change in the number of secondary electrons, which can be compared at concentrations less than and greater than 5 mg/mL, with increased secondary electron production exhibited around NPs within a distance of 10–100 nm from the surface of all nanospheres. It was found that there was a steep dose enhancement drop-off up to a factor of dose enhancement factor (DFE) ≤ 1 within a short distance of 100 nm from the surface of the GNPs, which revealed that the dose enhancement existed locally at nanometer distances from the GNPs. Overall, our results indicate that the physical interactions of protons with GNP clusters should not be considered as being directly responsible for the radio-sensitization effect, but should be regarded as playing a major role in NP properties and concentrations, which has a subsequent impact on local dose enhancement.

Radiation shielding properties of gallium alloys

Consideration of the photon interaction in the target medium is not sufficient for the estimation of absorbed dose in the different organs from a source of photons. For the accurate absorbed dose calculation specific absorbed fraction of energy is required. It is defined as the ratio of the energy absorbed by the target to the energy emitted by the source. The calculation of specific absorbed fraction of energy depends on the buildup factor values and the interaction of secondary electrons. X-rays and gamma rays undergo scattering on entering a medium, thus the energy gets degraded, due to which secondary radiation are produced. These secondary radiations can be calculated using buildup factor. Gallium alloys are less toxic and cost effective material compared to lead. Due to the importance of Gallium alloys in radiation shielding, the study of an interaction of X-rays and gamma radiation in these alloys becomes important.In the present work we have investigated the energy exposure buildup factors (EBF) and specific absorbed fraction (SAF) of energy for some gallium alloys such as Gallium alloy [Al-50%, Ga-50%], Galfenol [Fe-30%, Ga-70%] and Galinstan [Ga-68.5%, In-21.5%, Sn-10%]. With the increase in the penetration depth, the value of buildup factors increases. It is observed that the values of exposure buildup factors and Specific absorbed fraction of energy are larger for the gallium alloy Galinstan than the other studied gallium alloys. From this work it is clear that among the studied gallium alloys, in Galinstan absorption of X-rays and gamma radiations more compared to other studied alloys. Hence, we can conclude that the gallium alloy Galinstan is a good absorber of X-rays and gamma radiation among the studied gallium alloys. This work is useful in the shielding of radiation.

Comparison of Prospectively Generated Glioma Treatment Plans Clinically Delivered on Magnetic Resonance Imaging (MRI)-Linear Accelerator (MR-Linac) Versus Conventional Linac: Predicted and Measured Skin Dose.

Introduction: Magnetic resonance imaging-linear accelerator radiotherapy is an innovative technology that requires special consideration for secondary electron interactions within the magnetic field, which can alter dose deposition at air–tissue interfaces. As part of ongoing quality assurance and quality improvement of new radiotherapy technologies, the purpose of this study was to evaluate skin dose modelled from the treatment planning systems of a magnetic resonance imaging-linear accelerator and a conventional linear accelerator, and then correlate with in vivo measurements of delivered skin dose from each linear accelerator. Methods: In this prospective cohort study, 37 consecutive glioma patients had treatment planning completed and approved prior to radiotherapy initiation using commercial treatment planning systems: a Monte Carlo-based algorithm for magnetic resonance imaging-linear accelerator or a convolution-based algorithm for conventional linear accelerator. In vivo skin dose was measured using an optically stimulated luminescent dosimeter. Results: Monte Carlo-based magnetic resonance imaging-linear accelerator plans and convolution-based conventional linear accelerator plans had similar dosimetric parameters for target volumes and organs-at-risk. However, magnetic resonance imaging-linear accelerator plans had 1.52 Gy higher mean dose to air cavities (P < .0001) and 1.10 Gy higher mean dose to skin (P < .0001). In vivo skin dose was 14.5% greater for magnetic resonance imaging-linear accelerator treatments (P = .0027), and was more accurately predicted by Monte Carlo-based calculation (ρ = 0.95, P < .0001) versus convolution-based (ρ = 0.80, P = .0096). Conclusion: This is the first prospective dosimetric comparison of glioma patients clinically treated on both magnetic resonance imaging-linear accelerator and conventional linear accelerator. Our findings suggest that skin doses were significantly greater with magnetic resonance imaging-linear accelerator plans but correlated better with in vivo measurements of actual skin dose from delivered treatments. Future magnetic resonance imaging-linear accelerator planning processes are being designed to account for skin dosimetry and treatment delivery.

Dosimetric Comparison in Malignant Glioma Patients Clinically Treated on Hybrid Magnetic Resonance Imaging (MRI)-Linac (MRL) vs. Conventional Linac

The Magnetic Resonance Imaging (MRI)-Linac (MRL) is a hybrid machine integrating a high field strength 1.5T MRI with a linear accelerator, providing superior soft tissue visualization of tumors and organs-at-risk (OARs) during treatment delivery. A special consideration for MRL radiotherapy is accounting for interactions of secondary electrons generated within the magnetic field, which can alter dose deposition at air-tissue interfaces. We evaluated dosimetric outcomes in clinically treated malignant glioma patients who received at least one fraction of radiotherapy on both the MRL and a conventional Linac.Thirty-seven glioma patients treated on both the MRL and a conventional Linac for adjuvant chemoradiotherapy between July 2019 and February 2021 were analyzed. Planning was completed on treatment planning systems (TPS) using a Monte-Carlo algorithm that accounts for magnetic field effects (Monaco v5.40) for the MRL, and a convolution-superposition algorithm (Pinnacle v9.8) for the conventional Linac. Dosimetric parameters of interest from the target, OARs, and air-tissue interface volumes for each patients' clinical treatment plans were extracted and compared. For 10 representative patients, in vivo skin doses during a single fraction of MRL and conventional Linac treatment were obtained using an Optically Stimulated Luminescent Dosimeter (OSLD) placed in a defined location on the patient's skin near the Planning Target Volume (PTV). Student's t-test and Wilcoxon signed-rank test were used to compare parameters between Monaco and Pinnacle. Spearman's correlation was used to assess the relationship between in vivo OSLD measurements and TPS skin dose. Threshold for statistical significance was P < 0.05.Most patients were treated for high grade glioma (76% Grade III or IV, 24% Grade II), and median PTV was 257.4 cm3 (range, 37.1-570.3 cm3). MRL and conventional Linac had similar V100, V95, D98, and D95 for PTV, and D3cc for optic chiasm, optic nerves, and each cochlea (P = NS). However, clinically delivered Monaco plans had significantly greater doses within air cavities (mean Dmean higher by 1.3 Gy, P < 0.0001) and skin (mean Dmean higher by 1.9 Gy, P < 0.0001; mean D2cc higher by 8.1 Gy, P < 0.0001; mean V20 Gy higher by 7.2 cm3, P < 0.0001), compared to clinically delivered Pinnacle plans. In vivo OSLD skin readings were 14.5% greater for treatments delivered on the MRL (P = 0.0027), and were more accurately predicted by Monaco (r = 0.95, P < 0.0001) vs. Pinnacle (r = 0.80, P = 0.0096).In this prospective study of clinically treated glioma patients on both MRL and conventional Linac, the dosimetric impact of the magnetic field was minimal for the target and standard OARs. However, higher doses to skin and air cavities were observed. In vivo correlation of dose to skin was more accurately predicted with Monaco. Future MRL planning processes are being designed to account for skin dosimetry and treatment delivery.

Spatio-temporal plasma heating mechanisms in a radio frequency electrothermal microthruster

Low-power micro-propulsion sources are currently being developed for a variety of space missions. Electrothermal plasma thrusters are of specific interest since they enable spatial control of the power deposition to the propellant gas. Understanding the mechanisms whereby electrical power is coupled to the propellant will allow for optimization of the heating and fuel efficiencies of electrothermal sources. Previous studies of radio frequency (RF) plasmas have shown a dependence of the gas and electron heating mechanisms on the local collisionality. This is of particular importance to thrusters due to the large pressure gradients that exist between the inlet and outlet when expanding into vacuum. In this work, phase-resolved optical emission spectroscopy and numerical simulations were employed to study plasma heating in an asymmetric RF (13.56 MHz) electrothermal microthruster operating in argon between 186–226 Pa (1.4–1.7 Torr) plenum pressure, and between 130–450 V (0.2–5 W). Three distinct peaks in the phase-resolved Ar(2p1) electron impact excitation rate were observed, arising from sheath collapse heating, sheath expansion heating, and heating via secondary electron collisions. These experimental findings were corroborated with the results of two-dimensional fluid/Monte Carlo simulations performed using the Hybrid Plasma Equipment Model (HPEM). The influence of each mechanism with respect to the position within the plasma source during an α-γ mode transition, where plasma heating is driven via bulk and sheath heating, respectively, was investigated. Sheath dynamics were found to dictate the electron heating at the inlet and outlet, this is distinct from the center of the thruster where interactions of secondary electrons were found to be the dominant electron heating mechanism. Optimization of the heating mechanisms that contribute to the effective exhaust temperature will directly benefit electrothermal thrusters used on miniaturized satellite platforms.

Superthermal electron magnetosphere‐ionosphere coupling in the diffuse aurora in the presence of ECH waves

AbstractThere are two main theories for the origin of the diffuse auroral electron precipitation: first, pitch angle scattering by electrostatic electron cyclotron harmonic (ECH) waves, and second, by whistler mode waves. Precipitating electrons initially injected from the plasma sheet to the loss cone via wave‐particle interaction processes degrade in the atmosphere toward lower energies and produce secondary electrons via impact ionization of the neutral atmosphere. These secondary electrons can escape back to the magnetosphere, become trapped on closed magnetic field lines, and deposit their energy back to the inner magnetosphere. ECH and whistler mode waves can also move electrons in the opposite direction, from the loss cone into the trap zone, if the source of such electrons exists in conjugate ionospheres located at the same field lines as the trapped magnetospheric electron population. Such a situation exists in the simulation scenario of superthermal electron energy interplay in the region of diffuse aurora presented and discussed by Khazanov et al. (2014) and will be quantified in this paper by taking into account the interaction of secondary electrons with ECH waves.

Bases physiques du radiodiagnostic

An x-ray tube mainly emits low-energy X-rays, with few maximum energy E 0 (equal in keV to the voltage U in kV) x-rays. Aluminium filtration (mandatory minimum thickness of 1.5 to 2,5mm based on tube voltage) reduces soft X-rays and provides a mean energy equal to 2/3 E 0. The half value layer of a reference material characterizes the spectrum. X-ray attenuation in tissues is due to secondary electron interactions: photoelectric effect at low-energy, especially in dense materials with high Z number; compton effect at intermediate-energy, proportional to density. The optimization of acquisition parameters of a medically necessary examination is based on appropriate selection of the highest voltage (U in kV) providing the best contrast and lowest tube current (Q in mAs) providing a diagnostic image.

Focused electron beam induced etching of silicon using chlorine

A new beam-assisted process for removing silicon from a surface in the nanometer scalein a conventional scanning electron microscope is presented. This approach isbased on focused electron beam induced etching with pure chlorine gas being usedas the precursor. In contrast to the established etching process using a focusedion beam (with or without the addition of a precursor), no amorphizationand gallium implanting of the substrate takes place. The observed low etchrates facilitate removal with sub-nanometer precision. No spontaneous etchingof silicon as in the case of xenon difluoride was observed. Etch rates of up to4 nm min − 1 could be achieved as well as a minimum feature size of below 80 nm. The effect of etchingparameters like electron beam energy, electron beam accelerating voltage or pixelspacing were systematically examined. Finally, the underlying etching mechanism interms of secondary electron interactions and precursor replenishment is discussed.

Energy deposition model for I-125 photon radiation in water

In this study, an electron-tracking Monte Carlo algorithm developed by us is combined with established photon transport models in order to simulate all primary and secondary particle interactions in water for incident photon radiation. As input parameters for secondary electron interactions, electron scattering cross sections by water molecules and experimental energy loss spectra are used. With this simulation, the resulting energy deposition can be modelled at the molecular level, yielding detailed information about localization and type of single collision events. The experimental emission spectrum of I-125 seeds, as used for radiotherapy of different tumours, was used for studying the energy deposition in water when irradiating with this radionuclide.

Mechanistical studies on the electron-induced degradation of polymers: polyethylene, polytetrafluoroethylene, and polystyrene

Mechanisms of the electron-induced degradation of three polymers utilized in aerospace applications (polyethylene (PE), polytetrafluoroethylene (PTFE), and polystyrene (PS)) were examined over a temperature range of 10 K to 300 K at ultra high vacuum conditions (∼10(-11) Torr). These processes simulate the interaction of secondary electrons generated in the track of galactic cosmic ray particles in the near-Earth space environment with polymer material. The chemical alterations at the macromolecular level were monitored on-line and in situ by Fourier-transform infrared spectroscopy and mass spectrometry. These data yielded important information on the temperature dependent kinetics on the formation of, for instance, trans-vinylene groups (-CH=CH-) in PE, benzene (C(6)H(6)) production in PS, fluorinated trans-vinylene (-CF=CF-) and terminal vinyl (-CF=CF(2)) groups in PTFE together with molecular hydrogen release in PE and PS. Additional data on the radiation-induced development of unsaturated, conjugated bonds were collected via UV-vis spectroscopy. Temperature dependent G-values for trans-vinylene formation (G(-CH=CH-) ≈ 25-2.5 × 10(-4) units (100 eV)(-1) from 10-300 K) and molecular hydrogen evolution (G(H(2)) ≈ 8-80 × 10(-5) molecules (100 eV)(-1) from 10-300 K) for irradiated PE were calculated to quantify the degree of polymer degradation following electron irradiation. These values are typically two to three orders of magnitude less than G-values previously published for the irradiation of polymers with energetic particles of higher mass.

Modelling low energy electron interactions for biomedical uses of radiation

Current radiation based medical applications in the field of radiotherapy, radio-diagnostic and radiation protection require modelling single particle interactions at the molecular level. Due to their relevance in radiation damage to biological systems, special attention should be paid to include the effect of low energy secondary electrons. In this study we present a single track simulation procedure for photons and electrons which is based on reliable experimental and theoretical cross section data and the energy loss distribution functions derived from our experiments. The effect of including secondary electron interactions in this model will be discussed.

About the secondary electron emission from solid hydrogens: H 2, HD, D 2 and T 2

Reported by Sørensen and Schou [H. Sørensen, J. Schou, J. Appl. Phys. 53 (1982) 5230], the electron-induced secondary electron emission yield curves, δ = f( E 0), of condensed hydrogen and deuterium show a significant isotopic effect with a yield for D 2 at about ∼1.5 times larger than that for H 2 and very low values ( δ < 0.1 at E 0 = 3 keV) in contrast to the very large yields ( δ > 50 at E 0 = 3 keV) reported, elsewhere, for condensed rare gases. Combined to the use of a recent model, physical considerations on the interaction of secondary electrons with zero point fluctuations permit to suggest a coherent explanation for these facts. The values of SE attenuation for three isotopes are deduced (∼4 nm for H 2; ∼5.6 nm for D 2; ∼4.9 nm for HD) and the electron-induced secondary electron emission yield curves, δ = f( E 0), initially limited to an investigated energy range, 0.5 keV < E 0 < 3 keV, are extrapolated down to their maximum values ranging from δ max ∼ 0.95 at E max 0 ∼ 145 eV, for H 2 to δ max ∼ 1.13 at E max 0 ∼ 175 eV for D 2. From the estimate of the attenuation length of secondary electrons, the same extrapolation procedure is also applied to solid tritium. Some consequences on charging of thick films and of small clusters are deduced. Showing also an isotopic effect, the expected evolution of X-ray-induced secondary electron emission yields, δ X = f( hν), of condensed H 2, HD, D 2 and T 2 are finally estimated.

DNA strand breaks induced by low keV energy heavy ions

We present some results on the interaction of low energy atomic ions with DNA. Experiments consist of irradiation of dried DNA in vacuum with Ar ions at low keV energies for different time intervals. The DNA is placed back in solution and analysed by agarose gel electrophoresis. These experiments demonstrated the production of single and double strand breaks. The induction of these lesions could be due to several processes: direct collisions with DNA constituent atoms resulting in displacements, cascade recoil collisions of the constituent atoms, electron transfer processes between the ion and the DNA as well as breaks induced by molecular excitation and secondary electron interactions. Here we briefly discuss some aspects of direct and recoil collision processes.

Trench homogeneity in plasma immersion ion implantation

Neon was implanted into silicon pieces mounted onto aluminium trench structures using plasma immersion ion implantation (PIII). The spatially resolved depth profiles, obtained by elastic recoil detection analysis (ERDA), were compared with two-dimensional simulation results from particle-in-cell (PIC) calculations. Good qualitative agreement was obtained, however the absolute measured doses are too high. This can be explained by the interaction of secondary electrons with the plasma, increasing the ion density, and by density inhomogeneities within the plasma. Furthermore, the 2D simulation cannot adequately describe the 3D experiment as a significant flux of ions from the outside of the symmetry plane into it is observed in the experiment.

Transport of Secondary Electrons Through a Film of Condensed Water; Implications for Imaging Wet Samples

We have performed a theoretical and experimental study of the the effect that a surface layer of condensed water has on the emission of secondary electrons from the surface. This is an issue of considerable interest to users of the Environmental SEM (ESEM) when imaging wet samples. Previous work has been performed to investigate the effect of a layer of water on back scattered electrons (BSE), but secondary electron (SE) imaging is more commonly used in ESEM, so an understanding of the interactions of SE with water is important. The aim of this work is to quantify the thickness of water through which imaging is possible, by considering both the interactions of secondary electrons with the water, and the interactions of the water layer with the sample, which may affect the secondary electron emission coefficient, δ.The effects that a surface layer of water may have on electron emission from a sample surface can be split into three regimes.

Electron beams in research and technology

Fast electrons lose their energy by inelastic collisions with electrons of target molecules forming secondary electrons and excited molecules. Coulomb interaction of secondary electrons with valence electrons of neighboring molecules leads to the formation of radical cations, thermalized electrons, excited molecular states and radicals. The primary reactive species initiate chemical reactions in the materials irradiated. Polymer modifications using accelerated electrons such as cross-linking of cable insulation, tubes, pipes and moldings, vulcanization of elastomers, grafting of polymer surfaces, processing of foamed plastics and heat shrinkable materials have gained wide industrial acceptance. A steadily growing electron beam technology is curing of paints, lacquers, printing inks and functional coatings. Electron beam processing offers high productivity, the possibility to treat the materials at normal temperature and pressure, excellent process control and clean production conditions. On an industrial scale the most important application of fast electrons is curing of 100% reactive monomer/prepolymer systems. Mainly acrylates and epoxides are used to formulate functional coatings on substrates such as paper, foil, wood, fibre board and high pressure laminates. A survey is given about the reaction mechanism of curing, the characterization of cured coatings, and of some industrial application.

Energy Deposition and Ionization Fluctuations Induced by Ions in Small Sites: An Analytical Approach

A concise, analytical approach is developed for calculating energy deposition and ionization fluctuations in volumes within ion-irradiated media which have dimensions as small as 1 nm. The method accounts for both direct ion interactions with the site and interactions of secondary electrons which are produced by ions in the surrounding medium. Particular attention is given to the way the contributions of the two types of events are combined. Since energy deposition fluctuations are simply related to the fundamental quantities ZD and yD employed in microdosimetry theory, this new approach provides a convenient means to obtain these parameters. Results obtained with the analytical method show good agreement with Monte Carlo charged-particle track-structure calculations of yD for 0.5 to 20 MeV protons incident on spherical sites of water vapor with diameters ranging from 1 nm to 10 microns. In contrast to Monte Carlo techniques, the analytical method does not depend on knowing the intricacies of single ion and electron interactions with the target and can therefore be adapted to calculations with heavier incident ions and different target materials, including those of the condensed state.

Energetic secondary electrons and the nonthermal galactic radio background - A probe of the magnetic field in interstellar clouds

A previous analysis of the manifestations of charged-pion-decay secondary electrons in interstellar cloud material is extended to include those contributions to the Galactic radio and soft gamma-ray backgrounds that are directly attributable to energetic secondaries. The equilibrium distribution of secondary electrons in dense interstellar clouds is calculated, synchrotron emissivity from isolated interstellar clouds is examined, and it is shown how the value of the magnetic field in these clouds may be determined by observing the radio emission in their directions. The contribution that such clouds make to the integrated radio background is evaluated, and the Galactic distribution of bremsstrahlung gamma rays that arise from interactions of secondary electrons with thermal material in dense clouds is computed. The results indicate that a magnetic field of no more than 80 microgauss is characteristic of dense clouds and that the integrated synchrotron radiation from secondary electrons in interstellar clouds will contribute a significant fraction of the nonthermal brightness along the Galactic equator even if the mean cloud field is as low as 35 microgauss.

Beam-plasma interactions in a hollow cathode

A new hollow-cathode effect has been observed in the residual plasma of a mercury arc discharge. A pair of negatively biased plane parallel electrodes were found to give rise to an orderly sequence of high frequency electrical bursts. The bursts made negligible contribution to ionization in the plasma. Similar bursts were subsequently observed in a self-sustained hollow-cathode discharge. The timing of the bursts and their variation with pressure in the range 2 to 15 mTorr are in good agreement with a simple theory based on multiple-mode resonant interaction of secondary electrons with a decaying plasma. In the present tube, the frequencies of the oscillation within each burst are predicted to lie between 100 MHz and 20 GHz.