High-energy Secondary Electrons Research Articles

Electron Imaging of Nanoscale Charge Distributions Induced by Femtosecond Light Pulses.

Surface charging is ubiquitously observable during in situ transmission electron microscopy of nonconducting specimens as a result of electron beam/sample interactions or optical stimuli and often limits the achievable image stability and spatial or spectral resolution. Here, we report on the electron-optical imaging of surface charging on a nanostructured surface following femtosecond multiphoton photoemission. By quantitatively extracting the light-induced electrostatic potential and studying the charging dynamics on relevant time scales, we gain insights into the details of the multiphoton photoemission process in the presence of an electrostatic background field. We study the interaction of the charge distribution with the high-energy electron beam and secondary electrons and propose a simple model to describe the interplay of electron- and light-induced processes. In addition, we demonstrate how to mitigate sample charging by simultaneously optically illuminating the sample.

Development of Transition Edge Sensors for the Detection of Secondary Electrons

Secondary electrons generated when carbon ions go through the body are considered to be playing a major role in damaging the DNA of cancerous cells. To fully understand the mechanism of carbon ions, experimental and quantitative analysis on the energy and the behavior of these electrons are important. Therefore, we have started the development of TES arrays for the detection of secondary electrons. By performing simulations using Geant4, we have confirmed that the superconducting thin film of a TES is able to stop the electrons. We have also found that relatively high energy secondary electrons might have an influence on the energy spectrum of electrons having energies lower than 100 eV. We are currently fabricating a prototype 2-pixel TES array for the detection of electrons.

Self-discharge by Streaming Cosmic Rays

A new nonthermal phenomenon caused by streaming cosmic rays (CRs) in the universe is proposed. The streaming CRs drive the return current of thermal electrons to compensate for the CR current. Then, electric fields are induced by the resistivity of the return current. It is shown that the resistive electric fields can accelerate secondary electrons generated by the streaming CRs. This is the self-discharge by streaming CRs. In this work, the self-discharge condition and the condition for runaway acceleration of secondary electrons are presented. The self-discharge creates high-energy secondary electrons, resulting in enhancements of ionization and nonthermal emission including the Kα emission line of neutral iron. After the self-discharge, the return current of thermal electrons is replaced by the electric current of secondary electrons. Since some generation and amplification of magnetic fields are driven by the return current of thermal electrons, the self-discharge can significantly influence them.

Monte Carlo study of the relative role of energy absorption mechanisms in water under irradiation by photons in the energy range of 3–1000 Ry

Mechanisms of energy absorption in liquid water under 3–1000 Ry photon irradiation are studied by Monte Carlo simulation with accounting for the cascade decays of vacancies in inner electron subshells created by primary and secondary ionization processes. At incident photon energies above O1s ionization threshold, most of the energy is transferred to the medium by secondary electrons and photons produced in the processes of photoionization, electron impact ionization, and cascade decay of inner-shell vacancies. On the average, on the whole considered incident photon energy interval, 49.8% of the absorbed energy is transferred to the medium by high-energy secondary electrons and photons in inelastic processes of interaction with the atoms of the medium. Low-energy electrons and photons incapable of ionizing/exciting the atoms of the medium are responsible for 48.8% of energy absorption. Energy absorption in the processes of primary ionization by incident photons are significant only at small incident photon energies of a few Rydbergs, their average contribution to energy absorption on the whole considered energy interval makes only 1.4%. At incident photon energies above O1s-ionization threshold low-energy electrons are produced in large quantities; this can be important in radiotherapy since in the tissues of organisms, low-energy electrons may cause the DNA strand breaks via the mechanism of dissociative electron attachment.

Cosmic ray transport and radiative processes in nuclei of starburst galaxies

AbstractThe high rate of star formation and supernova explosions of starburst galaxies make them interesting sources of high-energy radiation. Depending on the level of turbulence present in their interstellar medium, the bulk of cosmic rays produced inside starburst galaxies may lose most of their energy before escaping, thereby making these sources behave as calorimeters, at least up to some maximum energy. Contrary to previous studies, here we investigate in detail the conditions under which cosmic ray confinement may be effective for electrons and nuclei and we study the implications of cosmic ray confinement in terms of multifrequency emission from starburst nuclei and production of high-energy neutrinos. The general predictions are then specialized to three cases of active starbursts, namely, M82, NGC 253, and Arp220. Both primary and secondary electrons, as well as electron–positron pairs produced by gamma-ray absorption inside starburst galaxies are taken into account. Electrons and positrons produced as secondary products of hadronic interactions are found to be responsible for most of the emission of leptonic origin. In particular, synchrotron emission of very high energy secondary electrons produces an extended emission of hard X-rays that represents a very interesting signature of hadronic process in starburst galaxies, potentially accessible to current and future observations in the X-ray band. A careful understanding of both the production and absorption of gamma-rays in starburst galaxies is instrumental to the assessment of the role of these astrophysical sources as sources of high-energy astrophysical neutrinos.

Simulations of hybrid direct current radiofrequency (dc/rf) capacitively coupled plasmas

Effects of applying a high negative direct current (dc) voltage in argon capacitive coupled plasmas on the flux of secondary electrons (SEs) to a radiofrequency (rf) biased electrode and plasma uniformity were investigated using particle-in-cell Monte Carlo collision simulation and results were compared with experimental results. High energy SEs originating at electrodes with high negative dc voltage due to ion-impact as well as electron-impact were accelerated out of the sheath through the sheath voltage drop. These SEs gain energy equal to the applied dc voltage and either travel to the wafer electrode where they are lost (dumped) or are trapped between electrodes depending on the voltage on bottom electrode. Trapping and dumping of ballistic electrons depends on the magnitudes of the voltages (dc and rf) applied to each electrode. High energy SEs alter the electron energy distribution function at the wafer, high energy electron flux to the wafer and the plasma density profile. With the application of negative dc voltage, wafer receives high energy electron flux with energies up to applied dc voltage. For the range of conditions examined in this paper, the plasma density increases with the application of the negative dc voltage due to ionization by trapping of high energy SEs. The electrons that originate at the negative dc electrode minimize the impact of enhanced radial electric fields at the rf electrode edge allowing dc/rf plasmas to be center peaked even at low (13.56 MHz) rf frequencies.

Quantitative comparison of simulated and measured signals in the STEM mode of a SEM

Abstract The transmission of electrons with energies 15 keV and 30 keV through Si and Au films of 100 nm thickness each have been studied in a Scanning Transmission Electron Microscope. The electrons that were transmitted through the films were detected using a multi-annular photo-detector consisting of a central Bright Field (BF) and several Dark Field (DF) detectors. For the experiment the detector was gradually offset from the axis and the signal from the central BF detector was studied as a function of the offset distance and compared with MC simulations. The experiment showed better agreement between experiment and several different MC simulations as compared to previous results, but differences were still found particularly for low angle scattering from Si. Data from Au suggest that high energy secondary electrons contribute to the signal on the central BF detector for low primary beam energies, when the STEM detector is in its usual central position.

Dynamics of the ion energy spectrum in EUV-induced hydrogen plasma

The dynamics of the ion energy spectrum in low-pressure (10–100 Pa) hydrogen plasma induced by extreme ultraviolet (EUV) pulses in the wavelength range of 10–20 nm was studied experimentally. The plasma was generated under cathode irradiation due to both direct gas ionization by EUV photons and impact ionization by high-energy secondary electrons. The dynamics of the spectra of ions incident on the cathode was measured using a time-resolved retarding field energy analyzer. It is shown that the ion spectrum dynamics is completely determined by the time evolution of the cathode sheath. At low gas pressures (<20 Pa), the ion spectrum at early moments after the EUV pulse has a peaked shape, typical of a collisionless plasma sheath, and is mainly determined by the cathode voltage. As the pressure increases, the peak broadens and low energy ions appear in the spectrum due to ion collisions in the cathode sheath. An increase in the role of collisions with decreasing plasma density is also observed in the time evolution of ion spectra.

The absorbed doses to water and the TLD-100 signal contributions associated with the neutron contamination of a clinical 18 MV photon beam

Neutrons produced by the photonuclear effect have to be considered as unwanted concomitants of radiotherapy photon beams. Therefore the response of TLD-100 to thermal neutrons via the L6i(n,α)3H reaction may disturb TLD-100 measurements of the absorbed dose to water DW in water (a) on the beam axis as part of the external quality control system by mailed TLD probes, and (b) at out-of-field positions utilizing their time-integrating properties. The present study aims at quantitatively estimating the possible disturbances and identifying the situations where TLD-100 has to be replaced by the TLD-600/TLD-700 system to get rid of the unwanted neutron induced TL signal. This work fills the gap of knowledge between on-axis and far out-of-field data. The absorbed doses to water and to a TLD-100 chip, caused by neutrons and photons at several positions in a water phantom, were determined using the EGSnrc and the FLUKA Monte Carlo codes, and for the H4e and H3 ions released in the L6i(n,α)3H reaction a relative TL efficiency of 0.3, compared with high-energy secondary electrons, has been adopted. It was found that (1) the total DW profile is negligibly neutron-affected up to 10cm off-axis distance, (2) the perturbation of the DW reading of a TLD-100 detector due to its neutron response can be neglected within the borders of the photon field, but becomes significant beyond the field edge, (3) the non-reference condition correction factor kNR for TLD-100-based DW measurements in water is accordingly affected at off-axis positions.

Improvement in luminance efficiency of organic light emitting diodes by suppression of secondary electron bombardment of substrate during sputter deposition of top electrode films

In this study, we investigated the degradation mechanisms of the luminance performance of organic light-emitting diodes (OLEDs) when their top electrode films were deposited by sputter deposition process. During the sputter deposition of the top electrode films, the suppression of the incidence of high-energy electrons on the substrate was attempted using various methods. As a result, we found that during electrode deposition, the incidence of the high-energy secondary electrons, which were emitted from the target surface, on the substrate was the main cause of the significant degradation of the luminance performance. It was also found that the application of a magnetic field by setting permanent magnets near the substrate holder and the insertion of a shield plate near the target cathode were effective in suppressing the incidence of secondary electrons on the substrate.

Radiation-induced solidification of ionic liquid under extreme electric field

An extreme electric field on the order of 1010 V m−1 was applied to the free surface of an ionic liquid to cause electric-field-induced evaporation of molecular ions from the liquid. The point of ion emission was observed in situ using a TEM. The resulting electrospray emission process was observed to create nanoscale high-aspect-ratio dendritic features that were aligned with the direction of the electric field. Upon removal of the stressing field the features were seen to remain, indicating that the ionic liquid residue was solidified or gelled. Similar electrospray experiments performed in a field-emission scanning electron microscope revealed that the features are created when the high-energy electron beam damages the molecular structure of the ionic liquid. While the electric field does not play a direct role in the fluid modification, the electric stress was critical in detecting the liquid property change. It is only because the electric stress mechanically elongated the fluid during the electrospray process and these obviously non-liquid structures persisted when the field was removed that the damage was evident. This evidence of ionic liquid radiation damage may have significant bearing on electrospray devices where it is possible to produce high-energy secondary electrons through surface impacts of emitted ions downstream of the emitter. Any such impacts that are in close proximity could see reflected secondary electrons impact the emitter causing gelling of the ionic liquid.

Abstract B08: Space radiation exposure induces stable epigenome alterations relevant to human lung cancer

Abstract Manned interplanetary travel is imminent, but currently inhibited by uncertainty surrounding the long-term human health risks associated with space radiation exposure, for which there is currently no effective means of shielding. Terrestrial exposure to ionizing radiation, such as γ- or X-rays, increases the age-related risk of most common human cancers, however the extent to which these risks can be extrapolated to astronauts in deep space remains unclear. Compared to terrestrial radiation, space radiation contains high charge and energy (HZE) ions with a high linear energy transfer (high-LET). As they traverse the cell, HZE ions interact directly with macromolecules along the particle trajectory, and emit high-energy secondary electrons (δ rays) that extend laterally for several microns, resulting in a wide and densely ionizing core track. HZE radiation creates a complex mixture of DNA damage (double strand breaks, single strand breaks, oxidative base damage, etc.) in tight clusters along the particle track that can be difficult to repair. Consequently, research thus far has focused on the influence of both terrestrial and space radiation on DNA. Here we sought to determine the acute impact and long-term consequences of space radiation exposure on the epigenome. Immortalized human bronchial epithelial cells (3KT) were exposed to low LET (X-ray) or HLET radiation (28Si, 56Fe). Cells were harvested after 48hrs or maintained in continuous culture for an additional 3 months, and DNA methylation patterns using the Illumina Human 450K platform. A linear mixed-effects model was applied to identify changes in DNA methylation significantly associated with dose, source, or time after exposure. Significantly, we found that radiation-induced methylation changes occur early and persist over time, reflecting a stable and heritable change to the epigenome. CpG sites affected by high and low LET radiation exposure also tended to affect CpG sites in different genomic compartments. Fe-affected CpG sites were enriched in CpG island ‘shores’ and underrepresented in gene bodies, whereas those affected by X-ray exposure were enriched in gene bodies and intergenic regions. Interestingly, Fe affected sites tended to affect CpG sites in regions with features of ‘open’ chromatin, such as promoters and distal enhancer elements, whereas sites affected by Si were overrepresented in heterochromatic regions. To probe the significance of our findings with respect to human lung cancer, we examined the methylation status of the radiation-sensitive CpG sites in primary human lung cancers analyzed in The Cancer Genome Atlas (TCGA) project. We found that the methylation status of Fe radiation sensitive sites could discriminate tumor from normal tissue for both lung adenocarcinomas and squamous cell lung carcinomas. No such relationship existed for the sites affected by low LET X-rays or 28Si radiation exposure. Thus, the high LET radiation exposure ‘signature’ reflects a cancer-specific methylation pattern observed in human primary lung cancers. From these data, we suggest that the stable imprint of a prior high LET radiation exposure is reflected in the DNA methylation pattern, and may prove useful as a biomarker of long-term, individual cancer risk. Citation Format: E. Kennedy, D.R. Powell, Z. Li, J.S.K. Bell, H. Feng, W. Dynan, B. Dwivedi, J. Kowalski, K.N. Conneely and P.M. Vertino. Space radiation exposure induces stable epigenome alterations relevant to human lung cancer. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Sep 24-27, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2016;76(2 Suppl):Abstract nr B08.

Role of the tail of high-energy secondary electrons in the Monte Carlo evaluation of the fraction of electrons backscattered from polymethylmethacrylate

This work describes a Monte Carlo algorithm which appropriately takes into account the stochastic behavior of electron transport in solids and the simulation of the energy distributions of the secondary and backscattered electrons from polymethylmethacrylate irradiated by an electron beam. The simulation of the backscattered and secondary electron spectra also allows calculating the backscattering coefficient and the secondary electron yield of polymethylmethacrylate as a function of the initial energy of the incident electrons. Results of the simulation are compared with the available experimental data. The importance of considering all the electrons emerging form the surface in calculating the secondary electron yield and the backscattering coefficient is highlighted. In particular, we will discuss the importance of taking into account the tail of high energy secondary electrons in the spectrum for the simulation of the backscattering coefficient.

Prototypes of self-powered radiation detectors employing intrinsic high-energy current.

The authors experimentally investigate the effect of direct energy conversion of x-rays via selfpowered Auger- and photocurrent, potentially suitable to practical radiation detection and dosimetry in medical applications. Experimental results are compared to computational predictions. The detector the authors consider is a thin-film multilayer device, composed of alternating disparate electrically conductive and insulating layers. This paper focuses on the experiments while a companion paper introduces the fundamental concepts of high-energy current (HEC) detectors. The energy of ionizing radiation is directly converted to detector signal via electric current induced by high-energy secondary electrons generated in the detector material by the incident primary radiation. The HEC electrons also ionize the dielectric and the resultant charge carriers are selfcollected due to the contact potential of the disparate electrodes. Thus, an electric current is induced in the conductors in two different ways without the need for externally applied bias voltage or amplification. Thus, generated signal in turn is digitized by a data acquisition system. To determine the fundamental properties of the HEC detector and to demonstrate its feasibility for medical applications, the authors used a planar geometry composed of multilayer microstructures. Various detectors with up to seven conducting layers with different combinations of materials (250 μm Al, 35 μm Cu, 100 μm Pb) and air gaps (100 μm) were exposed to nearly plane-parallel 60-120 kVp x-ray beams. For the experimental design and verification, the authors performed coupled electron-photon radiation transport computations. The detector signal was measured using a commercial data acquisition system with 24 bits dynamic range, 0.4 fC sensitivity, and 0.9 ms sampling time. Measured signals for the prototype detector varied depending on the number of layers, material type, and incident photon energy, and it was in the range of 30-150 nA/cm(2) for unit air kerma (1 Gy), which is viable for practical applications. The experiments had an excellent agreement with the computations. Within the examined range of 60-120 kVp, the energy dependence of the HEC (normalized to the x-ray tube output) was relatively small. Based on the experimental results for 100 ms sampling time, it would be possible to measure the time dependence of x-ray beams for x-ray tube current of 0.1 mA or higher. Significant advantages of the HEC device are that generation of its signal does not require external power supply, it can be made in any size and shape, including flexible curvilinear forms, and it is inexpensive. It remains to be determined, which of the potential applications in medical dosimetry (both in vivo and external), or radiation protection would benefit from such selfpowered detectors.

Direct measurement of the 3-dimensional DNA lesion distribution induced by energetic charged particles in a mouse model tissue

Charged particles are increasingly used in cancer radiotherapy and contribute significantly to the natural radiation risk. The difference in the biological effects of high-energy charged particles compared with X-rays or γ-rays is determined largely by the spatial distribution of their energy deposition events. Part of the energy is deposited in a densely ionizing manner in the inner part of the track, with the remainder spread out more sparsely over the outer track region. Our knowledge about the dose distribution is derived solely from modeling approaches and physical measurements in inorganic material. Here we exploited the exceptional sensitivity of γH2AX foci technology and quantified the spatial distribution of DNA lesions induced by charged particles in a mouse model tissue. We observed that charged particles damage tissue nonhomogenously, with single cells receiving high doses and many other cells exposed to isolated damage resulting from high-energy secondary electrons. Using calibration experiments, we transformed the 3D lesion distribution into a dose distribution and compared it with predictions from modeling approaches. We obtained a radial dose distribution with sub-micrometer resolution that decreased with increasing distance to the particle path following a 1/r2 dependency. The analysis further revealed the existence of a background dose at larger distances from the particle path arising from overlapping dose deposition events from independent particles. Our study provides, to our knowledge, the first quantification of the spatial dose distribution of charged particles in biologically relevant material, and will serve as a benchmark for biophysical models that predict the biological effects of these particles.

Numerical model of the plasma formation at electron beam welding

The model of plasma formation in the keyhole in liquid metal as well as above the electron beam welding zone is described. The model is based on solution of two equations for the density of electrons and the mean electron energy. The mass transfer of heavy plasma particles (neutral atoms, excited atoms, and ions) is taken into account in the analysis by the diffusion equation for a multicomponent mixture. The electrostatic field is calculated using the Poisson equation. Thermionic electron emission is calculated for the keyhole wall. The ionization intensity of the vapors due to beam electrons and high-energy secondary and backscattered electrons is calibrated using the plasma parameters when there is no polarized collector electrode above the welding zone. The calculated data are in good agreement with experimental data. Results for the plasma parameters for excitation of a non-independent discharge are given. It is shown that there is a need to take into account the effect of a strong electric field near the keyhole walls on electron emission (the Schottky effect) in the calculation of the current for a non-independent discharge (hot cathode gas discharge). The calculated electron drift velocities are much bigger than the velocity at which current instabilities arise. This confirms the hypothesis for ion-acoustic instabilities, observed experimentally in previous research.

SU-E-T-141: Effect of a Single Gold Nanoparticle with Different Sizes Inside a Small Water Phantom

Purpose: This study is to investigate the effects of the gold nanoparticles (GNP), a series of micrometre scale simulations have been constructed with Geant4 to track particles and simulate the effects of those particles as they pass through water phantom. Methods: The simulations were used to calculate the number of secondary electrons which are emitted from the particle tracks and the amount of energy which is deposited in the cell tissue. More electrons means that more water molecules can undergo hydrolysis and create potentially dangerous free radical molecules, therefore breaking up DNA and killing off cells or causing damaging mutations. Results: For the 20nm GNP, all three proton energies saw a small increase of electrons above the control, while the X-rays nearly tripled the number of electrons in the phantom. For the 50 nm GNP, the 3 and 2 MeV protons saw a small increase again, however the 1 MeV protons saw a decrease in electrons, the X-rays saw a large increase of nearly 4 times the number of electrons. For the 110nm GNP, all three proton energies saw a decrease in the total number of electrons in the phantom, while the X-rays saw an increase of 8 times as many electrons. Conclusion: From the range of GNP sizes used, it was found that the X-rays have a larger dose enhancement effect as the GNP size increases, the relation between electron emissions and GNP size was linear. This is because the majority of the dose from the X-rays is delivered to the cell tissue through the initial high energy secondary electrons, any dose lost from the Augerelectrons being trapped inside the GNP volume is small compared to the dose that escapes with the high energy electrons.

Monte carlo simulation of positron induced secondary electrons in thin carbon foils

Emission of secondary electrons induced by the passage of low energy positrons through thin carbon foils was studied by the Monte Carlo method. The positron and electron elastic cross sections were calculated by partial wave analysis. The inelastic positron-valence-electron was described by the energy loss function obtained from dielectric theory. The positron-core-electron interaction was modelled by the Gryzinski's excitation function. Positron transport inside the carbon foil was simulated in detail. Secondary electrons created by positrons and high energy secondary electrons through inelastic interactions were tracked through the foil. The positron transmission coefficient and secondary electron yielded in forward and backward geometry are calculated and dependences on positron energy and carbon foil thickness are discussed.

Proton beam writing: a platform technology for nanowire production

P-beam writing (proton beam writing) is a unique direct write 3D nano-lithographic technique which has been developed at the Centre for Ion Beam Applications (CIBA), in the Physics Department of the National University of Singapore. This technique employs a focused MeV proton beam which is scanned in a predetermined pattern over a resist (e.g. PMMA, SU-8 or HSQ), which is subsequently chemically developed. In e-beam writing as well as p-beam writing the energy loss of the primary beam is dominated by energy transfer to substrate electrons. Unlike the high energy secondary electrons generated during e-beam writing the secondary electrons induced by the primary proton beam have low energies (typically less than 100 eV) and therefore a limited range, resulting in minimal proximity effects. The low proximity effects exhibited by p-beam writing coupled with the straight trajectory and high penetration of the proton beam enables the production of high aspect ratio, high density 3D micro- and nano-structures with well defined smooth side walls to be directly written into resist materials. These structures can be used as templates to electroplate metallic nanowires.

Exposure parameters in proton beam writing for hydrogen silsesquioxane

In proton beam writing (PBW) a focused MeV proton beam is scanned in a predetermined pattern over a resist (e.g. PMMA, SU-8 or HSQ), which is subsequently chemically developed. In e-beam writing as well as p-beam writing the energy loss of the primary beam is dominated by energy transfer to substrate electrons. Unlike the high energy secondary electrons generated during e-beam writing the secondary electrons induced by the primary proton beam have low energy and therefore a limited range, resulting in minimal proximity effects. The low proximity effects exhibited by p-beam writing coupled with the straight trajectory and high penetration of the proton beam enables the production of high aspect ratio, high density 3D micro and nanostructures with well defined smooth side walls to be directly written into resist materials. This property together with the stability and focusing power of the end station ensures even exposures with nm smoothness and allows fabrication of details down to the 20nm level. In this paper, we present results like contrast and sensitivity for PBW using, hydrogen silsesquioxane (HSQ) and XR-1541, both are non-C based resists. Unlike PMMA and SU-8 resist HSQ shows aging effects, requiring optimized processing parameters in PBW.