Secondary Electron Energy Research Articles

SU‐GG‐J‐127: Monte Carlo Calculation of Secondary Electron Production from Gold Nanoparticles in Proton Beam Irradiation

Purpose: We examined the production efficiency and average kinetic energy of secondary electrons produced from a single gold NP upon proton irradiation Method and Materials: The production efficiency and average kinetic energy were investigated by using the Geant4 Monte Carlo simulation toolkit. The proton energy range corresponding to the 250‐MeV pristine Bragg peak is from 100 MeV to 0 MeV. We investigated proton beams with energies of 100 MeV, 50 MeV, and 20 MeV to represent the scope from 100 MeV to 0 MeV. Upon biological uptake efficiency, we chose gold NPs with diameters of 35 nm, 50 nm, 70 nm, and 100 nm. Results: We found that for a NP with a diameter of 70 nm, the secondary electron production efficiency increased as the proton energy decreased from 100 MeV to 50 MeV and then 20 MeV. The average kinetic energy of the secondary electrons increased with proton energy. We found that the larger the NP size, the higher the production efficiency and average kinetic energy of the secondary electrons. Finally, we found that among the samples we tested the smallest NP was the one with a diameter of 70 nm. Conclusion: Collectively, the secondary electron production efficiency and average kinetic energy from the NPs should also be taken into account when considering the potential use of NPs in proton radiotherapy.

A Monte Carlo Study of the Radiation Quality Dependence of DNA Fragmentation Spectra

We simulated the irradiation of human fibroblasts with gamma rays, protons and helium, carbon and iron ions at a fixed dose of 5 Gy. The simulations were performed with the biophysical Monte Carlo code PARTRAC. From the output of the code, containing in particular the genomic positions of the radiation-induced DNA double-strand breaks (DSBs), we obtained the DNA fragmentation spectra. Very small fragments, in particular those related to "complex lesions" (few tens of base pairs), are probably very important for the late cellular consequences, but their detection is not possible with the common experimental techniques. We paid special attention to the differences among the various ions in the production of these very small fragments; in particular, we compared the fragmentation spectra for ions of the same specific energy and for ions of the same LET (linear energy transfer). As found previously for iron ions, we found that the RBE (relative biological effectiveness) for DSB production was considerably higher than 1 for all high-LET radiations considered. This is at variance with the results obtainable from experimental data, and it is due to the ability to count the contribution of small fragments. It should be noted that for a given LET this RBE decreases with increasing ion charge, due mainly to the increasing mean energy of secondary electrons. A precise quantification of the DNA initial damage can be of great importance for both radiation protection, particularly in open-space long-term manned missions, and hadrontherapy.

Secondary electron doping contrast: Theory based on scanning electron microscope and Kelvin probe force microscopy measurements

The secondary electron emission flux in a scanning electron microscope is a powerful tool for delineation of electrically active dopant concentration, built-in potentials, and surface electric fields in semiconductor junctions. In all the secondary electron images of p-n junctions, the p-doped regions appear brighter than n-doped regions. We present a theory for the doping contrast in p-n junctions that is based on the secondary electron emission yield and surface band bending extracted from Kelvin probe force microscopy measurements. We show that the contrast is governed by the secondary electron escape depth, and their escape probability which is related to the secondary electron energy distribution and the effective electron affinity. It is found that the escape depth is the main factor determining the dopant contrast, and the escape probability has a smaller effect. In addition, our theory explains the logarithmic dependence of the measured contrast on the acceptor concentration in silicon reported by many groups.

Secondary electron generation in electron-beam-irradiated solids:resolution limits to nanolithography

We have investigated the secondary electron generation in electron-beam-irradiated solids by means of a Monte Carlo simulation. The slow secondary electron energy was found to be independent of the position and the incident energy of the electron beam, and the electron beam broadening in thin films due to secondary electrons was found to be at least 5 – 10 nm, setting limits to the nanolithographic resolution.

Theoretical study of dc-biased single-surface multipactors

The physical characteristics of a dc-biased single-surface multipactor are investigated, and a novel scheme of the multipactor that uses a photonic crystal cavity resonator maximizing insulation gap distance is introduced. With a restriction of no premature impact, a resonant rf phase minimizing rf electric field is analytically obtained, which is universal for any surface material. The surface material properties bring about a restriction of secondary electron energy by which some backscattered, high energy electrons may have no fixed resonant phases. With an assumption of zero initial velocity, analytical derivations and a parametric map for the phase locking are presented. In the map, without possibility of premature impact, a bifurcation of a stable resonant phase into two stable resonant phases appears when the ratio of rf to dc electric field is over a critical value. This distinctive phenomenon is demonstrated by a three-dimensional particle-in-cell simulation.

Determination of angular cross sections for electron dissociative ionization of the CCl4 molecule

Secondary electron energy and angle dependent differential cross sections for the product ions CCl3+, CCl2+, CCl+, C+, Cl+, Cl2+ and CCl3++ resulting from the dissociative ionization of CCl4 by electron collision have been calculated at fixed incident electron energy of 100 eV. The Jain-Khare semiempirical formulation that requires the oscillator strength data as input has been employed. The corresponding partial and total integral ionization cross sections have also been evaluated in the energy range varying from the ionization threshold up to 1000 eV. The partial and the total ionization cross sections are compared with the previously reported experimental and theoretical results leading to a reasonably good agreement.

Effects of electron-impact ionization on the damage to biomolecules irradiated by XFEL

The damage to biomolecules irradiated by an X-ray free electron laser is investigated with time-dependent rate equations, considering photoionization, Compton scattering, Auger decay and electron-impact ionization of the C atom and its ions. Averaged energies of secondary electrons, which are produced by electron-impact ionization, are calculated with the binary encounter dipole model (Kim Y-K. and Rudd M. E., 1994 Phys. Rev. A, 50 3954) to study electron-energy distribution in the biomolecule. We found that those energies for the C atom, C1+ and C2+ ions are about 20, 60 and 150 eV, respectively, at an incident electron energy of 12keV, while those energies are in the order of tens of electronvolts when the incident electron energy is about 250 eV. We also found that the importance of electron-impact ionization increases as the X-ray flux decreases.

Electron‐Collision‐Induced Dissociative Ionization Cross Sections for Silane

Secondary electron energy and angle dependent differential cross sections for the production of cations (n = 0–3), and H+ resulting from dissociative ionization of SiH4by electron collision have been evaluated at fixed incident electron energies of 100 and 200 eV. The semiempirical formulation of Jain and Khare which requires the oscillator strength data as a major input has been employed. In the absence of experimental data for differential cross sections, the corresponding derived integral partial and total ionization cross sections in the energy range varying from ionization threshold to 1000 eV revealed a satisfactory agreement with the available experimental and theoretical data. We have also evaluated the ionization rate coefficients on the basis of calculated partial ionization cross sections and Maxwell‐Boltzmann energy distributions.

Theoretical and experimental investigations of electron emission in C 6+ + H 2O collisions

Theoretical differential and total cross sections for the direct ionization process of water vapour by 6 MeV/u C 6+ ions are compared to new experimental measurements performed by the dedicated apparatus already used for measuring the energy and angular distributions of secondary electrons emitted from water vapour by fast heavy-ion impact [D. Ohsawa, H. Kawauchi, M. Hirabayashi, Y. Okada, T. Homma, A. Higashi, S. Amano, Y. Hashimoto, F. Soga, Y. Sato, Nucl. Instr. and Meth. B 227 (2005) 431]. In the present work, ab initio calculations have been carried out in the first Born approximation by using an accurate molecular wave function for describing the initial bound state of the target. The calculated cross sections exhibit good agreement with the present experimental measurements and compare relatively well to the existing semi-empirical results over the entire angular and energy ranges investigated here. Free from any adjustable parameter, the proposed theoretical approach describes in detail the complete kinematics of the water molecule ionization process by highly energetic carbon ions, and could therefore be easily used for modelling the heavy charged-particle transport in the biological matter.

Hydrogen in nano‐diamond films: experimental and computational studies

AbstractWe present studies related to the incorporation of hydrogen and its bonding configuration in diamond films composed of diamond grains of varying size which were deposited by three different methods: hot filament (HF), micro wave (MW) and direct current glow discharge (dc GD) chemical vapor deposition. The size of the diamond grains which constitute the films varies in the following way: hundreds of nm in the case of HF CVD (“sub‐micron size”, ∼300 nm), tens of nm in the case of MW CVD (3–30 nm) and a few nm in the case of dc GD CVD (“ultra nano‐crystalline diamond”, ∼5 nm). Secondary ion mass spectroscopy (SIMS) and high resolution electron energy loss spectroscopy (HR‐EELS) were applied to investigate the hydrogen trapping in the films. The hydrogen retention of the diamond films increases with decreasing grain size, indicating that most likely hydrogen is bonded and trapped in grain boundaries as well as on the internal grain surfaces. HR‐EELS analysis shows that at least part of this hydrogen is bonded to sp2‐ and sp3‐hybridized carbon, thus giving rise to typical C–H vibration modes. The vibrational spectroscopies show the increase of sp2 C–H modes in transition from sub‐micron to ultra nano‐crystalline grain size. These conclusions are supported by preliminary results of computer simulations which show that hydrogen atoms at the boundary between a nano‐diamond particle and an amorphous shell have lower energy than hydrogen atoms within either the nano‐diamond or the amorphous region. These results suggest that hydrogen is expected to be localized at the nano‐diamond/amorphous carbon interface as experimentally found. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Secondary electrons in rf and dc/rf capacitive discharges

We extend an analytic model of the effects of secondary electrons on symmetric (equal area) capacitive discharges to include a combination of rf and dc voltage sources. A ‘dc/rf sheath’ develops on the negatively biased dc electrode while a typical rf sheath develops on the other electrode. The analytical results for rf-only and dc/rf cases, including secondaries, are compared with the results of numerical simulations using a one-dimensional (1D) plane-parallel particle-in-cell (PIC) code, confirming the pressure and density regimes for which adding dc improves the overall discharge efficiency and can control the fluxes and energy distributions of the secondary electrons incident on the electrodes. From a 2D PIC simulation, the effects of secondaries and dc voltage on the uniformity of the ions and secondary electrons are investigated, with the dc adding an additional element of uniformity control. The uniformity of the secondary electron energy and angular distributions on the substrate electrode are determined from the 2D simulations.

Dosimetric characteristics of an unshielded p-type Si diode: linearity, photon energy dependence and spatial resolution

The unshielded Si diode PTW 60012, used for accurate measurements of the transversal dose profiles of narrow photon beams, has been investigated with regard to its linearity, photon energy dependence and spatial resolution. The diode shows a slight supralinearity, i.e. increase of the response with pulse dose, by 3% over the pulse dose range 0.1 to 0.8mGy. In p-type silicon, supralinearity results from the increased chance for radiation-induced electrons to escape recombination when the pulse dose increases. Over the energy range from 6 to 15 MV, the response decreases by about 4%. This small variation of the response results from partial compensation between the influences of the secondary electron energy on the mass stopping power ratio silicon/water and on electron backscattering from the silicon chip. The lateral response function of the examined diode has a full half width of 1.3mm. Dose profiles of 5mm half-width can still be recorded with negligible error.

Secondary electron spectra of gold under bombardment by very low-energy positrons

Measurements of the secondary electron energy spectra resulting from very low-energy positron bombardment of a polycrystalline Au surface are presented. The low-energy part of the secondary spectra contain significant contributions from two processes: (1) annihilation-induced Auger electrons that have lost energy before leaving the surface and (2) secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions (∼3 eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. These measurements were also used to obtain the first experimentally determined upper limit on the intensity of the spectrum of Auger-induced secondary electrons.

Plasma-sheath instability in Hall thrusters due to periodic modulation of the energy of secondary electrons in cyclotron motion

Particle-in-cell simulation of Hall thruster plasmas reveals a plasma-sheath instability manifesting itself as a rearrangement of the plasma sheath near the thruster channel walls accompanied by a sudden change of many discharge parameters. The instability develops when the sheath current as a function of the sheath voltage is in the negative conductivity regime. The major part of the sheath current is produced by beams of secondary electrons counter-streaming between the walls. The negative conductivity is the result of nonlinear dependence of beam-induced secondary electron emission on the plasma potential. The intensity of such emission is defined by the beam energy. The energy of the beam in crossed axial electric and radial magnetic fields is a quasiperiodical function of the phase of cyclotron rotation, which depends on the radial profile of the potential and the thruster channel width. There is a discrete set of stability intervals determined by the final phase of the cyclotron rotation of secondary electrons. As a result, a small variation of the thruster channel width may result in abrupt changes of plasma parameters if the plasma state jumps from one stability interval to another.

Differential and partial ionization cross sections for electron impact ionization of plasma processing molecules: CF4 and PF5

The semiempirical approach that requires the oscillator strength data as input has been employed to evaluate the single and double differential cross sections corresponding to the production of various cations, CFx+ (x=0–3), F+, CF2++ and CF3++ from CF4 and PFx+ (x=0–4), F+ and PFx++ (x=0–4) from the PF5 molecule, through dissociative electron ionization processes. The calculations are made as a function of secondary electron energy and angle at fixed incident electron energies of 100 and 200 eV. The corresponding partial and total ionization cross sections have also been derived in the energy range varying from ionization thresholds to 1000 eV. Comparison of the evaluated partial and total ionization cross sections is made with the experimental and theoretical data wherever available. The formula has been extended to the calculation of ionization rate coefficients as a function of electron temperature/energy using the Boltzmann energy distribution function.

Tribological effects of surface texturing on nitriding steel for high-performance engine applications

This study reports on the effects of surface modification by laser texturing on tribological performances of nitriding steel for high-performance engine applications. In particular, a comparison of tribological properties on untextured and textured 30NiCrMo12 nitriding steel was made among hydrodynamic, mixed and boundary lubrication regimes. Ordered arrays of circular dimples were created on nitriding steel surfaces by Laser Surface Texturing. Friction experiments were performed using a pin-on-disc apparatus at sliding speeds ranging from 1 to 12 cm s −1 and nominal contact pressures ranging from 1 to 10 MPa. Static counterpart was a custom pin of 100Cr6 steel. Two different configurations were studied: “single drop” lubrication and “dry contact”. Long sliding condition tests and Stribeck curves were obtained. Finally, Secondary Electron Microscopy imaging and Energy Dispersive Spectroscopy investigations were carried out to study morphology and chemistry at surfaces, before and after tribological tests. Maximum effects of Laser Surface Texturing on friction coefficient and wear have been observed in “single drop” configuration at high normal applied load and for long sliding time.

Role of secondary electrons and metastable atoms in the electron-beam activation of argon-silane mixtures

The energy and spatial degradation of the primary beam electrons and the production of high- energy secondary electrons in ionizing collisions are analyzed by solving the Boltzmann integral equation for the electron distribution function. The effect of the primary and secondary electrons on the direct ionization of an Ar- SiH 4 mixture, the production of metastable argon atoms, and the dissociation of monosilane molecules is investigated over a wide range of the beam electron energies, argon pressures, and monosilane concentra- tions. The influence of metastable Ar* atoms on the dissociation of SiH 4 is studied by using the balance equa- tion for metastable argon atoms and the equation for the ambipolar diffusion of ions and low-energy secondary (plasma) electrons in the beam plasma. It is shown that the main contribution to the activation of an Ar-SiH 4 mixture in an electron-beam plasma is provided by secondary electrons with energies higher than the excitation threshold for argon and the dissociation threshold for monosilane, whereas the contribution from metastable argon atoms, though potentially being comparable with that from secondary electrons, is less than in gas-dis- charge plasmas.

Hydrogen concentration and bonding in nano‐diamond films of varying grain sizes grown by different chemical vapor deposition methods

AbstractHydrogen in a variety of forms (molecular hydrogen, atomic hydrogen, hydrocarbon radicals) is involved in diamond formation. The present work studies the incorporation of hydrogen and its bonding configuration in diamond films composed of diamond grains of varying size which were deposited by three different methods: hot filament (HF), micro wave (MW) and direct current glow discharge (dc GD) chemical vapor deposition (CVD). The size of diamond grains which constitute the films varies in the following way: hundreds of nm in the case of HF CVD (∼300 nm), tens nm in the case of MW CVD (3–30 nm) and a few nm in the case of dc GD CVD (∼5 nm). Raman spectroscopy, secondary ion mass spectroscopy (SIMS) and high resolution electron energy loss spectroscopy (HR‐EELS) were applied to investigate the hydrogen trapping in the films. The retention of hydrogen in the films increases with decreasing grain size, indicating that most likely hydrogen is bonded and trapped in grain boundaries as well as on the films surfaces. Raman and HR‐EELS analysis show that hydrogen is bonded to sp2‐ and sp3‐hybridized carbon, giving rise to typical C–H vibration modes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Secondary Electron Yield for High-Energy Proton Bombardment on Aluminum Target

The phenomenon of secondary electron emission was discovered in the beginning of the 20th century. 1 It is well known that there are two mechanisms contributing to the secondary electron emission, 1 potential emission and kinetic emission. The former may proceed in front of the surface either by Auger neutralization or by resonance neutralization following Auger deexcitation when the potential energy of the projectile is two times larger than that of the work function. The latter is more important than the former in the MeV range of projectile energies. It is dominant in many ion-beam analyses since they are usually performed with MeV ion beams. Secondary electron emission is the dominant surface yield when MeV-energy protons enter metallic targets, with the yields of sputtered ions or reflected protons being insignificant. 2 The source of these electrons is the Coulomb interaction of the fast protons with electrons within about 100 A of the surface of the target. In the experiment described here, the secondary-electron emission coefficient is measured for 33, 39 and 42 MeV protons from the cyclotron accelerator passing through aluminum surfaces. When we measure the beam current, it is hard to get exact incident beam current because of excessive evaluation by reason of secondary electron. We try to get down it using negative electron voltage, it is very hard to get down because secondary electron energy is very high in case of high energy incident beam. So we need exact data for these but that is poor, and we can't correct its present situation. In this study, we try to measure this secondary electron using cyclotron accelerator it is many used for isotope production. 3,4

Use of titanium and zirconium in centrifugally cast heat resistant steel

Low carbon 25Cr–35Ni steel (HP type steel) modified with titanium and zirconium has been produced by centrifugal casting. The different phases present in the as cast and aged conditions were described by light optical and scanning electron microscopy with secondary electron imaging and energy dispersive spectroscopy. Results suggest that the use of titanium as a microalloying element reduces secondary precipitation during aging. Moreover, secondary precipitates in the microalloyed steel are much finer and more evenly distributed. On the other hand, zirconium oxides was found to be potential nucleation sites for primary titanium rich carbides contributing to an optimum distribution of these carbides in the tubes. These differences together with the higher stability of the titanium containing primary carbides are responsible for the improvement on ductility and creep resistance found in the present work.