Wide Range Of Electron Energies Research Articles

Electron impact excitation of the M-shell electrons from Zn-like through Co-like tungsten ions

A detailed study using fully relativistic distorted wave theory for electron impact excitation of Zn-like through Co-like tungsten ions is presented in a wide range of incident electron energies up to 50 keV. We have considered electron impact excitations, namely 3d–4p and 3d–nf (n = 4–8) transitions from the M-shell of the ground state in the Zn-like W44+, Cu-like W45+, Ni-like W46+ and Co-like W47+ ions. We report excitation cross sections for these dipole allowed transitions considered in all the four ions along with the linear polarization of the photon emissions due to the decay of the excited anisotropic states to the ground state. We also provide the analytic fits to our calculated cross sections for plasma application purposes.

Large-scale pseudostate calculations for electron scattering from neon atoms

We report large-scale $R$-matrix (close-coupling) with pseudostates calculations for electron scattering from Ne atoms. The present calculations were performed in the nonrelativistic $LS$-coupling approximation with a recently developed parallel version of our suite of $B$-spline $R$-matrix codes. The principal goal was to generate converged (with the number of states in the close-coupling expansion) results for angle-integrated elastic, ionization, and total cross sections. The cross sections for excitation, which are also required for the latter, are generated in this nonrelativistic model as the sum for all terms. The close-coupling expansion used in this work includes 679 target states, with the lowest-lying 55 states representing the Ne bound spectrum and the remaining 624 states representing the ionization continuum. Our results are in close agreement with available experimental data for the elastic and total cross sections over the wide range of electron energies between 0.1 and 200 eV. With the pseudostate approach, we also obtain accurate cross sections for ionization from both the ground and the metastable states of neon. Our results confirm the very strong influence of coupling to the target continuum on theoretical predictions for excitation cross sections in Ne at intermediate energies, an effect that was previously reported by Ballance and Griffin [J. Phys. B 37, 2943 (2004)].

Electron pair emission from surfaces: Diffraction effects

We have systematically studied the electron pair emission from a Cu(100) surface over a wide range of primary electron energies. We are able to identify contributions from valence states of different orbital characters. The relative intensity from states near the Fermi level (${E}_{F}$) and from the $3d$ levels is strongly affected by the primary energy. The primary energy dependence of the pair intensity resembles the behavior of the specularly reflected primary electron beam. We discuss these findings within a kinematical diffraction model. The azimuthal orientation of the sample has a strong effect on the coincidence energy spectra. We performed measurements along two high-symmetry directions and the difference of the energy spectra reveals intensity variations up to 50$%$. This is an immediate consequence of the symmetry of the surface. This, in turn, proves that the depletion zone in the pair emission is nonspherical.

Kilovoltage energy imaging with a radiotherapy linac with a continuously variable energy range

In this paper, the effect on image quality of significantly reducing the primary electron energy of a radiotherapy accelerator is investigated using a novel waveguide test piece. The waveguide contains a novel variable coupling device (rotovane), allowing for a wide continuously variable energy range of between 1.4 and 9 MeV suitable for both imaging and therapy. Imaging at linac accelerating potentials close to 1 MV was investigated experimentally and via Monte Carlo simulations. An imaging beam line was designed, and planar and cone beam computed tomography images were obtained to enable qualitative and quantitative comparisons with kilovoltage and megavoltage imaging systems. The imaging beam had an electron energy of 1.4 MeV, which was incident on a water cooled electron window consisting of stainless steel, a 5 mm carbon electron absorber and 2.5 mm aluminium filtration. Images were acquired with an amorphous silicon detector sensitive to diagnostic x-ray energies. The x-ray beam had an average energy of 220 keV and half value layer of 5.9 mm of copper. Cone beam CT images with the same contrast to noise ratio as a gantry mounted kilovoltage imaging system were obtained with doses as low as 2 cGy. This dose is equivalent to a single 6 MV portal image. While 12 times higher than a 100 kVp CBCT system (Elekta XVI), this dose is 140 times lower than a 6 MV cone beam imaging system and 6 times lower than previously published LowZ imaging beams operating at higher (4-5 MeV) energies. The novel coupling device provides for a wide range of electron energies that are suitable for kilovoltage quality imaging and therapy. The imaging system provides high contrast images from the therapy portal at low dose, approaching that of gantry mounted kilovoltage x-ray systems. Additionally, the system provides low dose imaging directly from the therapy portal, potentially allowing for target tracking during radiotherapy treatment. There is the scope with such a tuneable system for further energy reduction and subsequent improvement in image quality.

Resonance–parton duality in [formula omitted] off nucleons

Regge-pole-based descriptions of pion-electroproduction on nucleons have given a very good description of the longitudinal components of the cross sections. However, these very same models grossly underestimate the transverse components. A related problem appears in QCD-based scaling arguments that predict the predominance of longitudinal over transverse electroproduction of pions by terms ∝Q2. However, data from JLAB, Cornell, and DESY, covering a wide kinematical range 1<Q2<11GeV2 and 2GeV<W<4GeV, do not show this expected behavior. We address here this issue of the transverse response in pion-electroproduction by considering the contributions of high-lying (W>2GeV) nucleon resonances to pion production. The coupling strengths and form factors are obtained through resonance–parton duality. We show that in a wide range of electron energies and four-momentum transfers such a model describes all the available data very well.

Experimental ionization of atomic hydrogen with few-cycle pulses

We present experimental data on strong-field ionization of atomic hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10% level between the data and an ab initio simulation over a wide range of laser intensities and electron energies.

Electronic detectors for electron microscopy

Electron microscopy (EM) is an important tool for high-resolution structure determination in applications ranging from condensed matter to biology. Electronic detectors are now used in most applications in EM as they offer convenience and immediate feedback that is not possible with film or image plates. The earliest forms of electronic detector used routinely in transmission electron microscopy (TEM) were charge coupled devices (CCDs) and for many applications these remain perfectly adequate. There are however applications, such as the study of radiation-sensitive biological samples, where film is still used and improved detectors would be of great value. The emphasis in this review is therefore on detectors for use in such applications. Two of the most promising candidates for improved detection are: monolithic active pixel sensors (MAPS) and hybrid pixel detectors (of which Medipix2 was chosen for this study). From the studies described in this review, a back-thinned MAPS detector appears well suited to replace film in for the study of radiation-sensitive samples at 300 keV, while Medipix2 is suited to use at lower energies and especially in situations with very low count rates. The performance of a detector depends on the energy of electrons to be recorded, which in turn is dependent on the application it is being used for; results are described for a wide range of electron energies ranging from 40 to 300 keV. The basic properties of detectors are discussed in terms of their modulation transfer function (MTF) and detective quantum efficiency (DQE) as a function of spatial frequency.

Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron, and proton beam energies

Since the Gafchromic film EBT has been recently replaced by the newer model EBT2, its characterization, especially energy dependence, has become critically important. The energy dependence of the dose response of Gafchromic EBT2 film is evaluated for a broad range of energies from different radiation sources used in radiation therapy. The beams used for this study comprised of kilovoltage x rays (75, 125, and 250 kVp), 137Cs gamma (662 KeV), 60Co gamma (1.17-1.33 MeV), megavoltage x rays (6 and 18 MV), electron beams (6 and 20 MeV), and proton beams (100 and 250 MeV). The film's response to each of the above energies was measured over the dose range of 0.4-10 Gy, which corresponds to optical densities ranging from 0.05 to 0.74 for the film reader used. The energy dependence of EBT2 was found to be relatively small within measurement uncertainties (1 sigma = +/- 4.5%) for all energies and modalities. For relative and absolute dosimetry of radiation therapy beams, the weak energy dependence of the EBT2 makes it most suitable for clinical use compared to other films.

Elastic scattering of electrons from Rb, Cs and Fr atoms

Differential, integrated elastic, momentum-transfer and total cross sections as well as differential S, T and U spin parameters for scattering of electrons from rubidium, caesium and francium atoms in the incident energy range up to 300 eV are calculated using a relativistic Dirac equation. The projectile electron–target atom interaction is represented by both real and complex parameter-free optical potentials for obtaining the solution of a Dirac equation for scattered electrons. The Dirac–Fock wavefunctions have been used to represent the Rb, Cs and Fr target atoms. The results of differential cross sections and spin asymmetry parameter S for e-Rb and e-Cs have been compared with the available experimental and theoretical results. Detailed results are reported for the elastic scattering of electrons from the ground states of a francium atom for the first time in the wide range of incident electron energies. The results of electron-Fr elastic scattering show the similar features to those obtained in the case of e-Rb and e-Cs elastic scattering.

Electron-heliumS-wave model benchmark calculations. I. Single ionization and single excitation

A full four-body implementation of the propagating exterior complex scaling (PECS) method [J. Phys. B 37, L69 (2004)] is developed and applied to the electron-impact of helium in an $S$-wave model. Time-independent solutions to the Schr\"odinger equation are found numerically in coordinate space over a wide range of energies and used to evaluate total and differential cross sections for a complete set of three- and four-body processes with benchmark precision. With this model we demonstrate the suitability of the PECS method for the complete solution of the full electron-helium system. Here we detail the theoretical and computational development of the four-body PECS method and present results for three-body channels: single excitation and single ionization. Four-body cross sections are presented in the sequel to this article [Phys. Rev. A 81, 022716 (2010)]. The calculations reveal structure in the total and energy-differential single-ionization cross sections for excited-state targets that is due to interference from autoionization channels and is evident over a wide range of incident electron energies.

Analysis of mixed e,X radiation along the extraction facilities of electron accelerators

When an electron accelerator operates in the bremsstrahlung generation regime, the initial beam is transformed in the extraction facilities of the accelerator into a flux of secondary particles - electrons and bremsstrahlung photons (e,X radiation). The ratio of the radiation components in a given plane depends on the initial energy of the electrons and the thickness and materials used for the extraction components. This article describes a simplified method for evaluating the main characteristics of e,X radiation in the radiation formation channel. The method is based on measuring the thickness of the components of the channel in units of electron path lengths in the continual stopping approximation. This makes it possible to express the radiation characteristics of materials with a wide range of atomic numbers (7-73) and electron energies (5-100 MeV) in a unified form. The possibilities of the method are demonstrated by the results of a simulation using the PENELOPE/2006 system of computer programs.

Determination of electron inelastic mean free paths for poly[methyl(phenyl)silylene] films

The inelastic mean free paths (IMFPs) of electrons at a poly[methyl(phenyl)silylene] thin film surface were determined using elastic peak electron spectroscopy (EPES) and Monte Carlo calculations for a wide electron energy range, 200–1600 eV. We considered the surface composition determined from X-ray induced photoelectron spectra (XPS), the hydrogen concentration evaluated by EPES, and a correction for surface excitations. The results compare well to those calculated from the predictive TPP-2M and G1, formulae. Calculations carried out with the quantitative structure–property relationship of Cumpson and the formula of Ashley and Williams provide larger IMFP values, and can be useful only for a rough estimation.

Conditions for electron runaway under leader breakdown of long gaps

An original hydrodynamic model in which inelastic collisions in the equations of motion and energy balance play a decisive role is developed and applied to simulate electron avalanches in strong electric fields. The mean energy and drift velocity of electrons, as well as the ionization coefficient and electric field in a wide range of mean electron energies, are determined for helium and xenon. A criterion is derived for the runaway of the average electron in discharges with ionization multiplication. It is shown that runaway can take place at any value of E/p, provided that the momentum mean free path exceeds the gap length. The voltage corresponding to electron runaway is found for helium, xenon, and air as a function of the electric field, the electron mean energy, and the parameter pd. Conditions for the formation of a precursor in electronegative gases are analyzed. It is shown that the presence of a precursor with a high electric conductance is necessary for the formation of a new leader step. The voltage and time ranges corresponding to efficient electron runaway and X-ray generation during leader breakdown in air are determined.

KLn Dielectronic Recombination Process of B- Through He-like Cu Ions

The KLn dielectronic recombination processes of trapped highly charged B-like through He-like Cu ions are studied theoretically, and the theoretical results are used to analyse our previous experimental data at Heidelberg electron beam ion trap (EBIT). The theoretical resonant positions agree with the experimental resonant positions to a precision of 0.4%, in comparison with the resonant positions of those highest peaks between theory and experiment. The experimental spectra are then fitted using a formula with the theoretical resonant energies and strengths, the result shows good overall agreement between theory and experiment over a wide electron energy range. The distribution of highly charged states is obtained from the fitting parameters.

Monte Carlo calculations of beam quality correction factors kQ for electron dosimetry with a parallel-plate Roos chamber

Current dosimetry protocols (AAPM, IAEA, DIN) recommend the use of parallel-plate ionization chambers for the measurement of absorbed dose-to-water in clinical electron beams. For well-guarded plane-parallel chambers, it is assumed that the perturbation correction pQ is unity for all electron energies. In this study, we present detailed Monte Carlo simulations with the EGSnrc code for the widely used Roos parallel-plate chamber which is, besides other plane-parallel chamber types, recommended in all protocols. We have calculated the perturbation corrections pcav and pwall for a wide range of electron energies and for 60Co. While our results confirm the recommended value of unity for the cavity perturbation pcav, the wall-correction factor pwall depends on electron energy and decreases with increasing electron energy. For the lowest electron energies in this study (R50 ≈ 2 cm), pwall deviates from unity by up to 1.5%. Using the perturbation factors for the different electron energies and those for the reference beam quality, 60Co, we have calculated the beam quality correction factor kQ. For electron energies E0 > 9 MeV (R50 > 4 cm), the calculated values are in good agreement with the data published in the IAEA protocol. Deviations in the range of 0.5–0.8% are found for R50 < 3 cm.

Electron interaction with deoxyribose analogue molecules in gaseous phase

We present recent results on elastic and inelastic low/medium energy electron interaction with tetrahydrofuran, C4H8O, tetrahydrofurfuryl alcohol, C5H10O2 and 3-hydroxytetrahydrofuran, C4H8O2 molecules in gaseous phase. A comparative study of both absolute differential cross sections for elastic electron scattering and electron energy loss spectra, in a wide incident electron energy range from 50 to 300 eV, has been given. The measurements of the cross sections were performed both as a function of scattering angle and incident electron energy and normalized to the absolute scale according to relative flow measurements and reliable calculations. The experimental elastic DCSs are compared with the most recent theoretical results. Both for elastic and inelastic scattering a special attention has been paid to investigate potential differences in electron scattering processes upon substitution of one of the H-atom in tetrahydrofuran by the OH group (in 3-hydroxytetrahydrofuran) or CH2OH group (in tetrahydrofurfuryl alcohol).

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.

New aspects of dynamic electron diffraction in a crystal

An analytical theory of three-wave dynamic interaction of electrons with a crystal, which is valid in a wide range of electron energies (from several hundreds of eV to several MeV), and a quantum-dynamic concept of propagation of electron waves in a nonabsorbing crystal, which takes into account their dispersion and decay of the interference effects, are presented.

Water equivalence of polymer gel dosimeters

To evaluate the water equivalence and radiation transport properties of polymer gel dosimeters over the wide range of photon and electron energies 14 different types of polymer gels were considered. Their water equivalence was evaluated in terms of effective atomic number ( Z eff ), electron density ( ρ e ), photon mass attenuation coefficient ( μ / ρ ), photon mass energy absorption coefficient ( μ en / ρ ) and total stopping power ( S / ρ ) tot of electrons using the XCOM and the ESTAR database. The study showed that the effective atomic number of polymer gels were very close ( < 1 % ) to that of water except PAGAT, MAGAT and NIPAM which had the variation of 3%, 2% and 3%, respectively. The value of μ / ρ and μ en / ρ for all polymer gels were in close agreement ( < 1 % ) with that of water beyond 80 keV. The value of ( S / ρ ) tot of electrons in polymer gel dosimeters were within 1% agreement with that of water. From the study we conclude that at lower energy ( < 80 keV ) the polymer gel dosimeters cannot be considered water equivalent and study has to be carried out before using the polymer gel for clinical application.

Comparison between an event-by-event Monte Carlo code, NOREC, and ETRAN for electron scaled point kernels between 20 keV and 1 MeV

An event-by-event Monte Carlo code called NOREC, a substantially improved version of the Oak Ridge electron transport code (OREC), was released in 2003, after a number of modifications to OREC. In spite of some earlier work, the characteristics of the code have not been clearly shown so far, especially for a wide range of electron energies. Therefore, NOREC was used in this study to generate one of the popular dosimetric quantities, the scaled point kernel, for a number of electron energies between 0.02 and 1.0 MeV. Calculated kernels were compared with the most well-known published kernels based on a condensed history Monte Carlo code, ETRAN, to show not only general agreement between the codes for the electron energy range considered but also possible differences between an event-by-event code and a condensed history code. There was general agreement between the kernels within about 5% up to 0.7 r/r (0) for 100 keV and 1 MeV electrons. Note that r/r (0) denotes the scaled distance, where r is the radial distance from the source to the dose point and r (0) is the continuous slowing down approximation (CSDA) range of a mono-energetic electron. For the same range of scaled distances, the discrepancies for 20 and 500 keV electrons were up to 6 and 12%, respectively. Especially, there was more pronounced disagreement for 500 keV electrons than for 20 keV electrons. The degree of disagreement for 500 keV electrons decreased when NOREC results were compared with published EGS4/PRESTA results, producing similar agreement to other electron energies.