Catastrophic processes in dielectrics in irradiation by high-current electron beams

The results of the research in explosive decomposition of heavy metal azides initiated by electric (“streamer”) charges induced by high-current electron beam have been considered. A physical model for initiation of heavy metal azides explosive decomposition by electron beam has been suggested. The model suggests formation of strong electric field in the sample and its neutralization by ultrasound anode charges. The streamer front generates “hot spots” which start the formation of explosive decomposition sites in a condensed reactive material.     

Optical excitations in electron-induced desorption of sodium from NaCl surface

Optical excitation processes stimulated by electron bombardment of alkali halides have been investigated under various experimental conditions. As an example a 500 eV e⁻ beam was used to bombard a (100) NaCl crystal. The work included measurements of both excited and ground state sodium atom yields as a function of the target temperature and the beam intensity, a cross-beam experiment with two electron beams: parallel and perpendicular to the sample surface, and a modulated beam measurement in search for time correlations between primary electrons and excited state production.The results were compared with the H-centre migration model and the results of recent publications. In particular it was found that a direct excitation of sodium atoms took place above the surface in collisions with the flux of primary and secondary electrons.     

On the understanding of positive and negative ionization processes during ToF-SIMS depth profiling by co-sputtering with cesium and xenon

In this paper, ionization processes of secondary ions during ToF-SIMS dual beam depth profiling were studied by co-sputtering with 500 eV cesium and xenon ions and analyzing with 25 keV Ga⁺ ions. The Cs/Xe technique consists in diluting the cesium sputtering/etching beam with xenon ions to control the cesium surface concentration during ToF-SIMS dual beam depth profiling. Several depth profiles of a H-terminated silicon wafer were performed with varying Cs beam concentration and the steady state Si, Xe and Cs surface concentrations were measured in situ by Auger electron spectroscopy. It was found that the implanted Cs surface concentration increases with the Cs fraction in the beam from 0% for the pure Xe beam to a maximum Cs surface concentration for the pure Cs beam. Secondly, the variation of the silicon work function, due to the Cs implantation, was measured in situ and during depth profiling as the shift of the secondary ion kinetic energy distributions. Finally, the positive and negative elemental ion yields generated by the Ga analysis beam were recorded and modeled with respect to varying Cs/Xe mixture. We found that the Si⁻ and the Cs⁻ yields increase exponentially with the decrease of the silicon’s work function while that of Cs⁺ and Si⁺ decrease exponentially, as expected by the electron tunneling model.     

On the solution and migration of single Xe atoms in uranium dioxide - An interatomic potentials study

Atomic scale simulation techniques based on empirical potentials have been considered in the present work to get insight on the behaviour of single Xe atoms in the uranium dioxide matrix. In view of the high activation energies commonly observed for Xe migration, this work has focused on the so-called “static calculations” (i.e. energy minimization based calculation) of incorporation and migration energies of Xe in UO₂, using empirical interatomic potentials to describe atom interactions. A detailed study of these results enables to determine the solution and the migration properties of Xe in the different stoichiometry regimes, and can be applied as well for the in-pile behaviour of xenon.     

Monte Carlo study of molecular weight distribution changes induced by degradation of ion beam irradiated polymers

In this work we study a polymeric material that degrades upon irradiation due to the energy inhomogeneously deposited by heavy ion beams. Ion beam irradiation of polymers generates rather different effects than those induced by “classical” low ionizing particles such as electrons or gamma rays. This is due to the high electronic stopping power and the inhomogeneous distribution of deposited energy. This energy is transferred to the material within a small volume along the ion path forming the so called “nuclear track” or “latent track”. The track size primarily depends on the ion velocity, and it is determined by the secondary electrons (delta rays) generated along the ion trajectory. By means of Monte Carlo simulations we first obtained equilibrated polymer configurations using a coarse-grained model, and then investigated the spatially inhomogeneous chain scission process due to the passage of the ions. The number average molecular weight, weight average molecular weight and the polydispersity were calculated as a function of track radius, scission probability within the ion track and irradiation fluence. Finally we compared our results with a numerical implementation of a model for random homogeneous degradation.     

The role of radiolysis products in in situ luminescence of Li2O

A new phenomenon of an “excess luminescence” (EL) in Li₂O observed at 4.5–2.5 eV under light ion (H⁺, He⁺) irradiation during the rise of temperature (>573 K) was studied. The essence of the EL is in the rapid pulse increase of the luminescence intensity. It is proposed that this phenomenon is based on the thermo-dissociation of colloidal Li into Li lattice ions, F⁺ and F⁰ centers, and oxygen vacancies. Formed oxygen vacancies capture electrons during the irradiation and form excited F-centers, whose relaxation gives the EL. This phenomenon was reproduced using X-ray irradiation and a sample containing colloidal Li introduced by irradiation with electron accelerator to an absorbed dose of 105 MGy.     

TEM observations and finite element modelling of channel deformation in pre-irradiated austenitic stainless steels - Interactions with free surfaces and grain boundaries

Transmission electron microscopy (TEM) observations show that dislocation channel deformation occurs in pre-irradiated austenitic stainless steels, even at low stress levels (∼175 MPa, 290 °C) in low neutron dose (∼0.16 dpa, 185 °C) material. The TEM observations are utilized to design finite element (FE) meshes that include one or two “soft” channels (i.e. low critical resolved shear stress (CRSS)) of particular aspect ratio (length divided by thickness) embedded at the free surface of a “hard” matrix (i.e. high CRSS). The CRSS are adjusted using experimental data and physically based models from the literature. For doses leading to hardening saturation, the computed surface slips are as high as 100% for an applied stress close to the yield stress, when the observed channel aspect ratio is used. Surface slips are much higher than the grain boundary slips because of matrix constraint effect. The matrix CRSS and the channel aspect ratio are the most influential model parameters. Predictions based on an analytical formula are compared with surface slips computed by the FE method. Predicted slips, either in surface or bulk channels, agree reasonably well with either atomic force microscopy measures reported in the literature or measures based on our TEM observations. Finally, it is shown that the induced surface slip and grain boundary stress concentrations strongly enhance the kinetics of the damage mechanisms possibly involved in IASCC.     

Low-temperature irradiation effects in lithium orthosilicates

The emission spectra of lithium orthosilicates (Li₄SiO₄₎ ceramics have been measured in the range of 1.8–5.8 eV under irradiation by 6–30 eV photons or 1–30 keV electrons at 6–300 K. The tunnel recombination phosphorescence, as well as luminescence, stimulated by 1.5–2.5 eV photons has been detected in the sample preliminarily irradiated at 6 or 80 K. The main peaks of thermally stimulated luminescence (TSL) in the irradiated ceramics have been observed at 72, 118 and 265 K. The creation spectra of the 118 K TSL peak, as well as the excitation spectrum of photostimulated luminescence (PSL) span the region of the intrinsic absorption of a lithium orthosilicate (9–30 eV). The intensity of PSL and the TSL peaks in Li₄SiO₄ ceramics prepared in hydrogen/argon atmosphere is several times lower than that in the mainly investigated Li₄SiO₄ ceramics prepared in the atmosphere of dry argon. The optical characteristics of Li₄SiO₄ are compared with the ones known for Li₂O and SiO₂. Low-temperature luminescent methods are promising for the investigation of electron–hole processes and radiation defects serving as the traps for tritium released in D–T fusion reactor blanket systems.     

Development of function-graded proton exchange membrane for PEFC using heavy ion beam irradiation

The graded energy deposition of heavy ion beam irradiation to polymeric materials was utilized to synthesize a novel proton exchange membrane (PEM) with the graded density of sulfonic acid groups toward the thickness direction. Stacked Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) films were irradiated by Xe⁵⁴⁺ ion beam with the energy of 6 MeV/u under a vacuum condition. The induced trapped radicals by the irradiation were measured by electron spin resonance (ESR) spectroscopy. Irradiated films were grafted with styrene monomer and then sulfonated. X-ray photo-electron spectroscopy (XPS) spectra showed that the densities of sulfonic acid groups were controlled for injection “Surface” and transmit “Back” sides of the fabricated PEM. The membrane electrode assembly (MEA) fabricated by the function-graded PEM showed improved fuel cell performance in terms of voltage stability. It was expected that the function-graded PEM could control the graded concentration of sulfonic acid groups in PEM.     

Improved wear resistance of Al-15Si alloy with a high current pulsed electron beam treatment

A hypereutectic Al-15Si alloy (Si 15 wt.%, Al balance) was irradiated by high current pulsed electron beam (HCPEB). The HCPEB treatment causes ultra-rapid heating, melting and cooling at the top surface layer. As a result, the special “halo” microstructure centering on the primary Si phase is formed on the surface due to interdiffusion of Al and Si elements. The composition of the “halo” microstructure is distributed continuously from the center to the edge of the “halo”. Compared to an untreated matrix, the remelted layer underneath the surface presents single contrast because of the compositional homogeneity after HCPEB treatment. The thickness of the remelted layer increases slightly from 4.4 μm (5 pulses) to 5.6 μm (25 pulses). HCPEB treatment broadens and shifts the diffraction peaks of Al and Si. The lattice parameters of Al decreases due to the formation of a supersaturated solid solution of Al in the melted layer. Through analysis of Raman spectra and transmission electron microscopy (TEM), the amorphous Si (a-Si) and nanocrystalline Si are formed in the near-surface region under multiple bombardments of HCPEB. The relative wear resistance of a 15-pulse sample is effectively improved by a factor of 9, which can be attributed to the formation of metastable structures.     

Cross-section transmission electron microscopy of the ion implantation damage in annealed diamond

It has formerly been shown that low-damage levels, produced during the implantation doping of diamond as a semiconductor, anneal easily while high levels “graphitize” (above about 5.2 × 10¹⁵ ions/cm²). The difference in the defect types and their profiles, in the two cases, has never been directly observed. We have succeeded in using cross-section transmission electron microscopy to do so. The experiments were difficult because the specimens must be polished to ∼40 μm thickness, then implanted on edge and annealed, before final ion beam thinning to electron transparency. The low-damage micrographs reveal some deeply penetrating dislocations, whose existence had been predicted in earlier work.     

Transmission of low energy (< 10 eV) oxygen ions through ultrathin xenon films

In studies of desorption induced by electronic transitions (DIET) such as electron or photon stimulated desorption, it is important to know whether the desorbing species originate solely from the outermost surface layer, or also from layers beneath the surface. In order to gain better understanding of the charge transfer, elastic scattering, and other inelastic processes involved in this issue, we are currently performing a series of experimental studies of the transmission of low energy ions ( 7 eV) through ultrathin films (submonolayer to multilayer) of condensed gases. Here we report on the first quantitative measurements of the yield, angle, and energy of oxygen ions after transmission through ultrathin films of xenon. In our novel approach, a focused 300 eV electron beam bombards a target at 25 K consisting of an oxidized tungsten (100) crystal with adsorbed overlayers of xenon. In the absence of the xenon, O⁺ ions desorb in a sharp beam normal to the surface, as measured in a velocity and angle resolving ESDIAD apparatus (electron stimulated desorption ion angular distribution). When Xe layers are present, some oxygen ions penetrate several monolayers of xenon without significant change in energy and angle while others seem to be scattered by large-angle elastic scattering or to be attenuated from the O⁺ beam. The work presented is the first experimental study of the depth of origin of desorbing ions in DIET processes.     

Stopping power and ranges of electrons, protons and alpha particles in liquid water using the Geant4-DNA package

This paper presents stopping power and ranges of electrons, protons, and alpha particles in liquid water, calculated using the latest Geant4-DNA processes implemented in the Geant4 Monte Carlo simulation toolkit. Inelastic cross sections are obtained using the first Born approximation and semi-empirical formulas like Rudd’s model for ionisation and the Miller and Green formula for excitation. Elastic collisions and vibrational excitations are considered for tracking electrons until complete thermalisation (0.025 eV). A speed scaling procedure with an effective charge screening term was used to compute alpha particle and heavy ion cross sections. Geant4-DNA simulations were carried out using thin liquid water volumes to determine the linear energy loss (dE/dX), while larger volumes were used to obtain the particle range. While results converge for highly energetic particles, differences are observed for low energies when the applied theoretical models begin to diverge from each other. Results show a good agreement between the analytical calculations obtained from the models, the Geant4-DNA Monte Carlo simulation predictions and the data published in the ICRU reports. Geant4-DNA processes apply to the following energy ranges: 0.025 eV-1 MeV for electrons, 100 eV-100 MeV for protons and 1 keV-400 MeV for alpha particles in liquid water, however since experimental data for very low energies is scarce and very difficult to obtain these processes could not be thoroughly validated so they are recommended for energies above 1 eV for electrons, 1 keV for protons and 10 keV for alpha particles. Relativistic, highly charged ions were implemented in our own “house” version of the code and will be available in future releases of Geant4.     

Interferences in coherent electron emission from diatomic molecules

Recent studies of electron emission from molecular hydrogen by the impact of fast ions have shown the existence of interference effects. The interferences are manifested as oscillations in the velocity (or energy) distributions of the ejected electrons, and are analogous to the interference of light in Young’s two-slit experiment. The frequencies of the oscillatory structures depend strongly on the electron observation angle and to a lesser extent on the collision velocity. Additionally, secondary oscillations with ∼2-3 times higher frequencies attributed to scattering of the primary electron “wave” at the other atomic center are found to be superimposed on the primary oscillations. More recently, electron interference studies have focused on diatomic molecules more complex than H₂, including N₂ and O₂, for which only structures due to secondary interferences are apparently observed. Here, these various results are reviewed, outstanding questions identified, and future directions indicated.     

Nano-scale quasi-melting of alkali-borosilicate glasses under electron irradiation

Quasi-melting of micro- and nano-samples during transmission electron microscope irradiation of glassy materials is analysed. Overheating and true melting by the electron beam is shown not to be an explanation due to the ultra-sharp boundary between transformed and intact material. We propose that the observed fluidisation (quasi-melting) of glasses can be caused by effective bond breaking processes induced by the energetic electrons in the electron beam. The bond breaking processes modify the effective viscosity of glasses to a low activation energy regime. The higher the electron flux density the lower is the viscosity. Quasi-melting of glasses at high enough electron flux densities can result in shape modification of nano-sized particles including formation of perfect beads due to surface tension. Accompanying effects, such as bubble formation and foil bending are revisited in the light of the new interpretation.     

Total cross sections for ionizing processes induced by proton impact on molecules of biological interest: A classical trajectory Monte Carlo approach

In the current work, we present a study of ionizing interactions between protons and molecular targets of biological interest like water vapour and DNA bases. Total cross sections for single and multiple ionizing processes are calculated in the independent electron model and compared to existing theoretical and experimental results for impact energies ranging from 10 keV/amu to 10 MeV/amu. The theoretical approach combines some characteristics of the classical trajectory Monte Carlo method with the classical over-barrier framework. In this “mixed” approach, all the particles are described in a classical way by assuming that the target electrons are involved in the collision only when their binding energy is greater than the maximum of the potential energy of the system projectile-target. We test our theoretical approach on the water molecule and the obtained results are compared to a large set of data and a reasonable agreement is generally observed specially for impact energies greater than 100 keV, except for the double ionization process for which large discrepancies are reported. Considering the DNA bases, the obtained results are given without any comparison since the literature is till now very poor in terms of cross section measurements.     

Radiation induced deuterium absorption for RB-SiC,HP-SiC,silicon and graphite loaded during electron irradiation

Absorption, diffusion, and desorption of hydrogen isotopes are expected to occur during operation in future fusion reactors and these processes will strongly depend on the irradiation conditions, neutron flux and purely ionizing radiation. The main aim of the work is to address the electron irradiation induced absorption of hydrogen isotopes in RB-SiC. Deuterium loading was carried out with both the sample and the surrounding deuterium gas exposed to 1.8 MeV electron irradiation in order to evaluate the radiation enhanced deuterium absorption. Thermo stimulated desorption (TSD) measurements were carried out for both electron irradiated and unirradiated samples in order to evaluate the possible radiation enhanced retention of the previously loaded deuterium. The materials subjected to the deuterium loading process were also studied by SIMS. Noticeable radiation enhanced deuterium absorption was observed. Most of the deuterium absorbed during irradiation was thermally released at about 600 °C.     

Microstructural degradation mechanisms during creep in strength enhanced high Cr ferritic steels and their evaluation by hardness measurement

There are two creep regions with different creep characteristics: short-term creep region “H”, where precipitates and subgrains are thermally stable, and long-term creep region “L”, where thermal coarsening of precipitates and subgrains appear. In region “H”, the normalized subgrain size (λ-λ₀)/(λ^∗-λ₀) has a linear relation with creep strain and its slope is 10ε⁻¹. But, region L is the time range in which the static recovery and the strain-induced recovery progress simultaneously. In this region, the static recovery accelerates the strain-induced recovery, and subgrain size is larger than that line which neglects the contribution of the static recovery. In region “L”, the Δλ/Δλ^∗-strain present a linear relation with a slope 35ε⁻¹. There is a linear relation between hardness and subgrain size. Hardness drop, H₀ − H, as a function of Larson-Miller parameter can be a good measure method for assessment of hardness drop and consequently degradation of microstructure. Hardness drop shows an identical slope in creep region “H”, whereas hardness drop due to thermal aging and creep in region “L” show together a similar slope. In region “H”, degradation of microstructure is mainly due to recovery of subgrains controlled by creep plastic deformation, and precipitates do not have a major role. However, in creep region “L”, there are three degradation mechanisms that accelerate creep failure; (1) strain-induced recovery of subgrains due to creep plastic deformation, (2) static-recovery of subgrains and precipitates and (3) strain-induced coarsening of precipitates due to the appearance of static-recovery.     

The electroluminescence of Xe-Ne gas mixtures: a Monte Carlosimulation study

We have performed a Monte Carlo simulation of the drift of electrons through a mixture of gaseous xenon with the lighter noble gas neon at a total pressure of 1 atm. The electroluminescence characteristics and other transport parameters are investigated as a function of the reduced electric field and composition of the mixture. For Xe-Ne mixtures with 5, 10, 20, 40, 70, 90, and 100% of Xe, we present results for electroluminescence yield and excitation efficiency, average electron energy, electron drift velocity, reduced mobility, reduced diffusion coefficients, and characteristic energies over a range of reduced electric fields which exclude electron multiplication. For the 5% Xe mixture, we also assess the influence of electron multiplication on the electroluminescence yield. The present study of Xe-Ne mixtures was motivated by an interest in using them as a filling for gas proportional scintillation counters in low-energy X-ray applications. In this energy range, the X rays will penetrate further into the detector due to the presence of Ne, and this will lead to an improvement in the collection of primary electrons originating near the detector window and may represent an advantage over the use of pure Xe     

Tandem mirror reactor with thermal barriers

A new invention — the thermal barrier — promises to improve the tandem mirror fusion reactor. The thermal barrier consists of a region of reduced magnetic field strength, plasma density, and plasma potential between each end plug and the central cell of a tandem mirror. The depressed plasma potential serves to thermally insulate the plug electrons from the central cell electrons. With barriers and auxiliary electron heating in the plugs, the central cell confining potential can be generated with a lower plug plasma density, magnetic field strength, and beam injection energy than for the case without barriers. This paper summarizes the status of the rapidly evolving physics knowledge concerning tandem mirrors with thermal barriers, describes end plug components typical for tandem mirror reactors — yin-yang magnets, neutral beams, and ECRH heating systems, and discusses central cell design.