Low-velocity Ions Research Articles

Comparison of electronic stopping powers of 4H-SiC and 2H-GaN for low-velocity 〈0001〉 channeled ions with atomic numbers of 12 to 15

Measured electronic stopping powers along the 〈0001〉 direction (S e) of 4H-SiC and 2H-GaN for low-velocity 12Mg, 13Al, and 15P ions were reproduced with the modified El-Hoshy−Gibbons model that reduced not only the atomic numbers of projectiles and targets but also the impact parameter for small-angle collisions (based on the Kohn−Sham radii of projectiles) in the Firsov model. Unreported S e of 2H-GaN for low-velocity 14Si ions was then predicted to be between S e of 2H-GaN for 12Mg ions and S e of 4H-SiC for 13Al ions, indicating not only Al and Mg but also Si channeling being usable for fabricating cost-effective superjunctions.

Flux effect on defect evolution in uranium dioxide under electronic and ballistic regimes

The microstructure evolution of ion-irradiated UO2 was evaluated by in situ Raman spectroscopy. Transmission electron microscopy (TEM) was also conducted for selected irradiation conditions. The UO2 samples were irradiated with single- (900 keV I ions) or dual-ion beams (900 keV I and 27 MeV Fe or 36 MeV W ions) with different ratios between the high- and low-velocity ion flux (R). The analysis of the damage build-up shows that the electronic excitations deposited by the high-velocity ions affect the formation and evolution of defects induced by nuclear energy loss. In addition, the kinetics of dislocation evolution are more advanced when the flux ratio promotes electronic excitations. Consequently, a higher R value favors electronic excitations, leading to greater microstructural evolution.

Modified El-Hoshy−Gibbons model for electronic stopping cross sections in Si and SiC for low-velocity ions with atomic numbers between 5 and 15

The El-Hoshy−Gibbons model, which reduces not only the atomic numbers of projectiles (Z 1) and targets but also the impact parameter for small-angle collisions (R 0) in the Firsov model, was modified based on the relation between R 0 and the Kohn−Sham radii of projectiles (r KS); namely, the reduction factor y of R 0 was chosen to be 10 when R 0 was larger r KS and 5 in the case R 0 ≤ r KS. This modification improved the reproducibility of the periodic dependences of the electronic stopping cross sections in Si, as well as those in SiC, for low-velocity ions with 5 ≤ Z 1 ≤ 15.

Local electronic excitations induced by low-velocity light ion stopping in tungsten

Local electronic excitations induced by low-velocity light ion stopping in tungsten

Imaging the Ion-Molecule Reaction Dynamics of O- + CD4.

While neutral reactions involved in methane oxidation have been intensively studied, much less information is known about the reaction dynamics of the oxygen radical anion with methane. Here, we study the scattering dynamics of this anion-molecule reaction using crossed-beam velocity map imaging with deuterated methane. Differential scattering cross sections for the deuterium abstraction channel have been determined at relative collision energies between 0.2 and 1.5 eV and ab initio calculations of the important stationary points along the reaction pathway have been performed. At lower collision energies, direct backscattering and indirect complex-mediated reaction dynamics are observed, whereas at higher energies, sideways deuterium stripping dominates the reaction. Above 0.7 eV collision energy, a suppressed cross section is observed at low product ion velocities, which is likely caused by the endoergic pathway of combined deuteron/deuterium transfer, forming heavy water. The measured product internal energy is attributed mainly to the low-lying deformation and out-of-plane bending vibrations of the methyl radical product. The results are compared with a previous crossed-beam result for the reaction of oxygen anions with nondeuterated ̧methane and with the related neutral-neutral reactions, showing similar dynamics and qualitative agreement.

Application of polyimide films as a nuclear track detector (1): A systematic study of track registration sensitivity

This paper reports the variation of track registration sensitivity as a function of the stopping power of heavy ions in UPILEX-S® films, which is known as the most radiation tolerant polyimide (PI). The detection thresholds in the stopping power for etch pit formation are determined as 4,000, 4,100, 4,800, and 5600 keV/μm for 40Ar, 84Kr, 132Xe and 238U ions, respectively. Furthermore, we investigate the latent track structure in two kinds of PI films (UPILEX-S® and Kapton) by means of FT-IR spectroscopy. At the similar stopping power value, the radiation chemical yields (G value) for heavier ions are lower than those of lighter ions. This is due to the difference of the radial dose distribution for low and high velocity ions.

Цифровая осциллография полимерных мембран с протонной проводимостью

Polyvinyl alcohol-based cast membranes have promising application in creating commonly available fuel cells and fuel synthesizers, as well as inexpensive water sources and microelectromechanical systems. In order to control the properties of such membranes a deep understanding of the charge transfer mechanisms and their relationship with the structure formed at a certain composition and manufacturing technology is required. A method for studying the structure of the proton-conductive polymer membranes using digital oscillography of ion currents excited by the low-frequency rectangular pulses with the amplitudes below the threshold voltage of the ionic conductivity in a dehydrated membrane, and for the analysis of the resulting ion current pulses (spikes) in the frames of the model of a proton pump acting in each layer of the membrane is proposed in this paper. Fast Fourier transform of these oscillograms reveals from 2 to 4 branches or spike sequences corresponding to the phases with different ionic conductivity and makes it possible to determine the thicknesses of both high-conductivity phase layers (7–30 µm) and low-conductivity phase interlayers (1–7 µm) formed in the process of polymerization. The reason of spike merging into bursts is described in terms of successively induced increase in the excited proton density over a threshold value in highly conductive layers. The resonance observed in dry proton membranes at the frequencies of about 2.2 to 3.0 kHz is interpreted as the burst merge point with the further increase in impedance due to proton lagging and respective decrease in the effective thickness of active layers. The effective charge carrier concentrations (as small as 1012 to 1013 cm−3) and the velocity (from 5 to 18 cm/s for the highly conductive phases which turned out to be much higher than those observed in solutions) are estimated. The asymmetry of the cast membranes, which becomes apparent at low frequencies and causes the generation of a direct ion current in response to excitation by a purely alternating current, is studied. It is found that the apparent conductivity determining contribution to the total ohmic resistance is made by a thin interlayer with a very low ion velocity, presumably surface layer, rather than the main layers. The conclusion on the optimization of the production technology and the composition of the proton membranes for various applications is made.

Measurements of ion-electron energy-transfer cross sectionin high-energy-density plasmas.

We report on measurements of the ion-electron energy-transfer cross sectionutilizing low-velocity ion stopping in high-energy-density plasmas at the OMEGA laser facility. These measurements utilize a technique that leverages the close relationship between low-velocity ion stopping and ion-electron equilibration. Shock-driven implosions of capsules filled with D^{3}He gas doped with a trace amount of argon are used to generate densities and temperatures in ranges from 1×10^{23} to 2×10^{24}cm^{-3} and from 1.4 to 2.5 keV, respectively. The energy loss of 1-MeV DD tritons and 3.7-MeV D^{3}He alphas that have velocities lower than the average velocity of the thermal electrons is measured. The energy loss of these ions is used to determine the ion-electron energy-transfer cross section, which is found to be in excellent agreement with quantum-mechanical calculations in the first Born approximation. This result provides an experimental constraint on ion-electron energy transfer in high-energy-density plasmas, which impacts the modeling of alpha heating in inertial confinement fusion implosions, magnetic-field advection in stellar atmospheres, and energy balance in supernova shocks.

Time-synchronized laser-induced fluorescence in the near-field of a 600 Watt Hall thruster

We report on the results of an experimental campaign to measure time-varying velocity distributions in the near-field of a low power Hall thruster. We employ a sample-hold technique, enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The measurements are applied to study flow field dynamics in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44–49 kHz range. A very detailed experimental picture of the near-field emerges from these studies. The results indicate that velocity fluctuations lessen further downstream of the exit plane. Along the thruster axis where there is a general appearance of a central jet, there is evidence of a low velocity ion population in between the periodic bursts of high velocity ions, indicative of local ionization of neutrals outside of the thruster. One possible source of this residual ionization may be background chamber gas, which is not unexpected with the limited pumping capacity of ground test facilities.

Acceleration of heavy ions in inverse free electron laser

In conventional linear accelerators, the beam is accelerated with a synchronous harmonic of the radio frequency field where the electric field component is collinear with the beam direction. This approach requires the design of complex accelerating structures, especially for low-energy heavy ions. If the beam motion were sustainably coupled to transverse electromagnetic fields, this could significantly simplify the accelerating structure design, and even allow acceleration with free-space waves. However, despite the long history of the proposed concept for accelerating low-velocity ion beams, it has not found practical application, partially because of the complexity of the technical design. In this paper, we present a practical design approach for this undulator-based accelerator for low-energy heavy-ions, reminiscent of the inverse free electron laser operating principle, but in a different parameter space.

Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. The energy loss of H$^+$, H$_2 ^+$, He$^+$, B$^+$, N$^+$, Ne$^+$, $^{28/29}$Si$^+$ and Ar$^+$ in self-supporting silicon membranes was analysed along channelled and random trajectories in a transmission approach. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For heavier ions, the energy-loss difference between channelling and random geometry is generally found more pronounced, and, in contrast to protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channelled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit a Z$_1$ oscillation with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory.

Electronic stopping power under channeling conditions for slow ions in Ge using first principles

The electronic stopping power for low-velocity ions (including protons and $\ensuremath{\alpha}$ particles) in the semiconductor Ge is investigated with the aid of time-dependent density functional theory based on Ehrenfest dynamics. The purpose of this study is to further learn about the energy loss mechanisms of the slow ions. When the projectile ions pass through the crystal film along the $\ensuremath{\langle}100\ensuremath{\rangle}, \ensuremath{\langle}110\ensuremath{\rangle}$, and $\ensuremath{\langle}111\ensuremath{\rangle}$ channels, we analyze the channeling effect of the electronic stopping power in detail by investigating the channeling electronic density, the stopping force, and the trapped electrons. Our results are in good agreement with the available experimental data and the other theoretical calculations, which demonstrate that this approach is able to reasonably describe the electronic stopping power. Furthermore, these results can act as the reference data for the ion-beam irradiation of Ge in the low-energy regime.

Sputtering of LiF and other halide crystals in the electronic energy loss regime

Sputtering experiments were performed by irradiating LiF, NaCl, and RbCl crystals with various swift heavy ions like S, Ni, I, Au with energies between 60 and 210 MeV, C60 clusters between 12 and 30 MeV or Pb ions between 730 and 6040 MeV. Sputtered species are collected on arc-shaped catchers and subsequently analyzed by elastic recoil detection analysis or Rutherford backscattering analysis. The study focuses on angular distributions and total yields for LiF and covers a broad range of experimental parameters including cleaved or rough sample surfaces, ion fluence, beam incident angles, and different ion velocities leading to electronic energy loss (Se) values from 5 to 45 keV/nm. In most cases, the angular distribution has two components, a jet-like peak perpendicular to the surface sample superimposed on a broad isotropic cosine distribution whatever is the beam incident angle. The observation of the jet depends mainly on the surface flatness and angle of ion incidence. However, the jet does not appear clearly when irradiated with C60 cluster. The sputtering yield is stoichiometric and characterized by huge total yields of up to a few 105 atoms per incident ion. The yield follows a power law as function of electronic energy loss, Y follows an exponential law with Sen with n ~ 4. While the azimuthal symmetry for sputtering is observed at low ion velocity (~1 MeV/u), it seems to be lost at high velocity (>4 MeV/u). The data provide a comprehensive overview how the angular distribution and the total sputtering yield scale with the energy loss, beam incidence angle and ion velocity. Complementary experiments have been done with NaCl and RbCl targets confirming the observation made for LiF.

Ionization-induced annealing in silicon upon dual-beam irradiation

Silicon single crystals were irradiated at room temperature (RT) with single and dual low-velocity (i.e., 900 keV I) and high-velocity (i.e., 27 MeV Fe or 36 MeV W) ion beams in order to study synergistic effects between nuclear and electronic energy losses in semiconductors. The damage created by irradiation was quantified by using Rutherford backscattering spectrometry in the channeling mode and Raman spectroscopy, and it was visualized using transmission electron microscopy. Whereas single low-velocity ion irradiation leads to amorphization of the surface layer of Si crystals, the use of a dual low- and high-velocity ion beam prevents this phase transformation. However, a remaining disorder exists, the level of which depends on the ratio of the high- to low-velocity ion fluxes. The higher the ratio (1.6 in the present case), the lower the disorder level, with a 30% integrated disorder instead of 100% upon single low-energy irradiation. These results provide evidence that ionization-induced disorder annealing can occur at RT in Si.

TEM analysis of ion tracks and hillocks produced by swift heavy ions of different velocities in Y3Fe5O12

The size of ion tracks and hillocks produced during heavy ion irradiation of Y3Fe5O12 is analyzed by transmission electron microscopy. The cross sections of hillocks and ion tracks produced by ions with electronic stopping power Se in the range of 20–35 keV/nm are found to be comparable. In this range, the characteristic dimensions of the hillocks (both cross section and height) increase as a function of Se. The data show that there is a correlation between hillock height and hillock cross section, which is linked to the relation between lifetime of the melt along the ion trajectory and the maximum molten area. In addition, the results clearly show that the size of the hillocks produced by low velocity ions is larger than those produced by high velocity ions of the same Se, due to the so-called velocity effect.

Energy-transfer in collisions of low velocity ions with CO molecules

SynopsisThe energy transfer in collisions of low velocity ions with CO molecules is theoretically investigated. Energy loss spectrum of ions scattered by CO molecules is obtained with using the classical trajectory (CT) calculation and the sudden-limit models. Both of the calculations well reproduce an experimental spectrum with a considerable accuracy. Rotational excitation is found to be principal energy transfer process from translational to internal motions.

Profile of energy-loss spectrum in collisions of ions with heteronuclear diatomic molecules

SynopsisProfile of energy-loss spectrum in collisions of low velocity ions with heteronuclear diatomic molecules is theoretically investigated. With changing mass asymmetry in a target diatomic molecule, energy-loss spectrum is calculated with the classical trajectory calculation. Rotational excitation plays an essential role for energy-loss. The shape of the spectrum is found to show a drastic variation with mass asymmetry.

Electronic stopping power for slow ions in the low-hardness semimetal HgTe using first-principles calculations

The electronic stopping power for low-velocity ions (including protons, -particles, and ) is investigated in a novel semimetal HgTe system, where the data are obtained with the aid of Ehrenfest dynamics combined with time-dependent density functional theory. For the light projectile ions (protons and -particles), the linear and nonlinear behaviors of electronic stopping power in three different channel directions are analyzed in detail. In the case where the projectile ion is a proton, the linear results for the threshold velocity are correlated with an indirect band gap; the direction of the electronic stopping power depends on the radial drag force, the channeling electronic density and the trapped charge. More notably, we report an interesting channel-geometry fact, i.e. that the electronic stopping power of HgTe is powerfully modulated by the impact parameters. The parallel off-center tracks increase the electronic stopping power, making it more consistent with the SRIM data. In the case of an -particle as the projectile ion, nonlinear behavior that varies with velocity can be ascribed to the charge transfer, which is another mode of energy dissipation. In addition, when the slightly heavier projectile travels through the medium HgTe, the projectile can capture more free charges than the protons and -particles under the same circumstances. Especially, for the projectile in the off-channel, the electronic stopping power is close to the SRIM data with the decrease of the impact parameter. These results extend the study of radiation damage to a new field of materials.

Angular distribution of mass and charge flows in far zone of plasma beams generated by nanosecond vacuum surface flashover at 70 kV

In this work, we measure directional patterns of charge flow of ions and mass flow of neutrals in particle beam generated by vacuum surface flashover in linear configuration at voltage of 70 kV. We used a generator, which provide a discharge current of 2.8 kA. The samples used are polymethylmethacrylate, polytetrafluorethylene and polyethylene. We found that the directional patterns of mass and charge flows in linear configuration have distinct axial asymmetry. The corresponding angular distributions are significantly wider in the plane which is normal to the discharge than in the plane which contains the discharge. Angular distributions of charge flow are wider than distributions of neutral mass flow in the same planes. Besides, the asymmetry in ion current amplitudes at the cathode side and at the anode side is observed. We found that the share of low-velocity ions in ion current drops drastically in the plane which contains the discharge at the angles of 15 degrees and larger for all the tested materials.

Analyses and design of nuclear emulsions for dark matter detection

Studies were made on a nuclear emulsion system with 40-nm AgBrI grains for dark matter detection. This system is characterized by its efficient detection of a nuclear recoil to form a latent image center composed of several Ag atoms on the grain and by reduction of an AgBrI grain with the center to form an Ag grain by development, achieving a degree of magnification of ~106. Dark matter detection with this system has two problems: rapid recombination between electrons and positive holes with high concentrations (≥1000 per grain at its maximum) for short times (10–100 fs) in the first step and re-halogenation of once formed latent image centers by halogen molecules formed from positive holes, taking place for long times (up to several days) in the second step of dark matter events. In the framework of detective quantum efficiency (DQE) as a criterion for system evaluation, theoretical and experimental approaches were used to characterize and design emulsions for dark matter detection, in which low-velocity ion beams were used to simulate the behavior of nuclear recoil induced by dark matter events. Instead of chemical sensitization used in conventional photographic emulsions, the immersion of the emulsion layers in an aqueous solution of sodium sulfite (i.e. HA treatment) was most effective for latent image formation by ion beams. Ion beam experiments and photoelectron spectroscopy revealed that HA treatment could overcome both of the problems. Ideas for the reduction of fog in nuclear emulsions for dark matter detection considering DQE are proposed.