Amorphous Targets Research Articles

A relation between the nuclear scattering probability in a bent crystal and the mean volume reflection angle

For volume reflection of charged particles governed by the continuous potential of atomic planes in a bent crystal, we calculate the probability for the particle to undergo a nuclear interaction. It is found to differ from the corresponding probability in an amorphous target by an amount proportional to the crystal bending radius and the particle mean deflection angle, independently of the shape of the inter-planar continuous potential. That result is also applied to the description of the final beam angular divergence owing to the multiple Coulomb scattering. The theoretical predictions are in agreement with the results of recent CERN experiments.

Electrostatic focusing Spindt-type field emitter array for an image sensor with a high-gain avalanche rushing amorphous photoconductor target

A new electrostatic focusing Spindt-type field emitter array (FEA) was simulated and fabricated to develop a compact FEA image sensor with a high-gain avalanche rushing amorphous photoconductor target. Although the conventional double-gated FEAs can generate focused electron beams by applying a lower voltage to the focusing electrode (second gate) than that of the gate electrode, the emission current drastically decreases. These experimental results show that by increasing the thickness of the gate electrode the emission current can be improved without deterioration of the focusing characteristics.

Topography effects and monatomic ion sputtering of undulating surfaces, particles and large nanoparticles: Sputtering yields, effective sputter rates and topography evolution

AbstractAn analysis is provided of the change in topography and consequent apparent change in sputtering yields of surfaces as a result of ion sputtering using Yamamura et al.'s model for the angular dependence of the sputtering yield for monatomic primary ions. This is extended to provide a general scheme for any monatomic primary ion and any amorphous elemental solid target. Details are provided of the topographic changes for surfaces with an undulating cosine profile sputtered with primary ions normal to, and at 45° to, the average surface and also for spherical particles. Smoothing occurs at normal incidence and both smoothing and roughening can occur at 45°. Predictions show how the sputtering rate of the material varies with the depth sputtered as a result of the topographical changes, the rate either increasing or decreasing. The effect of roughening usually increases the sputtering rate with values increasing up to four times. The effect of sample rotation is a fast smoothing. Conditions are described for the optimal profiling of particles with a layered structure and calculated profiles are evaluated. The effect of rotation on particles is discussed in relation to different instrumental geometries. In this work, recoil mixing, redeposition, diffusion and the topographic effects at the size of the individual ion cascade are ignored. © Crown copyright 2011. Reproduced by permission of the Controller of HMSO and the Queen's printer for Scotland.

Generation of an amorphous graphite substrate by cumulative deuterium bombardment using molecular dynamics with full nonbonded interactions

A deuterated amorphous carbon target is generated from a fresh graphite layer via hyperthermal irradiation with deuterium ions using molecular dynamics (MD) simulations. We use interatomic potentials that include nonbonded (long-range) interactions for maximum accuracy and simulate cumulative bombardment up to doses of 5.80 × 1016 ions cm−2 at 1000 K. The graphite target goes through several stages of erosion and swelling, leading to complete amorphization and significant density loss. The calculations show a transition from graphitelike hybridization to a mixture of diamondlike and linear hybridizations with dose. It is concluded that the current sample sizes obtained directly by cumulative irradiation affordable with MD are not sufficiently large to be used for sputtering calculations under steady-state conditions.

Optimization of large amorphous silicon and silica structures for molecular dynamics simulations of energetic impacts

A practical method to create optimized amorphous silicon and silica structures for molecular dynamics simulations is developed and tested. The method is based on the Wooten, Winer, and Weaire algorithm and combination of small optimized blocks to larger structures. The method makes possible to perform simulations of either very large cluster hypervelocity impacts on amorphous targets or small displacements induced by low energy ion impacts in silicon.

A predictive model for the electronic stopping force for molecular dynamic simulation (I)

We have examined a predictive model for the electronic stopping force ( dE/dx) e to be used in the classical molecular dynamic (MD) simulation. The essential term ( dE/dx) proton of ( dE/dx) e is based on the Lindhard–Winther theory, while the effective charge follows the Brandt–Kitagawa model. The ( dE/dx) proton term is expressed by the electron local density ρ( r) defined by the Muffin-tin model and the Hartree–Fock–Slater approximation. This model had been proposed to explain the impact-parameter dependence of ( dE/dx) e for channeling ions passing through a semiconductor. Here the energy dependence of the averaged 〈 dE/dx〉 e after thin-film transmission was examined, where the electron–phonon interaction can be ignored in the computation. The present work uses a nitrogen ( 14N) beam passing through a diamond with energies from 10 to 100 keV, which is fully included in the region. Because of the channeling components in the crystalline target, the calculated data 〈 dE/dx〉 e was 10% smaller than that of measured values in an amorphous target. The exponent of the energy dependence of 〈 dE/dx〉 e (∝ E 0.473) was a little gentler than that assumed in conventional models as ( dE/dx) e (∝ E 0.5). We have confirmed that this predictive model without a free parameter will be useful in a new system, even for a MD simulation that will take into account the electron–phonon interaction for a non-metallic target.

A POSITRON SOURCE USING CHANNELING IN CRYSTALS FOR LINEAR COLLIDERS

An alternative way to the conventional positron source using intense electron beams, of some GeV, impinging on thick amorphous targets is presented. This source is using two successive targets: the first one is a tungsten crystal, which <111> axis is aligned with the beam direction, and the second is an amorphous target put at some distance from the crystal. The enhanced radiation due to electron channelling produces a large amount of photons which, consecutively, create high quantity of e +- e - pairs in the amorphous target. Between the two targets a sweeping magnet takes off all or part of the charged particles coming out from the crystal. An optimization procedure is carried out based on two important parameters: a) the distance between the two targets; b) the minimum energy above which the charged particles coming out from the crystal are allowed to hit the amorphous target. This optimization, which first step is presented here, aims to obtain the highest accepted yield (> 1 e +/ e -) after the capture system and the minimum Peak Energy Density Distribution (PEDD) (PEDD < 35 J/g) in order to avoid thermal gradients, which are destructive for the target.

Low-energy particle interaction with plasma-irradiated metal surfaces

In fusion experiments, plasma erodes walls of devices containing various elements including carbon. The eroded carbon penetrates into edge plasma and is transported backwards forming the carbon-adsorbed layers on the surfaces of components like mirrors for optical plasma diagnostics. To characterize the carbon-adsorbed layers by measuring energy distribution of particles reflected from the surface, we have used the experimental setup equipped with a magnetic deflection momentum analyzer and the time-of-flight energy analyzer of neutrals. Ion beams in the energy range 1–2 keV irradiated the samples of Mo and W with carbon deposition on them that were prepared in separate plasma chambers. The beams produced ions and neutrals with characteristic emission angle and energy distribution depending upon conditions of the sample surfaces. Experimental results are compared with the numerical calculation model ACAT (Atomic Collisions in Amorphous Targets) and the evaluations on how the structure of deposition layers may affect the particle reflection on solid surfaces were made.

High-Gain Avalanche Rushing amorphous Photoconductor (HARP) detector

We have been studying a very sensitive image sensor since the early 1980s. In 1985, the author found for the first time that an experimental pickup tube with an amorphous selenium photoconductive target exhibits high sensitivity with excellent picture quality because of a continuous and stable avalanche multiplication phenomenon. We named the pickup tube with an amorphous photoconductive layer operating in the avalanche-mode “HARP”: High-gain Avalanche Rushing amorphous Photoconductor. A color camera equipped with the HARP pickup tubes has a maximum sensitivity of 11 lx at F8. This means that the HARP camera is about 100 times as sensitive as that of CCD camera for broadcasting. This ultrahigh-sensitivity HARP pickup tube is a powerful tool for reporting breaking news at night and other low-light conditions, the production of scientific programs, and numerous other applications, including medical diagnoses, biotech research, and nighttime surveillance. In addition, since the HARP target can convert X-rays into electrons directly, it should be possible to exploit this capability to produce X-ray imaging devices with unparalleled levels of resolution and sensitivity.

Coupling of morphology to surface transport in ion-beam-irradiated surfaces: normalincidence and rotating targets

Continuum models have proved their applicability to describe nanopatterns produced byion-beam sputtering of amorphous or amorphizable targets at low and medium energies.Here we pursue the recently introduced ‘hydrodynamic approach’ in the cases ofbombardment at normal incidence, or of oblique incidence onto rotating targets, known tolead to self-organized arrangements of nanodots. Our approach stresses the dynamical rolesof material (defect) transport at the target surface and of local redeposition. Byapplying results previously derived for arbitrary angles of incidence, we deriveeffective evolution equations for these geometries of incidence, which are thennumerically studied. Moreover, we show that within our model these equations areidentical (albeit with different coefficients) in both cases, provided surface tension isisotropic in the target. We thus account for the common dynamics for both typesof incidence conditions, namely formation of dots with short-range order andlong-wavelength disorder, and an intermediate coarsening of dot features thatimproves the local order of the patterns. We provide for the first time approximateanalytical predictions for the dependence of stationary dot features (amplitude andwavelength) on phenomenological parameters, that improve upon previous linearestimates. Finally, our theoretical results are discussed in terms of experimental data.

Dynamic screening and charge state of fast ions in plasma and solids

AbstractThis paper addresses the effect of target plasma electrons on the charge state of energetic ions, penetrating a target composed of bound as well as plasma electrons. Dynamic screening of the projectile Coulomb potential by the plasma electrons brings about a depression in the ionization energy of the ionic projectiles, as has been verified experimentally. This in turn makes the ionization cross-sections larger, while making the recombination cross-section smaller, thereby causing an increase in the ion charge state compared to the case of a gas target. The effect of the plasma environment, where the valence electrons are treated as plasma, is illustrated here for a 2 MeV carbon beam penetrating amorphous carbon targets of varying densities.

Time dependence of secondary electron yield and of surface potential during charging of amorphous silica target

We study herein the charge evolution of a silica amorphous target submitted to an electronic bombardment. During the bombardment, the injected electronic charge and the primary energy as well as the surface bombardment of the target are controlled. The dynamics of the injected charge are described by combining measurements of the secondary electron and X-ray emission using a scanning electron microscope. Thus, we are able to access the evolution of the surface potential and secondary electron yield. Monte-Carlo calculations are used to simulate situations similar to our experiences and to study the effect of certain parameters (density of traps, energy of activation) inaccessible to the measures with our devices.

Dependence of Thickness on Avalanche Characteristics of Te-Doped Amorphous Selenium Photoconductive Target

The thickness dependence of the avalanche characteristics of a tellurium (Te)-doped amorphous selenium (a-Se) high-gain avalanche rushing amorphous photoconductor (HARP) target is investigated. To improve the quantum efficiency of the a-Se HARP photoconductive target, a Te-doped a-Se photoconductive layer is sandwiched within a-Se HARP target. The avalanche multiplication factor and hole ionization coefficient of the a-Se HARP target are obtained using the result of photocurrent measurement. The multiplication factor in the avalanche mode exponentially increases with increasing electric field by avalanche multiplication phenomena over the threshold field. The quantum efficiency of the 8-µm-thick a-Se HARP target in the avalanche mode is higher than that of the thin HARP target below 2 µm thickness. Also the spectral response, decay lag, and light-transfer characteristics are studied.

Monte Carlo Simulation of Ion Implantation Profiles Calibrated for Various Ions over Wide Energy Range

Monte Carlo simulation is widely used for predicting ion implantation profiles in amorphous targets. Here, we compared Monte Carlo simulation results with a vast database of ion implantation secondary ion mass spectrometry (SIMS), and showed that the Monte Carlo data sometimes deviated from the experimental data. We modified the electron stopping power model, calibrated its parameters, and reproduced most of the database. We also demonstrated that Monte Carlo simulation can accurately predict profiles in a low energy range of around 1 keV once it is calibrated in the higher energy region.

Enhanced output current density of an active-matrix high-efficiency electron emission device array with 13.75μmpixels

An active-matrix array of high-efficiency electron emission device (HEED) with a sufficient output density has been developed for the use as imaging probe in a high-gain avalanche rushing amorphous photoconductor target. Previously, it was demonstrated that a prototyped image sensor with 20×20μm2pixels could pick up a high definition image with an ultrahigh sensitivity under low-light-level condition. Based on it, an efficient active-matrix HEED with 13.75×13.75μm2pixels is fabricated for pursuing higher sensitivity and resolution. The improvement in the device isolation method enables to enlarge the relative emitting area, and then the emission current density per pixel reaches 3.6A∕cm2 that is about four times of that obtained from the previous one. The active-matrix HEED array with small pixels is available for application to the compact ultrahigh-sensitivity image sensor without affecting on the image definition.

Low-energy particle interaction at carbon nanowalls on W surface

We measured the characteristics of the reflected particles from a carbon nanowall (CNW) deposited on a W surface following the injection of 1–2 keV H + and O + ions. The reflected ion energies and intensities indicated a contribution from multiple scattering in the target. The reflection angular dependence of the reflected ion intensities reached the maximum around the mirror angle and showed a sharp distribution, which may be attributable to the effect due to the aligned structure of the CNW. The energies and intensities of the reflected ions decreased with the time of ion bombardment. The intensities and energies of the reflected ions were, however, recovered to some degree by baking the sample, indicating the surface modification due to retention of the injected particles during the injection. We used the Monte Carlo simulation code ACAT (Atomic Collision in Amorphous Target) to study these processes theoretically and the calculated results supported the experimental results.

Suppression of the yield of hard photons generated by electrons with energies above 100 GeV in oriented crystals

The yield of hard gamma quanta generated by a beam of electrons with energies above 100 GeV propagating through an oriented crystal has been studied as a function of the crystal thickness and electron energy. The phenomenon of suppression of the yield of hard photons in an oriented crystal as compared to that in an amorphous target of the same thickness is predicted.

Stripe domains and spin reorientation transition in Fe78B13Si9 thin films produced by rf sputtering

Magnetic thin films have been obtained by rf sputtering from an amorphous Fe78B13Si9 target. The samples have been produced with thickness t ranging in the interval 25–1000 nm. Microstructural investigations indicate that the films have different microstructures varying from fully amorphous to partially nanocrystalline with increasing t. Magnetic hysteresis loops have been measured by means of high-sensitive magnetometry. A tailorable spin reorientation transition (SRT) from in-plane single-domain-like to out-of-plane multidomain state with increasing film thickness was observed. Magnetic force microscopy images have been obtained for all samples indicating that for t≤80 nm the magnetization lies in the film plane. For larger thickness, a stripe domain pattern has been observed, indicating the presence of a magnetic anisotropy axis perpendicular to the film plane. In this work, SRT and stripe domain structure have been studied as a function of thickness and sample microstructure.

Molecular dynamics simulations of hypersonic velocity impact protection properties of CNT/a-SiC composites

The hypersonic velocity impact protection properties of carbon-nanotube-reinforced amorphous silicon carbide composites are investigated using molecular dynamics simulation. The projectile-impact-induced damage to target materials is analyzed for composite targets having perpendicular and parallel alignments of carbon nanotubes with respect to impact direction, and compared to that of pristine amorphous silicon carbide target. It was found that, in the considered range of impact velocities, the penetration depth is approximately the same for both CNT-reinforced composites and pristine a-SiC target. At short time-scales, damage to a target is defined by penetration depth of projectile and density rearrangements taking place in the target material. In the composite with carbon nanotubes aligned parallel to the impact direction, a channeling of damage to deeper regions of the target occurs. In the case of perpendicular alignment, the damage is confined to a narrow region underneath the penetrating projectile. For both considered composite samples, we found that a significant damping of the amplitudes of compressive shock-waves takes place, thus reducing the special extent of damage in composites as compared to that in the pristine a-SiC target.

Hypersonic velocity impact on a-SiC target: A diagram of damage characteristics via molecular dynamics simulations

Dynamic damage response characteristics of an amorphous silicon carbide target due to hypersonic velocity impacts of diamond projectiles are investigated using molecular dynamics simulations. In a certain range of radii of the projectile, four distinct regimes of damage are uncovered and summarized in a penetration depth diagram. The regimes correspond to shallow crater formation, deep penetration into the target, deep penetration with local melting of the target, and complete disintegration of the projectile. In the third regime, a logarithmic dependence of the penetration depth as a function of the projectile velocity has been found and explained by an analytical model.