Triple differential cross section measurements for the electron impact ionization of helium: Comparison with a second Born approximation

Triple differential cross sections for the ionization of helium by electrons of 500 eV impact energy have been measured in a coplanar, asymmetric geometry and for energies of the ejected electrons of 2.5, 5, 10 and 20 eV. A comparison is made between experimental data and a second Born treatment of Byron, Joachain and Piraux [1] for atomic hydrogen but for corresponding momenta of the incoming and the two outgoing electrons and for corresponding momentum transfer during the collision. The second Born describes quite well the shifts of the binary peak and the recoil peak to larger angles, the asymmetry of the recoil peak and the ratios of the intensities of binary to recoil peaks for different scattering parameters. — A quantitative estimate is made (for high impact energy) of the contributions to the total cross section of those ionizing collisions, in which the recoil peak intensity represents a considerable quantity. Probably most of these events can not be described by a single collision approximation.     

Point-defect properties of,and sputtering events in,the {001} surfaces of Ni3Al. II. Sputtering events at and near surfaces

Atomic recoil events at and near {001} surfaces of Ni₃Al due to elastic collisions between electrons and atoms have been simulated by molecular dynamics to obtain the sputtering threshold energy as a function of atomic species, recoil direction and atomic layer of the primary recoil atom. The minimum sputtering energy occurs for adatoms and is 3.5 and 4.5?eV for Al and Ni adatoms on the Ni–Al surface (denoted ‘M’), respectively, and 4.5?eV for both species on the pure Ni surface (denoted ‘N’). For atoms within the surface plane, the minimum sputtering energy is 6.0?eV for Al and Ni atoms in the M plane and for Ni atoms in the N surface. The sputtering threshold energy increases with increasing angle, θ, between the recoil direction and surface normal, and is almost independent of azimuthal angle, ?, if θ<60°; it varies strongly with ? when θ>60°, with a maximum at ??=?45° due to ?{110}? close-packed atomic chains in the surface. The sputtering threshold energy increases significantly for subsurface recoils, except for those that generate efficient energy transfer to a surface atom by a replacement collision sequence. The implications of the results for the prediction of the mass loss due to sputtering during microanalysis in a FEG STEM are discussed.     

Violation of the Franck-Condon principle due to recoil effects in high energy molecular core-level photoionization

Carbon 1s photoelectron spectra of methane are measured over a photon energy range between 480 eV and 1200 eV. Additional components appear between the individual symmetric stretching vibrational components and are attributed to the excitations of asymmetric stretching and bending vibrations due to recoil of the high-energy photoelectron emission. This recoil effect is the evidence for the violation of the Franck-Condon principle which states that neither the positions nor the momenta of the nuclei change during the ionization event.     

Measurement of the transverse kinetic energies of argon recoil ions produced in 120 MeV -Ar collisions

Measurements were carried out to deduce the transverse kinetic energies of highly charged argon recoil ions produced in single collisions of 120 MeV ions with argon atoms in which the post collision charge states of the projectiles were not determined. A time of flight spectrometer was designed and fabricated to detect the charge states of recoils. Experimental procedures for optimizing the spectrometer for extraction, transmission and detection of recoils are described. A simple approach for determining the transverse kinetic energy of the recoil ions from FWHM of the peaks is reported. This method is shown to be independent of the choice of collision partners and requires only the knowledge of the physical values of “optimized parameters” of time-of-flight spectrometer used in the experiment. The transverse kinetic energy of the recoil ions determined from the present approach is found to vary from 0.03 eV for to 4.02 eV for Ar¹⁰⁺. These values are compared with the results reported by earlier workers and are shown to follow a q²-behaviour up to a charge state q =8+ of the recoil ions. Received: 5 February 1998 / Revised: 8 June 1998 / Accepted: 11 June 1998     

Feasibility of detection of solar neutrinos and other astrophysical particles using superconducting filaments

Neutrinos from the sun or from a supernova will scatter coherently from target nuclei, and the associated nuclear recoil energy could in principle be detected using the significant local temperature rise produced at low temperatures in materials with a negligible electronic specific heat. Heavy ‘dark matter’ particles such as photinos might also be detected in this way. Drukier and Stodolsky [7] have suggested the use of a target in the form of superheated grains of superconductor which would be switched to the resistive state by individual neutrino scattering events, producing small but detectable local magnetic flux changes. The present paper considers the alternative scheme of a target consisting of coils of fine single or multi-filament superconducting wires, allowing the local resistive transitions to be detected as voltage pulses at the coil input. Calculations are presented of neutrino event rate versus energy deposited as a function of the target (A, Z) value, and the required filament diameter as a function of temperature and recoil energy, taking into account the latent heat requirements of the superconductor at transition. The possibility of using electrically parallel arrays of filaments is analysed, and the magnitude of the external voltage pulse is estimated for a range of type 1 and type 2 superconducting materials, including the effect of propagation of the normal zone. It is concluded that measurable voltage signals could in general be obtained with both type 1 and type 2 superconductors, and for operating temperatures in the region 10–100 mK typical filament diameters would range from 10–40 micron (for 30 eV recoil energy sensitivity) 40–160 micron (for 3 keV recoil energy sensitivity).     

Angular-resolved energy distribution of secondary ions emitted from a silicon target sputtered by a xenon ion beam

This paper reports preliminary results obtained on an experimental apparatus dedicated to the study of angular resolved energy distribution of particles emitted from a sputtered target. Secondary ions emitted during the bombardment of a silicon target by xenon ions at a primary energy of 10keV have been studied. In its low energy part the distribution reaches a maximum around 8eV, and then decreases according to an E ^–1 law. In the range 200eV to 1000eV, a second maximum appears whose height depends on the emission angle. Apart from this range, the angular distributions have a cosine square-like shape. On the contrary, the angular distribution of ions with energy between 200eV and 1000eV is pointed in a forward direction near the specular reflection direction of the ion beam. It is assumed that the measured ions correspond to two ionic populations: secondary ions sputtered according to the linear cascade theory and recoil silicon target ions.     

Experimental study of single- and double-electron-capture by Ne ions from He gas

In the present work, the total cross-sections for single- and double-electron-capture by low-energy highly-charged Ne^q+ (q =2-6) recoil ions from He atomic gas have been measured in the energy range 600–1000 eV/q. These measurements are compared with other available experimental results. The charge state dependence of the single-electron-capture cross-sections for Ne^q+ (q = 2-6) recoil ions incident on He at energy of 1000 eV/q is compared with the theoretical predictions of the absorbing sphere model and the classical model.     

Athermal migration of vacancies in iron and copper induced by electron irradiation

Abstract

Irradiation with high-energy particles induces athermal migration of point defects, which affects defect reactions at low temperatures where thermal migration is negligible. We conducted molecular dynamics simulations of vacancy migration in iron and copper driven by recoil energies under electron irradiation in a high-voltage electron microscope. Minimum kinetic energy required for migration was about 0.8 and 1.0 eV in iron and copper at 20 K, which was slightly higher than the activation energy for vacancy migration. Around the minimum energy, the migration succeeded only when a first nearest neighbour (1NN) atom received the kinetic energy towards the vacancy. The migration was induced by higher kinetic energies even with larger deflection angles. Above several electron-volts and a few 10s of electron-volts, vacancies migrated directly to 2NN and 3NN sites, respectively. Vacancy migration had complicated directional dependence at higher kinetic energies through multiple collisions and replacement of atoms. The probability of vacancy migration increased with the kinetic energy and remained around 0.3–0.5 jumps per recoil event for 20–100 eV. At higher temperatures, thermal energies slightly increased the probability for kinetic energies less than 1.5 eV. The cross section of vacancy migration was 3040 and 2940 barns for 1NN atoms in iron and copper under irradiation with 1.25 MV electrons at 20 K: the previous result was overestimated by about five times.     

Dependence of yield of radiation-induced defects and their energy capacity on initial-state density

Experimental dependences are obtained for the yield and energy capacity of defect formation on the densities of the initial states. It turns out that for fixed proton energy, a) the energy capacity of the defects decreases down to ε₀ = 500 eV/def; b) the energy yield increases practically linearly and is saturated at a specific energy capacity of 2.5·10²⁰ eV/cm³. It is demonstrated that these conclusions directly follow from the symmetry of a given integral transformation of the radial distribution of the excitation density in the track ρ_E (r) into the radial distribution of the concentration of radiation-induced defects n_E (r).     

Recoil effect of photoelectrons in the Fermi edge of simple metals

High energy resolution photoelectron spectroscopy of conduction electrons in the vicinity of the Fermi edge in Al and Au at excitation energies of 880 and 7940 eV was carried out using synchrotron radiation. For the excitation energy of 7940 eV, the observed Fermi energy of Al shows a remarkable shift to higher binding energy as compared with that of Au, with accompanying broadening. This is due to the recoil effect of the emitted photoelectrons. The observed spectra are well reproduced by a simple model of Bloch electrons based on the isotropic Debye model.     

Energy sharing and sputtering in low-energy collision cascades

Using a non-linear transport equation to describe the energy-sharing process in an isotropic collision cascade, we have numerically calculated sputtered particle velocity spectra for several very low energy (<10 eV) primary recoil distributions. Our formulation of the sputtering process is essentially that used in the linear model and our equations yield the familiar linear model results in the appropriate limit. Discrepancies between our calculations and the linear model results in other cases may be understood by considering the effects of the linear model assumptions on the sputtering yield at very low energies. Our calculations are also compared with recent experimental results investigating ion-explosion sputtering. The results of this comparison support the conclusion that in insulators sputtering is initiated by very low energy recoil atoms when the energy of the incident beam is high enough that the stopping power is dominated by the electronic contribution. The calculations also suggest that energy spectra similar to those for evaporation may result from non-equilibrium processes but that the apparent temperatures of evaporation are not related in a simple way to any real temperature within the target.     

Molecular dynamics study of structural damage in amorphous silica induced by swift heavy-ion radiation

In this paper, the radiation defects induced by the swift heavy ions and the recoil atoms in amorphous SiO₂ were studied. The energy of recoil atoms induced by the incident Au ions in SiO₂ was calculated by using Monte Carlo method. Results show that the average energies of recoils reach the maximum (200?eV for Si and 130?eV for O, respectively) when the incident energy of Au ion is 100?MeV. Using Tersoff/zbl potential with the newly built parameters, the defects formation processes in SiO₂ induced by the recoils were studied by using molecular dynamics method. The displacement threshold energies (E_d) for Si and O atoms are found to be 33.5 and 16.3?eV, respectively. Several types of under- and over-coordinated Si and O defects were analyzed. The results demonstrate that Si₃, Si₅, and O₁ are the mainly defects in SiO₂ after radiation. Besides, the size of cylindrical damage region produced by a single recoil atom was calculated. The calculation shows that the depth and the radius are up to 2.0 and 1.4?nm when the energy of recoils is 200?eV. Finally, it is estimated that the Au ion would induce a defected track with a diameter of 4?nm in SiO₂.     

Atomic collision cascades on void evolution in vanadium

Molecular dynamics simulations were performed to study void evolution subject to unidirectional self-bombardment and radiation-induced variation of mechanical properties in single crystalline vanadium. 3D simulation cells of perfect body-centered cubic (BCC) vanadium, as well as those with one, two, four, and six voids, were investigated. For the no void case, the maximum number of defects, maximum volumetric swelling, and the number of defects left in bulk after a sufficiently long recovery period increased with higher primary recoil energy. For the cases containing voids, a primary recoil energy was carefully assigned to an atom so as to initiate a dense collision spike in the voids center, where some self-interstitial atoms gained kinetic energy by secondary replacement collision sequence traveling along the ? 111? direction. It is found that the larger or the greater the number of voids contained initially in the box, the larger the normalized void volume, and the smaller the volumetric swelling after sufficient recovery of systems. In the single void case, the void became elongated along the bombarding direction; in the multiple void cases, the voids coalesced only when the intervoid ligament distance was short. After sufficient relaxation of the irradiated specimen, a hydrostatic tension was exerted on the box, where the voids were treated as dislocation sources. It is shown that with higher primary recoil energy, the yield stress dropped in cases with smaller or fewer voids but rose in those with larger or greater number of voids. This radiation-induced softening to hardening transition with increasing dislocation density can be attributed to the combined effects of the defect-induced dislocation nucleation and the resistance of defects to dislocation motion. Moreover, as the primary recoil energy increased, the ductility of vanadium in the no void case decreased, but was only slightly changed in the cases containing void.     

Ne原子(e,2e)反应中反冲离子动量分析

利用冷靶反冲离子动量谱仪,对电子轰击Ne原子的单电离反应(e,2e)进行了研究,实验测量了70—3300eV入射能量情况下,反应过程中产生的一价反冲离子的动量分布,并对反冲离子的总动量进行了还原。介绍了一个简单的碰撞机制,据此着重分析了反冲离子纵向动量和横向动量二维谱形成的原因,该碰撞机制能够较好地解释较高能量入射时的实验结果。最后根据反冲离子的动量,估算了出射电子的能量范围,为下一步进行电子、离子的符合测量奠定了基础。     

Electronic recoil effects in high-energy photoelectron spectroscopy

It is shown that the recoil energy imparted to the residual ion by the outgoing fast photoelectron leads to noticeable modifications of X-ray excited photoelectron spectra of molecules containing light atoms. The vibrational band envelopes may differ considerably from those predicted by the Franck-Condon principle. Al Kα excited valence electron bands are shown to exhibit, in addition to the translational recoil energy of the molecule, energy shifts of up to several tenth of an eV due to recoil-induced vibrational and rotational excitation. This effect has to be kept in mind when ionization potentials are determined from ESCA spectra. The recoil-induced vibrational and rotational excitation depends on the orbital quantum numbers of the ionized electron. Simple formulae for the shift of the centroid and the broadening of the band due to recoil effects are given for the special case of diatomic molecules.     

Absolute triple differential cross sections for electron impact ionization of helium: Comparison between experimental and theoretical results at 256 eV collision energy

Triple differential cross sections have been measured for electron impact ionization of helium at 256 eV collision energy, 3 eV energy of the slow outgoing electrons and scattering angles of the fast outgoing electrons of 4°, 6°, 8°, and 10°. The data have been put on absolute scale by extrapolating the generalized oscillator strength to zero momentum transfer. In this optical limit the triple differential cross sections can be normalized by using the well-known cross sections for photoionization. The experimental data are compared with results of different theoretical approaches. For nearly all calculated curves rather good agreement with the measurements is obtained for the relative shape of the binary peak, while often its absolute cross section is overestimated. Concerning the recoil peak, larger discrepancies are found with respect to both, relative shapes and cross sections. A perceptible improvement can be stated for calculations which have been performed in a distorted wave approximation and in second Born approximation.     

Energy dependence of electron-induced radiation damage in tungsten

Abstract

The energy dependence of low dose damage production in commercial and high purity polycrystalline tungsten wires was studied near 350 K with 1.6 to 2.4 MeV electrons. From resistivity measurements at 291 K the threshold energy for the onset of observable damage was determined as 50 × 2 eV. An ‘effective’ threshold of 52 ±2 eV was also determined by directly fitting the energy dependence of the damage rates to theoretical displacement cross sections calculated from step-function displacement probabilities. A decrease of two orders of magnitude in impurity content reduced damage rates by about a factor of two but did not affect threshold. These results combined with current defect recovery models for tungsten, low temperature threshold data, and computer-calculated bcc damage theory suggest: (1) Observed damage consisted of equal concentrations of vacancies and impurity-trapped Stage I free interstitials. (2) Across Stage II (100 K to 600 K) onset threshold should be within 50 ±2 eV. (3) Minimum recoil energy required for free interstitial production near 0 K is 53 ± 5 eV. (4) Threshold has little dependence on crystal direction. An empirical method is presented for predicting threshold energies in the bcc transition metals by assuming the directional dependence of threshold is directly proportional to that of Young's modulus. By the use of one universal proportionality constant (1.2 × 10^?11 eV.cm²/dyne), thresholds for a number of metals and directions are calculated and shown to have significantly better agreement with experiment than the best available theoretical estimates.     

On the formation of defect clusters in neutron-irradiated Si

The formation of radiation-induced defect clusters in neutron-irradiated silicon have been studied by solving the semilinear parabolic reaction-diffusion coupled equations. It is found that most of primary displacement defects (interstitial and vacancy) would be annihilated by direct I–V recombination in an extremely short time, and a lot of divacancies would be formed meantime. In particular, the production of 4-vacancy defects is independent of the concentration of sinks and impurities in the sample, and of the energy of recoil particles. The threshold energy of vacancy cluster formation has also been investigated. The results are discussed and compared with experiment observations.     

Investigation of the recoil fluxes in a SiO2/Si target by means of monte carlo simulation

Abstract

In order to study the effect of an interface on the recoil flux distribution we investigated the angular and energy distribution of Si and O recoil fluxes in a SiO₂/Si target implanted with 50 keV Sb⁺ by Monte Carlo simulation. At the interface the calculations show steps in the recoil fluxes, currents and average energies of the Si and O recoils. Relatively large backflow of Si recoils was found at the interface, but 0.3 nm in front of and behind the interface the net flow was positive. The energy-angle distributions contain a low energy near isotropic and a high energy forward directed part. Behind the interface the high energy part of the O recoil distribution is present. The effect of the interface on the Si recoils is to decrease the average recoil energy and to steepen the energy distribution.     

Energieverteilungen bei der Festkörperzerstäubung durch Ionenbeschuß

The energy distributions of the neutral particles sputtered from polycrystalline targets of Al, Ti, Ni, Cu and Ag by normally incident Ar⁺-ions in the 1 keV region have been determined for ejection energies below 20 eV in a direction close to the normal to the target surface. The experimental method employed is strongly connected to the characteristic properties of a low pressure electrodeless hf plasma used as an effective ion source as well as an ionizing medium for the ejected target atoms. The resulting curves always show a maximum at most probable ejection energies between 1 and 5 eV, being approximately half of the surface binding energy. These curves are converted to the corresponding energy distributions for the recoil atoms within the target by an energy dependent factor. It is found that the energy distribution within the bombarded solid decreases monotonically with the inverse square of the energy of the recoils.