Knock-on Collisions Research Articles

Dependence of knock-on collision electron emission on the orientation of fast diclusters in solids

The cluster-orientation dependence of the emitted knock-on collision electron distributions when fast diclusters interact with condensed matter is investigated. The scattering by each centre of the dicluster is described by means of the phase shifts of a velocity dependent Yukawa potential describing the dynamic screening, and the multiple scattering effects are approximated within the second Born approximation. The applicability of this approximation as the charges and velocities of the diclusters are changed is also discussed, and numerical comparisons of our results with experiment are made.

Quantitative analysis of knock-on and thermal damage of biological specimens by electron irradiation in transmission electron microscopy

High-energy electrons are able to transfer momentum to nuclei, which results in displacement on to the interstitial lattice sites with a maximum transferred energy of 4 · 10 4 eV for carbon at 100 keV. Moreover, most of the energy dissipated in energy losses is converted into heat, which results in melting and evaporation. The specimen temperature rise was calculated by the heat conduction theory and confirmed by the specimen drift due to the thermal damage. The damage can be reduced by a small area of illumination, the use of a metal-coated microgrid and small area scanning. A further displacement due to the knock-on collision and the resulting etching rate of biological specimens was measured. The damage is proportional to the current density in c cm -2 at the specimen. The allowable maximum dose was obtained from the measurement of an etching rate with the weight loss and dry density of the specimens. It was found that the images affected by the electron irradiation, in which -H, C-H, C-N bonds break molecules in proteinaceous biological specimens are removed, and the remaining molecules are changed to stable carbon-rich molecules by deposition, polymerization and contamination. In addition, defect images were observed in high contrast, when compared with unaffected images taken with a small area scanning method.

Observation of exit surface sputtering in TiO2 using biased secondary electron imaging

A dedicated ultra-high-vacuum 100 kV scanning transmission electron microscope has been used to characterize the electron-beam-induced damage which occurs on the surfaces of rutile crystals as a result of extensive electron irradiation. Biased secondary electron images from the entrance and exit surfaces indicate that the latter surface is damaged at a substantially greater rate than the former. It is concluded that knock-on collisions play an important role in the observed exit surface sputtering.

Trapping of atmospheric oxygen on metal surfaces under Ar + ion bombardmeNT

Various metals were bombarded with 8 keV Ar + ions in an oxygen atmosphere at a pressure of 1 × 10 −3 Pa. The chemical changes on the surface were analyzed using XPS. It was observed that titanium easily incorporates oxygen. This results in the formation of titanium dioxide with a nearly stoichiometric composition. The oxygen-trapping was also confirmed for other metal targets. It was proven that the metals with a Gibbs energy (Δ G) for oxide formation lower than −80 kcal mol easily trap oxygen, while those with a Δ G value higher than −60 kcal mol do not. We conclude from our observations that the trapping of atmospheric oxygen under Ar + ion bombardment can be attributed to chemical reactions at the metal-gas interface rather than to physical processes such as knock-on collisions.

Electron density fluctuations induced by ion clusters in condensed matter

Valence electron density fluctuations induced around a trailing ion by a leading ion of a swift dicluster moving through condensed matter depend on the orientation of the dicluster axis with respect to the beam axis. These density fluctuations may appear experimentally in the angle dependence of the knock-on collision electrons emitted under bombardment of molecular ions. We present linear-response theory calculations of the density fluctuations at the position of the trailing ion, as a function of the orientation. Second-order perturbation theory calculations of the dependence on the angle of the scattering probabilities of target valence electrons by a two-center Coulomb field moving with different velocities are presented. The applicability of linear theories as the charges and velocities of diclusters are changed is discussed.

Preparation of nitride films by Ar+-ion bombardment of metals in nitrogen atmosphere

A preparation of thin nitride films on metal surfaces was presented. The nitrogenation was carried out by bombarding the metals with 8-keV Ar+ ions in nitrogen at 1.0×10−3 Pa. Among the 13 metals examined, the nitrides of nine metals were produced. The approximate thickness of the TiN film was ∼7 nm, which is comparable to the range of the Ar+ ion. The saturated values of the surface N/Me (Me:metal) are different among various metals examined, and are related to the chemical reactivity of the metal with nitrogen. The N/Ti ratio was inversely proportional to the Gibbs energy of nitride formation. These results suggest that the nitride was produced not only through the knock-on collision by the Ar+ ions but also through the thermochemical reaction at the nitrogen–metal interface, which is promoted by the ion bombardment.

Electron beam stimulated nonthermal crystallization of CdS surface layers: Observations by real-time atomic-resolution electron microscopy

The transformations of partially amorphous CdS surfaces induced by 100–400 keV electrons have been followed in real time by atomic-resolution electron microscopy. A definite sequence of atomic rearrangements leading to nucleation and growth of CdS and Cd crystals was observed. These processes are believed to be due to nonthermal mechanisms involving inelastic electron collisions rather than direct knock-on collisions between electrons and the atomic nuclei since there is no threshold over the range of incident electron energies. These effects may be responsible for the degradation of electron beam pumped CdS lasers which has been reported elsewhere.

Angular Distribution of Scattered Electron and Medium Energy Electron Spectroscopy for Metals

The angular distribution (AD) of scattered electrons produced by medium energy incident electrons ( E P =50∼300 eV) from polycrystalline Ti, Fe, Ni, Cu and Au were obtained by the angle-resolved medium energy electron spectrometer. The AD of the energy loss peaks are similar figures to AD of the elastically reflected electron peaks. Therefore, the exchanged electrons produced by the knock-on collision between the incident electrons and those of metals without momentum transfer are observed as the energy loss spectra (ELS). This interpretation differs from the inconsequent interpretation by the dielectric theory or the interband transition. The information depth and penetration length are obtained from AD of the Auger electron peaks. The contribution of the surface to spectra is 3% at the maximum for E P =50 eV. The true secondary peaks representing the secondary electron emission spectroscopy (SES) are caused by the emissions of the energetic electrons (k T e ≧4 eV), and SES is the inversion of ELS. The e...

Beam interactions, contrast and resolution in the SEM

SUMMARYBackscattered and secondary electrons are both used in the SEM for imaging purposes. The backscattered signal is the result of high angle elastic scattering events, while the secondary signal is the result of knock‐on inelastic collisions. The characteristic differences between images in the two modes arise from the details of the relevant interactions in the two cases. In order to examine this in a quantitative manner Monte Carlo electron trajectory simulation techniques have been used. Calculations of the ultimate resolution and depth of information of the secondary and backscattered images are presented, together with simulations of the edge brightness effect in high resolution secondary images and an analysis of the microanalytical application of atom number contrast in the backscattered mode.

On charge exchange and knock-on processes in the exosphere of Io

One direct consequence of magnetospheric interaction of Io is the strong dynamical coupling of its neutral atmosphere with the corotating plasma. The absorption of the thermal ions and the associated neutral injection is an improtant issue not yet explored. As far as nonthermal escape of the neutral atmosphere is concerned, three processes stand out. That is, apart from sputtering, exospheric interactions like atom-ion knock-on collision and charge exchange recombination could be a significant source of the neutral clouds in the Jovian system. Using a current electrodynamic model of Io, both the absorption rate of the corotating thermal plasma and the production rates of new exospheric ions and the fast neutrals are considered. It is found that the source strength of the neutral atoms and molecules with speeds of about 100 km/sec could amount to 10 to the 26th/sec whereas exospheric neutrals emitted at lower speed (of about 10 km/sec) amounts to 4 x 10 to the 25th/sec. The generation of the new ions in connection with the streaming of the magnetospheric plasma around Io could also produce an asymmetric sputtering with a neutral flux of about 10 to the 27th/sec emitted from the region of Io which faces Jupiter. These results may be related to a number of sodium observations.

Ionization yield in xenon due to electron impact

The ionization yield in xenon for complete energy degradation of electrons with initial energy up to 1 keV has been calculated by solving the generalized Fowler equation. The expression for the energy spectrum of secondary electrons from the O shell was obtained by using the empirical scaling functions to weight the Williams-Weizsäcker cross section for glancing collisions and the Mott cross section for knock-on collisions. The total ionization and excitation cross sections were taken from the recent evaluation reported by De Heer et al. Contributions from the inner-shell ionization and the Auger process were explicitly taken into account. The results are expressed in terms of the quantity W, the mean energy required to produce an ion pair. The W value is found to decrease with increasing incident energy, finally approaching a constant value of 23.1 eV for electrons with an energy above 200 eV. The results are generally in good agreement with the available experimental work.

Ionization yield in krypton due to electron impact

The ionization yield in krypton for complete energy degradation of electrons with initial energy up to 1 keV has been calculated by solving the Fowler equation. The main expression for the energy spectrum of secondary electrons was obtained by utilizing the qualitative features of two nearly distinct domains of glancing collisions and knock-on collisions. Empirical scaling functions were used to weight the two kinds of contributions. The total ionization and excitation cross sections were taken from the recent evaluation reported by De Heer et al. Contributions to the ionization yield from M and N shells and from the Auger electrons were explicitly included. The results are expressed in terms of the quantity W, the mean energy required to produce an ion pair. The asymptotic value of W is obtained to be 23.6 eV for an incident energy above 200 eV. The results are in good agreement with the available experimental data.

Preferential sputtering as a method of producing metallic iron, inducing major element fractionation and trace element enrichment

The mechanism responsible for the formation of finely divided metallic iron in lunar samples has been the subject of considerable debate. A review of the data currently available from laboratory simulation studies would appear to favour an origin involving preferential sputtering of oxygen atoms by the solar wind. Using a model presented here we find that it is thermodynamically predictable that preferential sputtering would lead to the formation of free iron. Likewise titanium should be enriched in particle surfaces due to the formation of Ti 2O 3 from TiO 2. Other major elements will be fractionated according to the ratio of their heat of sublimation to the heat of dissociation of their oxide. The changes induced in the surfaces of fine grains will be of considerable importance when these particles are incorporated into complex grains. Thus, we suggest that preferential sputtering may be implicated in the major element fractionations observed for glassy agglutinates. A 1% component of Fe∮ is able to account easily for the enrichment in the total iron observed. If sufficient exposure time is available preferential sputtering may be able to produce metallic iron-rich layers ca. 40Åthick on the surfaces of lunar grains. Most of this metal would derive from helium sputtering rather than hydrogen bombardment. Provided reduction takes place predominantly in fine particles with a mean grain size <10 μm diameter which are subsequently incorporated into glassy agglutinates, preferential sputtering could account for all the free iron in the lunar soil. For elements of atomic number ca. Z = 30 and above, thermodynamic effects become insignificant compared to momentum transfer considerations. Heavy (Pb, Hg) and medium heavy elements (Rb, Sr) could all be enriched by preferential sputtering. Monovalent elements (e.g. K, Rb) in silicates will have a lower binding energy and knock-on collisions with incoming solar wind atoms could cause their increased abundance in fine fractions of lunar soil.

Sputtering of chemisorbed gas (nitrogen on tungsten) by low-energy ions

Flash filament techniques and mass spectrometry have been used to measure sputtering yields for nitrogen chemisorbed on tungsten and bombarded with noble-gas ions in the energy range up to 500 eV. The experimental results show that primarily nitrogen atoms rather than molecules are sputtered. Despite a high binding energy (∼6.7 eV/atom), we find high sputtering yields and low threshold energies. The results are found to be in reasonable agreement with simple theoretical estimates. It is sugguested that, in the investigated energy range, adsorbed nitrogen atoms are sputtered primarily as a consequence of direct knock-on collisions with impinging and/or reflected noble-gas ions. Estimates are also given for the yield of nitrogen atoms knocked off by sputtered tungsten atoms. This latter process is expected to dominate at much higher energies.

Radial Cutoff LET and Radial Cutoff Dose Calculations for Heavy Charged Particles in Water

A model has been presented to calculate radial energy deposition around the trajectory of a heavy charged particle in water. The model originates from the basic concept of knock-on collisions and glancing collisions in stopping-power theory. Energy deposition by diffusive motion of low-energy electrons has been included in the calculation. Available experimental data agrees reasonably well with the results of the present model.

Monte Carlo calculation of the total energy loss of electrons from 0.3 to 1 MeV in aluminium foils

Since low-energy electrons straggle in the beam extraction window, their total energy loss in this window cannot be given by the product of the theoretical energy loss rate of electrons passing through matter, and the foil thickness. This energy loss is calculated by the Monte Carlo method, including the energy loss by knock-on collisions and the difference between real path length and foil thickness. The total energy loss thus obtained is, for instance, about two times as large as the product of the theoretical energy loss rate based on Bethe's formula and the foil thickness, for an initial energy of 300 keV and a foil, 150 μm thick, made of Al, and decreases gradually with increasing initial energy up to about 1 MeV, while the theoretical energy loss rate is almost constant for electrons having kinetic energy above ≈ 300 keV.

Possible Source Mechanism for Low-Energy Galactic Electrons

A calculation is made of the expected secondary electron flux resulting from the knock-on collisions of the primary nuclear beam with the interstellar gas. The model includes ionization losses and a statistical Fermimechanism energy gain. Comparison is made with recent satellite experimental data.

Excitation Function for theC12(π−,π−n)C11Reaction

The excitation function for the reaction ${\mathrm{C}}^{12}({\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}n){\mathrm{C}}^{11}$ was measured from 53 to 1610 MeV by bombarding targets of plastic scintillator with pions. The intensity of the pion beam was monitored with a two-counter telescope and 40-Mc/sec scaling system. The scintillator target was mounted on a phototube and became the detector for the ${\mathrm{C}}^{11}$ positron activity. Corrections were made for muon contamination in the beam, coincidence losses in the monitor system, ${\mathrm{C}}^{11}$ activity produced by stray background at the accelerator, ${\mathrm{C}}^{11}$ activity produced by secondaries in the target, and the efficiency of the ${\mathrm{C}}^{11}$ detection system. The ${\mathrm{C}}^{12}({\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}n){\mathrm{C}}^{11}$ cross sections rise from a threshold at about 50 MeV to a peak of about 70 mb at 190 MeV after which they decrease to 30 mb at 373 MeV and are relatively constant at higher energies. The (${\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}n$) peak occurs at the same energy as the resonance in free-particle ${\ensuremath{\pi}}^{\ensuremath{-}}n$ scattering at 190 MeV. Calculations based on a "knock-on collision" mechanism and "sharp-cutoff" nuclear density reproduce the shape of the experimental excitation function, but the magnitudes of the calculated cross sections are low by a factor of about 5 or 6. This simple model indicates that the ${\mathrm{C}}^{12}({\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}n){\mathrm{C}}^{11}$ reaction occurs in the nuclear surface region at all bombarding energies.

Search forμ→e+e+e

We have made a search for the neutrinoless decay mode $\ensuremath{\mu}\ensuremath{\rightarrow}e+e+e$. Positive poins from the Berkeley 184-in. synchrocyclotron were stopped in a lithium target. The detection of three decay particles not in coincidence with an incoming pion served to trigger two thin-plate spark chambers viewing the decays. The energy of the decay particles was measured with a combination of NaI counters and range telescopes. The spark chamber pictures were first scanned for events with three tracks meeting at a common point in the target. A requirement that no angle between any of the three tracks could be greater than 160\ifmmode^\circ\else\textdegree\fi{} eliminated essentially all of the main source of background, namely, knock-on collisions of electrons passing through the target. Further requirements for the three tracks to be coplanar and for each particle to have at least 16 MeV eliminated all but one ambiguous event. For this event insufficient energy information was available. If we assume that one event to be $\ensuremath{\mu}\ensuremath{\rightarrow}e+e+e$, and with a calculated detection efficiency of 0.0083, an upper limit for the branching ratio for this mode can be set at 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ with a 90% confidence level.

Interaction of High-Energy Protons and Alpha Particles with Iodine-127

Iodine was bombarded with protons ranging in energy from 0.25 to 6.2 Bev and with 0.25-, 0.50-, and 0.72-Bev alpha particles. Reactions of the type ($p, \mathrm{pxn}$), ($p, 2pxn$), ($p, p{\ensuremath{\pi}}^{+}$), ($p, p2{\ensuremath{\pi}}^{+}$), ($p, n{\ensuremath{\pi}}^{\ensuremath{-}}$), and ($\ensuremath{\alpha}, \ensuremath{\alpha}\mathrm{xn}$) to produce iodine, tellurium, antimony, and cesium isotopes were investigated. Upper limits in the range 0.01 to 0.1 mb were found for cross sections of reactions to produce ${\mathrm{Sb}}^{127}$ and ${\mathrm{Cs}}^{127}$. For ${\mathrm{Te}}^{127}$, upper limits in the range of 1 to 2 mb were found. This and other studies of reactions in which the product has the same mass number as that of the target are discussed in terms of the initial interaction. At all incident energies studied, the cross section for the formation of ${\mathrm{I}}^{126}$ via the ($p, \mathrm{pn}$) or ($\ensuremath{\alpha}, \ensuremath{\alpha}n$) reaction is significantly higher than that of the other ($p, \mathrm{pxn}$) or ($\ensuremath{\alpha}, \ensuremath{\alpha}\mathrm{xn}$) reactions, with the possible exception of ($\ensuremath{\alpha}, \ensuremath{\alpha}4n$). The ($p, \mathrm{pn}$) and probably the ($\ensuremath{\alpha}, \ensuremath{\alpha}n$) reactions appear to be due primarily to knock-on collisions with surface neutrons. The excitation functions for the production of iodine isotopes by proton bombardment decrease between 0.25 and 0.72 Bev but remain relatively constant for higher energies. The ($p, 2pxn$) reactions show a similar effect, but with the excitation functions becoming constant at about 2 Bev. These results are compared with Monte Carlo calculations of the proton-initiated nucleon cascade and of the subsequent evaporation of light particles.