Computer simulation of CAICISS spectra of clean Cu(1 0 0) surface and Cu(1 0 0) surface by Pt deposition

The clean Cu(1 0 0) surface and Pt/Cu(1 0 0) surface by Pt deposition at room temperature have been investigated using the computer simulation of coaxial impact-collision ion scattering spectroscopy (CAICISS). The computer simulations employing the ACOCT program code, which treats the atomic collisions three-dimensionally and is based on the binary collision approximation (BCA), were carried out for the case of 3 keV He⁺ ions incident along the 〈1 0 0〉 and 〈1 1 0〉 azimuths of the clean Cu(1 0 0) and Pt/Cu(1 0 0) surfaces. The comparisons between ACOCT results and experimental CAICISS data show that the experimental results on the clean Cu(1 0 0) surface are relatively well reproduced by the ACOCT simulations including the inward relaxation of 1.2% in the first interlayer spacing and the outward relaxation of 1.6% in the second interlayer spacing, and that the ACOCT simulations for the Pt deposition with coverages of 2.35 ML and 2.75 ML on the Cu(1 0 0) surface appear the concentrations of 0.24 ML of Pt sitting 2.3 Å and 0.25 ML of Pt sitting 2.5 Å above the outermost atomic layer, respectively.     

New local model for electronic energy loss and its application to computer simulations of channeling

We used the average of the Thomas-Fermi (TF) electron distribution instead of that of Hartree-Fock (HF) electron distribution as the screening length of an isolated atom. Based on the Firsov theory, we proposed a new Firsov formula of the electronic energy loss which has a simple form ΔE_e(E, b) ∞ S_e(E) exp(γb)/(1 + βb)⁶, where S_e(E) is the electronic stopping cross section, b = p/a, p and a are the impact parameter and the screening length, respectively, and β and γ are the fitting parameters. Using the present screening lengths with the shell effect and the new Firsov formula, the depth distributions of channeling were simulated by the ACOCT code for 20 keV B⁺ ions impinging along the [1 1 0] channel direction of silicon (1 1 0) surface. The ACOCT depth profiles of channeling using the new Firsov (solid) local model for the AMLJ potential are in good agreement with the experimental ones.     

Experiments and simulations on the effect of channeling on the ranges of 1 keV deuterons in silicon

Simple extrapolation of the Lindhard analytic theory of channeling predicts very wide channeling angles ( 10°) at low keV energies; however, the effect is highly sensitive to the exact atomic interaction potential chosen, as shown by computer simulations. Amorphous, polycrystalline, and single-crystalline 110, 111 and 100 silicon samples have been implanted with 1 keV deuterons in channeled and random directions. The ranges are measured by the ERD biE × biB (elastic recoil detection) ion beam technique and compared to simulations with the codes MARLOWE, TRIM and BABOUM. Some significant conclusions are that the channeling angle is approximately 10° in the 110 direction, and that polycrystalline material gives significant “accidental” channeling.     

Evidence of nanostructure formation in Ge oxide by crystallization induced by swift heavy ion irradiation

Ge oxide films were irradiated with 150 MeV Ag ions at fluences varying between 10¹² and 10¹⁴ ions/cm². The irradiation-induced changes were monitored by FT-IR spectroscopy, atomic force microscopy, X-ray diffraction and photoluminescence spectroscopy. The FT-IR spectra indicate stoichiometric changes and an increase in Ge content on irradiation. X-ray diffraction shows a crystallization of the irradiated films and presence of both Ge and GeO₂ phases. The Ge nanocrystal size, as calculated from Scherrer’s formula, was around 30 nm. The morphological changes, observed in atomic force microscopy, also indicate formation of nanostructures upon ion irradiation and a uniform growth is observed for a fluence of 1 × 10¹⁴ ions/cm².     

Structure analysis of Ni (1 1 0) surface using computer simulation of NICISS

The relaxation at a Ni (1 1 0) surface has been estimated using the computer simulation of the 180 neutral impact-collision ion scattering spectroscopy (NICISS). The computer simulations employing ACOCT program code based on the binary collision approximation (BCA) are performed for the case of 1.5 keV He⁺ ions incident along the direction of the Ni (1 1 0) surface. It is found that the experimental results are well reproduced by the ACOCT simulations including the inward relaxation of 7% of the first interlayer spacing and the outward relaxation of 5% of the second interlayer spacing at the Ni (1 1 0) surface.     

Evaluations of proton inelastic mean free paths for 12 elemental solids over the energy range from 0.05 to 10 MeV

The systematical calculations of the inelastic mean free paths (MFPs) of 0.05-10 MeV protons in 12 elemental solids (Al, Si, Ni, Cu, Mo, Rh, Ag, W, Os, Ir, Pt, Au) have been performed. The calculations are based on the algorithm derived from Ashley’s optical-data model including the higher-order corrections to stopping power (SP) for protons. The prominence and necessity of the higher-order corrections are demonstrated by calculating the proton SPs for the 12 solids using Ashley’s optical-data model and by comparing the calculated SPs with the experimental results, the tabulated values and other corresponding theoretical evaluations. The algorithm of evaluating the proton inelastic MFP is described. In this algorithm, the Barkas-effect correction and the Bloch correction are taken into account, the minimum impact parameter from Lindhard is used in the Barkas-effect correction, and an empirical estimation of a free parameter involved in the Bloch correction to the inelastic MFP is proposed. The evaluated inelastic MFPs of 0.05-10 MeV protons for the 12 solids under two different cases, i.e. the higher-order corrections not being considered and the Barkas-effect correction and the Bloch correction being included, are presented in the tabulated form and are first results for these solids. These numerical results provide an alternative basic data for the Monte Carlo studies on low-energy proton transport in these 12 solids.     

Classical and quantum mechanical rainbow-scattering of fast He atoms from a KCl(0 0 1) surface

Fast He atoms with energies from 200 eV up to 16 keV are scattered under grazing polar angles of incidence from a flat and clean KCl(0 0 1) surface. For scattering along low-index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. From classical and semiclassical trajectory calculations based on individual Hartree-Fock pair and density functional theory (DFT) potentials, we obtain corresponding rainbow angles for comparison with the experimental data. The calculations were performed taking into account the rumpling of K and Cl in the topmost surface layer. Fair agreement with the experimental data is found for scattering along 〈1 0 0〉 for DFT as well as individual pair potentials calculated from Hartree-Fock wave functions.     

Bonding effect on channeling of C ions in a carbon nanotube

The channeling phenomenon of carbon ions in single-wall carbon nanotubes (SWCNTs) is investigated by using the molecular dynamics simulation with analytical potentials.The relationship between the channeling critical angles in the SWCNT and the bonding interaction is analyzed.It was found that,at 200-5000 eV and 10°-20° of incident angle,the ions with the bonding interaction or chemical effect,have decreased dechanneling probabilities and increased critical angles,compared to that of non-bonding ions.So the bonding effect cannot be ignored in the channeling mechanism of carbon ions through a SWCNT.     

On a peculiarity of low-energy ion scattering from a well-ordered bcc W(2 1 1) surface

A peculiarity in the backscattering of keV He⁺ ions by a well-ordered high-purity W(2 1 1) surface is reported. Besides the normal elastic binary collision peak and the low-energy tail due to backscattering in deeper layers, an extra peak is observed for an inelastic loss of about 95 eV. This unusually large loss has a constant value over a wide range of primary energies (1.5-4.5 keV). An extra peculiarity is that the peak is only observed for the scattering in normal incidence towards the (2 1 1) plane. It is also not seen for polycrystalline W. The energy loss may be due to a quasi-double or -triple collisions of He particles with the row-trough structure of W(2 1 1) involving electronic excitation of both He and W atoms. Alternatively it may be due to a special channeling/dechanneling process for the incident ions.     

Channeling of low energy light and heavy ions in single-wall nanotubes

The Monte Carlo simulation program has been used to study low energy channeling in the single-wall nanotube and its rope, in comparisons between beam sizes and between light (He) and heavy (Ar) ions. The simulation mainly shows that the critical angle Ψ_C = 48 E^−1/2 (E is incident energy) for the He (light) ion channeling but Ψ_C = 18 E^−1/2 for the Ar (heavy) ion channeling, in the (17,0) zigzag single-wall nanotube. Thus, it might be found in the simulation that Ψ_C strongly depends on the ion mass.     

Stopping cross-sections of liquid ethanol for swift He ions

Energy loss spectra of 4.0 MeV helium ions after penetrating through liquid phase ethanol have been measured at scattering angles from 10 to 50 mrad. Experimental data were fitted with Monte Carlo simulations using a GEANT4 toolkit, and stopping cross-sections were deduced in an energy range from 1.9 to 4 MeV. Present results are in fairly good agreement with previous experimental data and SRIM2008 values. By applying the Bragg’s additivity rule, the mean excitation energy was deduced as I = 62.0 ± 1.8 eV for liquid ethanol. The present I-value is compared with other experimental and calculated results.     

The sputter cross section of a surface-vacancy island

Using molecular-dynamics simulation we investigate the effect of surface-vacancy islands on ion-induced sputtering. As an exemplary case, the sputtering of a Pt(1 1 1) surface by 5 keV Ar⁺ ions incident at 83° towards the surface normal is investigated. We find that only the ascending step of the island induces sputtering. Wide vacancy islands exhibit the direct-hit, indirect-hit and channeling zones previously identified for surface steps and adatom islands. A special role is played by the descending step edge. Even though it is not sputtered itself, it deflects ion trajectories and may direct them to the ascending step edge thus enhancing sputtering. We derive a simple criterion based on the shadow cone of the descending step to decide whether a vacancy island contributes to sputtering or not.     

Structural and optical properties of 6H-SiC helium-implanted at 600 K

Single crystals of 6H-SiC were implanted at 600 K with 100 keV He ions to three successively fluences and subsequently annealed at different temperatures ranging from 873 to 1473 K in vacuum. The recovery of lattice damage was investigated by different techniques including Rutherford backscattering spectrometry in channeling geometry, Raman spectroscopy and Fourier transform infrared spectroscopy. All three techniques showed that the damage induced by helium ion implantation in the lattice is closely related to the fluence. Rutherford backscattering spectrometry/channeling data on high temperature implantations suggest that for a fluence of 3 × 10¹⁶ He⁺/cm², extended defects are created by thermal annealing to 1473 K. Apart from a well-known intensity decrease of scattering peaks in Raman spectroscopy it was found that the absorbance peak in Fourier transform infrared spectroscopy due to the stretching vibration of Si-C bond shifted to smaller wave numbers with increasing fluence, shifting back to larger wave numbers with increasing annealing temperature. These phenomena are attributed to different lattice damage behavior induced by the hot implantation process, in which simultaneous recovery was prevailing.     

Radiation flux and damage at accelerator-driven spallation neutron sources

Radiation damage (displacement, helium, and hydrogen production) at proton-driven spallation neutron sources is analyzed and compared for SNS SB (316SS at the nose of the Hg-container vessel), SNS PEW (Al6061 at a hypothetical proton entrance window), and SINQ EW (Al-3 wt% Mg entrance window at Target 5). Spallation neutrons at the three components exhibit differential fluxes, ?′, that increase monotonically with decreasing energy E. For SINQ EW, ?’ is roughly proportional to 1/E, which is attributed to the moderating effect of the D₂O coolant and moderator tank. For 316SS at SNS SB, the calculated total displacement production rate due to protons and neutrons is 34 dpa/yr at full power, with about 37% due to protons. For the Al at SNS PEW and SINQ EW, however, the total rate is 4-5 dpa/yr, with about 90% due to protons. He and H production in all three components is dominated by the incident protons. For He, comparison of experimental and calculated production cross sections for protons on 316SS and Al indicates the need to employ the non-default Jülich ILVDEN option in running LAHET. The resulting total production rates for SNS SB, SNS PEW, and SINQ EW are about 3000, 2400, and 1900 appmHe/yr, respectively. These rates are 1.5-2 times the rates previously calculated using the default GCCI ILVDEN option. The high mobility of H atoms promotes H escape from thin targets of 316SS and Al. For 0.1 cm-thick samples, we tallied the H where it comes to rest using IOPT 14, and obtained production rates at SNS SB, SNS PEW, and SINQ EW of 11500, 4300, and 3500 appmH/yr, respectively.     

Ion mass dependence for low energy channeling in single-wall nanotubes

An Monte Carlo (MC) simulation program has been used to study ion mass dependence for the low energy channeling of natural- and pseudo-Ar ions in single-wall nanotubes. The MC simulations show that the channeling critical angle Ψ_C obeys the (E)^−1/2 and the (M₁)^−1/2 rules, where E is the incident energy and M₁ is the ion mass. The reason for this may be that the motion of the channeled (or de-channeled) ions should be correlated with both the incident energy E and the incident momentum (2M₁E)^1/2, in order to obey the conservation of energy and momentum.     

Ion radiation damage in copper

Transmission electron microscopy techniques have been used to study dislocation loop type damage as a function of depth in copper single crystals irradiated with MeV Cu, Ni and He ions at room temperature. By comparing the location of the peak in the experimental depth profiles with calculated damage energy curves, the electronic stopping powers of Cu and Ni ions in copper were deduced. The deduced electronic stopping powers have been compared with those predicted by Lindhard et al., Bricc, and the Northcliffe and Schilling tables. It was found that the deduced stopping powers agreed well with the Northcliffe and Schilling values corrected for Z₂ oscillations by the method proposed by Ward et al. In the case of 1 MeV He ions, good agreement was obtained between the observed damage peak position and that calculated using the experimental electronic stopping of Ziegler and Chu and that of White and Mueller.     

LIGHT ION REFLECTION STUDIED BY MONTE CARLO SIMULATION AND TRANSPORT THEORY

The reflection of light ions, such as H+,3He+ and 4He+, with energies of 0.1- 10 keV, from Cu and Ni surface has been studied by Monte Carlo simulation and transport theory. The Monte Carlo simulation gives the detail energy spectra for the reflected particles and their angular distribution for different incident angles. It shows that the reflected particle energy spectra can be approximately described by an analytical formula for the whole energy range, all the incident angles and different ion- target combination studied here. The reflected particle energy vs its average reflection angle to the surface normal can almost be expressed by a universal curve for all cases studied here. The reflection energy spectra are used for the calculation of the reflection coefficient by transport theory including the realistic surface correction. The present work is compared with both experimental measurement and other simulation codes.     

Fe-implanted SiC as a potential DMS: X-ray diffraction and rutherford backscattering and channelling study

Single crystalline (0 0 0 1)-oriented 6H-SiC samples were implanted at 380 °C with low-energy Fe ions (in the 100 keV range) with the aim of synthesizing so-called diluted magnetic semiconductors. X-ray diffraction and Rutherford backscattering spectrometry and channeling are used to study the microstructural changes in these Fe-implanted SiC crystals submitted to furnace annealing and laser processing, both treatments being performed in order to eliminate the implantation-induced defects.     

Enhanced beam deflection in bent crystals using multiple volume reflection

This paper presents simulations of the trajectories of high-energy ions through several bent crystal layers. At certain layer alignments volume reflection occurs from each layer and the resultant multiple volume reflection angle is correspondingly increased, along with the range of entrance angles over which ions undergo volume reflection. Another feature is that the range of entrance angles for which bent crystal channeling occurs is also increased in passing through several bent layers. The use of several bent crystal layers to produce multiple volume reflection provides an alternative approach to the design of a space shield or radiation protection at accelerators based on bent crystals.