Calculation of Auger-neutralization probabilities for He-ions in LEIS

In Low Energy Ion Scattering (LEIS), Auger-neutralization is an omnipresent charge exchange mechanism, especially when noble gas ions are used as projectiles, with a primary energy below the threshold energy, E_th, for collision induced charge exchange processes (neutralization and reionization). Recent experiments revealed a significant dependence of the ion survival probability, P⁺, on the crystal plane, when He⁺ ions are scattered from a metal surface. This is in contrast to the fact, that the neutralization probability in LEIS is usually assumed to be independent of the chemical environment of the collision partner (absence of matrix effects). In order to investigate this crystal effect, an existent theory on Auger-neutralization (based on a Linear Combination of Atomic Orbitals) is adapted to the LEIS geometry. With this model, Auger-neutralization rates are calculated for a Ag(1 1 0) surface. Trajectories for He particles scattered from this surface into different azimuth directions are obtained by means of Molecular Dynamics simulations. Subsequently, the ion survival probability is calculated and compared to measurements. Good agreement is obtained which gives confidence in the applicability of this model in the LEIS regime. Moreover, it was possible to obtain detailed information on the properties of the neutralization process.     

Oxygen deficiency and excess of rutile titania (1 1 0) surfaces analyzed by ion scattering coupled with elastic recoil detection

Oxygen deficiency and excess of rutile titania (TiO₂) surfaces are important factors for catalytic activities of metal nano-particles on the TiO₂ supports. Medium energy ion scattering (MEIS; 80 keV He⁺) coupled with elastic recoil detection analysis (ERD; 150 keV Ne⁺) can determine the numbers of bridging O (O_br) vacancies (V_O) and excess O atoms adsorbed on the 5-fold Ti rows of TiO₂(1 1 0) surfaces. The amounts of V_O and adsorbed O were derived by H₂O and ¹⁸O₂ exposure followed by ERD and MEIS analyses, respectively. The present analysis revealed that only about a half of V_O are filled and a comparable amount of O atoms are adsorbed on the reduced TiO₂(1 1 0) surface after exposure to O₂ (1000 L; 1 L = 1 × 10⁻⁶ Torr s) at room temperature (RT). We also detected the adsorbed O for the hydroxylated TiO₂(1 1 0) after ¹⁸O₂ exposure at RT. Finally, it is shown that the O adsorbed on the Ti rows reacts with CO probably to form CO₂ at RT. Based on the results obtained here, we clarify the reason why only a half of V_O are filled by exposing reduced surface to O₂ at RT and what is the primary source of subsurface excess electronic charge, which acts as a leading part of the surface electrochemistry and gives the defect state in the band gap seen in the valence band spectra for reduced and hydroxylated TiO₂(1 1 0) surfaces.     

Neutralization rate for slow Ar ions in front of KCl(0 0 1)

Charge state distributions of reflected ions are measured when 5 keV Ar^q+(q = 0−2) ions are incident on a clean KCl(0 0 1) surface at grazing angle, θ_i. Although the charge state distribution does not depend on the incident charge state at larger θ_i, significant dependence of the charge state distribution on incident charge state is observed at smaller θ_i. The ionization of Ar⁰ is completely suppressed at θ_i < 20 mrad, while large neutralization probability is observed for Ar⁺ incidence. These features allow us to derive the position-dependent neutralization rate of Ar⁺ in front of KCl(0 0 1). The obtained neutralization rate decreases exponentially with distance from the surface as it is usually assumed.     

Surface and sub-surface oxidation of α-Cu-(17 at.%)Al(1 0 0) studied by X-ray photo-electron spectroscopy and low energy He scattering spectroscopy

In this study the initial stages of oxidation of the α-Cu-(17 at.%)Al(1 0 0) single crystal oriented alloy surface was investigated by X-ray photo-electron spectroscopy and low energy He⁺ scattering spectroscopy. It was found that oxygen adsorption can be divided in two sequential stages: (i) a fast process, up to 15–20 L exposure, in which oxygen adsorbs on the alloy’s surface resulting mostly in the formation of Al–O chemisorbed bonds; and (ii) a slower process during which oxygen adsorbs forming Cu–O and Al–O chemisorbed bonds concurrent with diffusion of O to the sub-surface and Al segregation to the surface region. The surface oxidation rate is much higher than the sub-surface one. The rate of Al segregation increases with oxygen exposure and involves both surface and sub-surface regions. Annealing of the oxidized alloy surface results in a pronounced segregation of Al and formation of an aluminum oxide layer.     

Anisotropic rearrangement of the substrate atoms during Ar bombardment on Pd(0 0 1) surface

Using a three-dimensional molecular dynamics (MD) simulation, we investigated the atomic scale rearrangement that occurs on a Pd(0 0 1) surface after energetic bombardment by Ar at room temperature. High energy Ar bombardment provoked the significant rearrangement of Pd atoms in a ballistic manner with a fourfold symmetric lateral distribution aligned along the 〈1 1 0〉 direction. The MD simulation of uniform Ar bombardment at normal incidence on a Pd surface reproduced the experimentally observed fourfold symmetric nano-scale surface structure. The present result supports that the ballistic rearrangement of the substrate atoms plays an important role in the ion induced surface structure evolution.     

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 regimes in ion surface scattering

We report on surface channeling experiments of singly charged ions on single crystal surfaces of Pt(1 1 0) and Pd(1 1 0). Using a time-of-flight system installed in forward direction we analyze the energy distribution of the scattered projectiles. By variation of the primary energy and the angle of incidence we investigate effects of the perpendicular energy on the channeling features. The perpendicular energy is defined as E = E₀sin²ψ with ψ the angle of incidence. In combination with precise azimuthal rotations of the crystal, we are sensitive to axial channeling and obtain information about the limits of axial surface channeling. From a comparison with detailed trajectory calculations we find that axial channeling effects are most pronounced for a perpendicular energy between 5 and 20 eV. As a result, we obtain an exemplary channeling map for the interaction of nitrogen ions with the (1 × 2) reconstructed Pt(1 1 0) surface identifying different channeling regimes.     

Mapping ion beam induced current changes in a commercial MOSFET

We demonstrate a novel nuclear microprobe imaging and analysis modality for micrometre-scale field effect transistor devices probed with focused beams of MeV ions. By recording the drain current as a function of time during ion irradiation it is possible to identify current transients induced by the passage of single ions through the sensitive structures of the device. This modality takes advantage of the fact that the ionization produced by the passage of a single ion acts in an equivalent way to a transient change in the gate bias which therefore modulates the drain current as a function of time. This differs from the traditional ion beam induced charge technique where the ionization drifts in an internal electric field and induces a single charge pulse in an electrode applied to the device. Instead a richer variety of phenomena are observed, with different time constants which depend on the proximity of the ion strike to the channel of the device. The signals may be used to examine device function, radiation sensitivity or to count ion impacts within the channel.     

110keV56Fe^1^＋离子注入麦胚中的能量沉积分布

测量１１０ＫＥｖ５６Ｆｅ＾１＾＋离子注入麦胚中的射程为２５７．１ｎｍ。根据测得的＾５＾６Ｆｅ＾１＾＋离子相对浓度随深度的分布和注入离子的总面密度，求得了＾５＾６Ｆｅ＾１＾＋离子绝对浓度随深度的分布。采用ＴＲＩＭ８８程序计算得到了不同浓度上注入离子的阻止本领，从而求得了随深度的能量沉积分布。并讨论了细胞损伤的可能机理。     

Plasmon excitation and electron promotion in the interaction of slow Na ions with Al surfaces

We studied the energy distributions of electrons emitted in the interaction of slow Na⁺ ions with polycrystalline Al surfaces. To study sub-threshold plasmon excitation we performed measurements as a function of emission angle which showed the excitation of bulk plasmons, confirming the kinetic nature of the excitation process.

Electron spectra show narrow transition lines due to Auger decay in vacuum of sodium projectiles excited in the 2p-shell by electron promotion in collisions with Al target atoms. Several previously unidentified transition lines could be attributed to the autoionization of doubly excited projectiles.     

Patterned microstructures formed with MeV Au implantation in Si(1 0 0)

Energetic (MeV) Au implantation in Si(1 0 0) (n-type) through masked micropatterns has been used to create layers resistant to KOH wet etching. Microscale patterns were produced in PMMA and SU(8) resist coatings on the silicon substrates using P-beam writing and developed. The silicon substrates were subsequently exposed using 1.5 MeV Au³⁺ ions with fluences as high as 1 × 10¹⁶ ions/cm² and additional patterns were exposed using copper scanning electron microscope calibration grids as masks on the silicon substrates. When wet etched with KOH microstructures were created in the silicon due to the resistance to KOH etching cause by the Au implantation. The process of combining the fabrication of masked patterns with P-beam writing with broad beam Au implantation through the masks can be a promising, cost-effective process for nanostructure engineering with Si.     

Measurement of neutron capture cross-section of the Ga(n, γ)Ga reaction at 0.0536 eV energy

The neutron capture cross-section for the ⁷¹Ga(n,  γ)⁷²Ga reaction at 0.0536 eV energy was measured using activation technique based on TRIGA Mark-II research reactor. The ¹⁹⁷Au(n, γ)¹⁹⁸Au monitor reaction was used to determine the effective neutron flux. Neutron absorption and γ-ray attenuation in gallium oxide pellet were corrected in determination of cross-section. The cross-section for the above reaction at 0.0536 eV amounts to 2.75 ± 0.14 b. As far as we know there are no experimental data available at our investigated energy. So far we are the first, who carried out experiment with 0.0536 eV neutrons for cross-section measurement. The present result is larger than that of JENDL-3.3, but consistent within the uncertainty range. The value of ENDF/B-VII is higher than this work. The result of this work will be useful to observe energy dependence of neutron capture cross-sections.     

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.     

Verification of SEU resistance in 65 nm high-performance SRAM with dual DICE interleaving and EDAC mitigation strategies

A dual double interlocked storage cell(DICE)interleaving layout static random-access memory(SRAM)is designed and manufactured based on 65 nm bulk com-plementary metal oxide semiconductor technology.The single event upset(SEU)cross sections of this memory are obtained via heavy ion irradiation with a linear energy transfer(LET)value ranging from 1.7 to 83.4 MeV/(mg/cm2).Experimental results show that the upset threshold(LETth)of a 4 KB block is approximately 6 MeV/(mg/cm2),which is much better than that of a standard unhardened SRAM with an identical technology node.A 1 KB block has a higher LETth of 25 MeV/(mg/cm2)owing to the use of the error detection and correction(EDAC)code.For a Ta ion irradiation test with the highest LET value(83.4 MeV/(mg/cm2)),the benefit of the EDAC code is reduced significantly because the multi-bit upset pro-portion in the SEU is increased remarkably.Compared with normal incident ions,the memory exhibits a higher SEU sensitivity in the tilt angle irradiation test.Moreover,the SEU cross section indicates a significant dependence on the data pattern.When comprehensively considering HSPICE simulation results and the sensitive area distri-butions of the DICE cell,it is shown that the data pattern dependence is primarily associated with the arrangement of sensitive transistor pairs in the layout.Finally,some sug-gestions are provided to further improve the radiation resistance of the memory.By implementing a particular design at the layout level,the SEU tolerance of the memory is improved significantly at a low area cost.Therefore,the designed 65 nm SRAM is suitable for electronic systems operating in serious radiation environments.     

Preparation of ion-track membranes of poly(p-phenylene terephthalamide): Control of pore shape by irradiation with different ion beams

The preparation of ion-track membranes of thermally stable poly(p-phenylene terephthalamide) (PPTA) was performed by ion beam irradiation followed by chemical etching with a sodium hypochlorite solution. Cylindrical pores were observed in the membrane irradiated with ¹⁹⁷Au and ²³⁸U ions at an energy of 11.1 MeV/n. In contrast, funnel shape pores appeared in the membrane irradiated with ⁸⁴Kr, ¹⁰²Ru and ¹²⁹Xe ion at energies of 6.2, 3.6 and 3.5 MeV/n, respectively. The ¹⁹⁷Au and ²³⁸U ion irradiation was found to exhibit more than four times larger sensitivity to the track etching under the same etching conditions. Consequently, the pore shape can be controlled by the masses and energies of the irradiated ions, in close relation to the etching sensitivity of the track.     

A method to measure transit time spread of photomultiplier tubes with Cherenkov light

With the technology of coincidence method, transit time spreads (TTSs) of PMTs are measured with Cherenkov light on the photocathode window of photomultiplier tube (PMT) produced by cascade γ radiation source,~(60)Co. TTSs with single and multi-photoelectron which are analyzed by convolution of Poisson and Gaussian are obtained. The study to XP2020 and XP2020Q PMTs shows that TTSs are consistent with data supplied by Philips.TTSs are inversely proportional to the square roots of number of photoelectrons. The method is feasible when single and multi-photoelectron can be distinguished.     

Experimental determination of photofission cross-sections of Th using electron accelerator

Photofission cross-section of ²³²Th was measured using Bremsstrahlung radiation energy 7.4–9.2 MeV with energy step of 0.3 MeV by employing Lexan polycarbonate film as Solid State Nuclear Track Detectors (SSNTD). The photon intensity from the Microtron accelerator facility was estimated to be 10¹⁰ photons/s at a distance of 15 cm from the Bremsstrahlung converter using EGS-4 code (Nelson et al., 1985). Photofission cross-sections were evaluated using fission fragment angular distribution measurements. The present experimental results were compared with EMPIRE-2.19 (Herman et al., 2005) code prediction of RIPL-1 and RIPL-2 ( and ) and a new analytical formula (Gupta and Saxena, 2005) for fission barrier.     

A pulse shape discrimination of CsI(T1) crystal with 6He beam

The performance test of a CsI(T1) crystal (70×27×23 mm~3) was performed by applying the pulse shape discrimination technique for identification oflight charged pafflcles , The crystal is coupled to a photomultiplier tube during an experiment with ~6He beam. The pulse waveform is fully recorded by employing a high precision digital oscilloscope. The fast and slow gates are used for the pulse shape discrimination and the best values for the gate widths were determined to be 0.5 μs and 1.67 μs, respectively. The ~6He, ~4He and ~3He are successfully discnminated with this technique.     

Contribution of near-edge processes to attenuation of the characteristic X-rays in elements with 48 ≤ Z ≤ 83

Attenuation of the characteristic K X-rays in the ₄₈Cd, ₅₀Sn, ₅₂Te, ₆₄Gd, ₆₅Tb, ₆₆Dy, ₆₈Er, ₇₄Ta, ₇₅Re, ₇₉Au, ₈₂Pb and ₈₃Bi elements have been measured with especial emphasis for the X-ray energies (E_in) in the region of respective K-shell/L_i subshell (i = 1, 2, 3) ionization threshold (B_K/B_Li). The characteristic X-rays were obtained from different fluorescent target elements excited by the X-rays and γ-rays emitted from the ⁵⁵Fe and ²⁴¹Am radioisotopes, respectively. The measurements were performed using an energy-dispersive detection set up involving a low-energy Ge detector. The measured attenuation coefficients for the X-rays with energies away from ionization thresholds of the attenuator element are found to be in good agreement with the available theoretical coefficients, which incorporate contributions of the photoionization, and the Rayleigh and Compton scattering processes. However, the measured attenuation coefficients are found to deviate significantly from the theoretical values for the X-rays with energies in vicinity of B_K/Li. The observed alteration is attributed to the X-ray Absorption Fine Structure (XAFS) for negative B_K/Li − E_in values, and the K-shell/L_i subshell resonant Raman scattering (RRS) process for positive B_K/Li − E_in values. Systematic of the K-shell/L_i subshell RRS contribution to attenuation of the X-rays are discussed in terms of the respective oscillator density and fraction of electrons available in the K-shell/L_i subshell Lorentzian profile of the attenuation element below E_in.