Proton induced L23 Auger electron emission from aluminium

The Auger electron emission efficiency from aluminium irradiated by protons in the 10 to 100 keV range is calculated with the simulation program MARLOWE on the basis of the impact parameter dependent ionization probability suggested by Brandt and Lapicki. The results show that the Auger emission efficiency scales with the total L₂₃ ionization cross section. The dependence of this cross section on the incident energy is estimated by matching the calculated efficiencies to the experimental values. The obtained cross section is used to estimate the incident angle dependence of the Auger yield, which is compared with experiment. The model is found to overestimate the emission efficiency from a polycrystal for incidences close to the surface normal by less than 20 percent. This is found to result from the difference between the distributions of scattering centers close to the surface in the experiment and in the simulation model. It is thus suggested that the discrepancy between simulation and experiment is due to the defect state of a real surface.     

Online time-differential perturbed angular correlation study with an O beam - Residence sites of oxygen atoms in highly oriented pyrolytic graphite

The online time-differential perturbed angular correlation (TDPAC) method was applied to a study of the physical states of a probe ¹⁹F, the β⁻ decay product of ¹⁹O (t_1/2 = 26.9 s), implanted in highly oriented pyrolytic graphite. The observed magnitude of the electric field gradient at the probe nucleus, ∣V_zz∣ = 2.91(17) × 10²² V m⁻², suggests that the incident ¹⁹O atoms are stabilized at an interlayer position with point group C_3v. Exhibiting observed TDPAC spectra having a clear sample-to-detector configuration dependence, we demonstrate the applicability of the present online method with a short-lived radioactive ¹⁹O beam.     

Raman scattering and SEM studies of graphite and silicon carbide surfaces bombarded with energetic protons,deuterons and helium ions

Experimental investigations on the chemical and physical effects of 10–15 keV H₁⁺, D₁⁺ and He⁺ ion bombardments to fluences up to 10¹⁹ ions/cm² on graphite and SiC have been conducted using the techniques of Raman scattering and scanning electron microscopy (SEM). Raman scattering data for ion bombarded graphite reveal the formation of an amorphous surface layer as indicated by the appearance of a broad band in the spectrum centered at 1525 cm which replaces the bands due to microcrystalline carbon at 1585 cm^?1 and 1360 cm^?1. The microcrystalline structure could be partially restored upon vacuum annealing at 1040°C for several hours. A weak, broad band centered at 2150 cm also appears after bombardment which is indicative of the formation of ?C = C? bonds. Surfaces of “KT” SiC were also amorphized on ion bombardment as indicated by changes in the Raman spectra. Chemical trapping of the incident h₁⁺ and D₁⁺ ions to form bulk C-H, C-D and Si-H species was observed. Preferential sputtering of Si leaving a carbon rich surface region also occurred. Blister formation was observed in the SEM studies.     

Electron emission by highly charged neon and xenon ions on fusion-relevant tungsten and graphite surfaces

Here we report highly charged ⁴⁰Ne^q+ (q = 3-8) and ¹²⁹Xe^q+ (q = 10-30) ion-induced secondary electron emission on the tungsten and highly oriented pyrolytic graphite (HOPG) surfaces. The total secondary electron yield is measured as a function of the potential energy of incident ion. The experimental data is used to separate contributions of kinetic and potential electron yields. We estimate roughly 10% of ion’s potential energy is consumed in potential electron emission. The rest of the ion’s potential energy is responsible for the sputtering and material modification.     

Deuterium trapping in deep traps of differently oriented pyrolytic graphite exposed to D2 gas at 1473 K

Due to their importance for tritium inventories in future DT fueled fusion machines, experimental data on H isotope diffusion, absorption and retention in deep traps (E_b ≅ 4.3 eV) of graphites exposed to hydrogen at elevated temperatures have been reviewed. Deuterium retention was studied in edge- and basal-oriented pyrolytic graphite (PG) and polycrystalline RG-Ti-91 damaged by irradiation with 200 keV carbon ions. Deuterium loading was done by soaking in D₂ gas at 1473 K, and the resulting D retention was measured by nuclear reaction analysis. The microstructure was studied by cross-sectional TEM, SEM and microprofilometry. The concentration of strong traps created by irradiation and estimated by the amount of accumulated deuterium was shown to saturate with the damage above ≈1 dpa at about 1000 appm. In non-damaged and damaged graphites deuterium diffuses via porous grain boundaries and along basal planes within crystallites, while its migration through the graphite lattice along the c direction was found to be negligible. Radiation modifications of PG retard deuterium diffusion and decrease the rate of its chemical erosion by a factor of five. The amount of deuterium accumulated in strong traps in graphites is mainly influenced by their macro- and microstructure, while the degree of graphitization seems to be less important. Derivations are made of the susceptibility of damaged graphites, in particular, CFCs to the retention of hydrogen isotopes in deep traps.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

Secondary electron emission from a thin carbon foil induced by equi-velocity protons and electrons of 0.5-3.5 MeV/u

The statistical distributions of the number of simultaneously forward-emitted secondary electrons (SEs) from a thin carbon foil have been measured with H⁰ and H⁺ projectiles of 0.5-3.5 MeV in coincidence with the foil-transmitted protons, irrespective of projectile incident charge state. From these results, the probabilities of the simultaneous emission of n electrons and the SE yields by loss electrons have been evaluated. At energies of 2.5-3.5 MeV, the obtained emission probabilities and the SE yields induced by loss electrons are larger than the corresponding ones by protons of the same velocity. Furthermore, the material parameter is found to be almost constant and is slightly larger than that for the protons. On the other hand, the forward SE yields by loss electrons is decreasing to zero with decreasing the incident energy. This behavior is consistent with the range-energy relation of electrons in carbon.     

Secondary nuclear reactions in the bombardment of tin by fast protons

A radiochemical method has been used to study the capture of products arising in disintegration of target nuclei.Using the measurements of yields of radioactive isotopes of tellurium (Z=52) and iodine (Z=53) obtained from tin (Z=50) irradiated by protons with energy from 170 to 660 Mev excitation curves are plotted for the secondary reactions which lead to the formation of products with charges 2 and 3 units greater than the charge of the original nucleus. The cross sections for these reactions increases with increasing energy of the incident protons: (, x n) and (Li, xn) are respectively 18.5 ± 5 and 0.17 ± 0.1 mbarns for E_p=170 Mev and 50 ± 6.5 and 1.6 ± 0.5 mbarns for E_p=660 Mev. The cross sections for lithium capture by tin in a comparable proton energy interval are in good agreement with the results of investigations of similar reactions in copper, tin and lead but are found to be 50 times smaller than the cross sections obtained by Marquez and Perlman.The observed cross sections for the secondary lithium capture can be explained only by assuming that these nuclei acquire energies greater than that which can be obtained In evaporation or fission of the target nuclei.Secondary reactions of the (, xn) type can be explained satisfactorily on the basis of an evaporation mechanism for the formation of the helium nuclei.The authors are indebted to B. V. Kurchatov, V. G. Solov'ev, and I. Iu. Levenberg for help in carrying out the present work and to V. P. Dzhelepov, M. G. Meshcheriakov and G. A. Leskin for a number of valuable critical remarks.     

Secondary electron emission from the entrance and exit surfaces of thin carbon foils under fast ion bombardment

The total secondary electron emission (SEE) yield from the entrance and exit surfaces of thin carbon foils under fast ion (¹⁶O, ¹⁹F, ³⁵Cl) bombardment has been measured as a function of the ion energy and the ion beam current intensity. Using a retarding field, the energy distribution of secondary electrons integrated over almost all angles of emission in the backward and forward directions has also been measured. It is found that total forward emission is larger than backward emission by factors of up to 2.5, 2.7 and 3.4 for ¹⁶O⁺³, ¹⁹F⁺³ and ³⁵Cl⁺⁵, respectively. It is suggested that the enhancement of forward SEE may be partly due to effects from the instantaneous charge state of the heavy ion beam in the solid in addition to the binary collisions of the projectile with individual electrons in the target. It is also shown that the total SEE yield from the entrance and exit surfaces of the target foils decreases with ion beam current intensity; this may be a beam-induced temperature effect. The total SEE yield in both the forward and backward directions is less sensitive to surface conditions for high velocity ions than for low velocity ions and the total yield from both surfaces of the foils is proportional to the ion stopping power in the target, where the constant of proportionality depends on the properties of material.     

Influence of the target thickness on the backward and forward electron emission characteristics induced by protons incident on thin carbon foils

The forward (γ_F) and backward (γ_B) electron emission yields have been measured for protons incident on thin carbon foils for incident energies between 2 and 7 MeV as a function of the target thickness. Comparisons with theoretical results obtained by Monte Carlo simulations are presented. In particular, the Meckbach factor R_γ = γ_F/γ_B is discussed.     

离子束增强沉积碳膜及其在抑制电子发射中的应用

栅电子发射是影响行波管工作性能和制约其使用寿命的主要因素。国内研制的导弹雷达制导用行波管因存在严重的栅电子发射现象,寿命只有70h,不能付诸军事应用。采用离子束增强沉积的方法在栅网表面沉积一层碳膜后,可有效地抑制栅电子发射,使行波管寿命超过了1000h,在军事和航天领域获得了成功应用。报导了低能离子束增强沉积类金刚石薄膜和高能离子束增强沉积类石墨碳膜的制备技术、结构性能及其在抑制栅电子发射中的应用和机理研究的结果。     

Kinetic electron emission from metals induced by impact of slow atomic particles

Available experimental data on the kinetic electron emission from metals bombarded by low energy atomic particles below the classical threshold were analyzed in terms of one-electron non-adiabatic model and in terms of phenomenological many-electron models. Total electron yields as a function of the particle velocity for several distinctly different substrate-particle systems were successfully interpreted using a phenomenological model. The essential microscopic mechanism of the model is the valence electron Auger interaction which leads to the Boltzmann - like energy distribution of the excited electrons in the impact zone.     

Ion induced high energy electron emission from copper

We present measurements of secondary electron emission from Cu induced by low energy bombardment (1-5 keV) of noble gas (He⁺, Ne⁺ and Ar⁺) and Li⁺ ions. We identify different potential and kinetic mechanisms and find the presence of high energetic secondary electrons for a couple of ion-target combinations. In order to understand the presence of these fast electrons we need to consider the Fermi shuttle mechanism and the different ion neutralization efficiencies.