Fusion neutron induced radioactive emissions from surfaces

Neutron induced direct nuclear recoil sputtering ratios have been measured for a variety of 14.8 MeV (d, t) neutron induced reactions in Nb, Mo, V and 316 SS. Absolute recoil sputtering ratios for forward and backward sputtering are reported. Forward sputtering ratios are typically in the range of 10^?9–10^?7 recoil atoms per (d, t) neutron while backward sputtering ratios are usually several orders of magnitude lower. Some of the implications of radioactive particle ejection in the first wall region of fusion reactors are discussed. It is shown that radioactivity ejected into the fusion reactor coolant channels by direct nuclear recoil and by lattice dynamic neutron sputtering, may have a significant effect on the design and maintenance of fusion reactors.     

Particle induced X-ray emission (PIXE) analyses on metal samples with structured surfaces

The X-ray yields obtained in PIXE measurements from metal samples with bent or relief surface have been investigated experimentally and by a computer simulation. It was found, that in the fundamental parameter method correct analytical results will be obtained, if two conditions are fulfilled: (1) the inclination of the tangential plane of the bent surface at the point where the ion beam hits this surface has to be known; and (2) the radius of curvature of the surface structure must be larger than 3 to 4 times the ion beam radius used.     

Argon ion impact desorption cross sections of chlorine and carbon from molybdenum

Total desorption cross sections have been measured for Cl (σ_Cl) and C(σ_C) on molybdenum by argon ion bombardment for an incidence angle of 60° from the surface normal. For the bombardment an ion gun with low current density (i₀ ~ 1 × 10 ^?7 A cm^?2) at low system pressure (~10^?9 Torr) was used. The detection was performed by AES and the data were sensitivity factor corrected. The AES analysis of the surface after adsorption showed that Mo, C and Cl contributed to more than 94% of the atomic composition. With known i₀, it is possible to obtain σ from the adsorbate signal vs ion bombardment time curve. For ion energies between 0.2 keV to 1.0 keV the measured value for σ_Cl and σ_C are 0.5?3 × 10^?15 cm² and 0.2?4 × 10^?15 cm², respectively. The possible effects of the surface roughness due to prebombardment are discussed.     

K X-ray emission induced by C and O ion impact on selected lanthanoids

Measurements of K-shell X-ray production cross sections by ¹²C⁴⁺ (beam energies between 12 MeV and 14 MeV), and ¹⁶O⁵⁺ ions (with energies between 12.5 MeV and 15 MeV) are presented. The target elements were selected lanthanoids (Ce, Gd, Dy, Ho and Er). The resulting measurements are evaluated through comparisons with the eECPSShsR-UA theory, the MECPSSR model and the adiabatic perturbation (also known as direct molecular orbital, MO) theory, using a scaling based on the reduced velocity parameter . Consideration is given to multiple ionization effects and electron capture contribution to K-shell ionization. An evaluation with previously published values is also given. It is shown that the behavior of the ratios of experimental to theoretical cross sections is different for both ions. The models do not seem to be accurate to predict the X-ray production cross sections for ¹²C⁴⁺ ions, while the MECPSSR theory predicts much better the experimental data for ¹⁶O⁵⁺ than the eECPSShsR-UA.     

Hydrogen ion desorption from amorphous carbon films induced by resonant core electron excitations

In order to get insight into the mechanism of structural change in tetrahedral amorphous carbon (ta-C) films that is induced by soft X-ray illumination at photon energies near the carbon core edge, the desorption of ions from ta-C films, as a possible process taking place concurrently with the photo-induced restructuring, was studied by time-of-flight (TOF) measurements of photo-ions as a function of photon energy. The results show that (1) the main ions detected are H⁺, (2) the desorption efficiency spectra exhibit a resonant peak at 286-287 eV which is common to all detected ions, and (3) is 3 eV lower than the resonant peak in the efficiency spectrum of photo-induced restructuring. These rule out the hypothesis that it is the photo-induced C-H bond rupture that causes the resonant soft X-ray-induced restructuring in ta-C films.     

Ion angular distribution of species desorbed from single crystal surfaces by electron impact

The measurement of the angular distribution of desorbing positive ions produced by electron impact desorption (ESDIAD) is of fundamental importance in understanding molecular structure in the chemisorbed layer. In this short review, two applications of ESDIAD to structural problems in the adsorbed layer will be described. Examples of strong chemisorption and weaker physical adsorption effects will be discussed. In addition, interactions between adsorbed species, leading to changes in bonding geometry will be described. The apparatus used for this work allows digitized acquisition of ion angular distributions in the absence of background effects due to soft X-ray emission stimulated by electron impact.     

X-ray characterization of surfaces irradiated with highly charged ions

Highly charged ions (HCI) approaching, touching or penetrating dielectric surfaces extract many electrons of the solid leading to the formation of permanent surface modifications. The ions which capture the electrons in their outermost shells form hollow atoms which emit X-rays during their decay to the ground state. In this paper one presents experiments showing that these X-rays) allow diagnosing the electric nature of the surfaces. HCI while modifying the structure of surfaces may then also be used to diagnose these changes on line or off line.     

Characterisation of ferromagnetic magnetic storage media surfaces by complementary particle induced X-ray analysis and time of flight-energy dispersive elastic recoil detection analysis

Thin (10 nm–1 μm) films of ferromagnetic material constitute an important class of materials that are difficult to analyse by conventional ion beam analytical (IBA) techniques because they are based on the ferromagnetic elements (Co, Fe, Mn, Ni, and Cr). The similar or overlapping isotope masses makes it difficult to separate the elemental signals using time of flight and energy dispersive elastic recoil detection (ToF-E ERD). In this exploratory study we have investigated the use of Particle Induced X-ray Emission (PIXE) measurements to refine the mass dispersive depth profile information from ToF-E ERD. The surfaces of two commercial magnetic media were investigated. One sample was a double density diskette with a coating of ferrite particles in an organic binder. The other sample was a complex C/Co/Cr/Ni–P/Al multilayer structure taken from a standard hard disc. The Lund nuclear microprobe with a 2.55 MeV proton beam was used for PIXE analysis. ToF-ERD measurements were carried out using a 55 MeV ¹²⁷I¹⁰⁺ ion beam incident at 67.5° to the surface normal. The time of flight and kinetic energy of recoils ejected at 45° to the ion beam direction was measured in a detector telescope. The findings demonstrate that by detailed analysis of the PIXE spectra it is possible to remove the ambiguities in mass assignment of the ToF-ERD data associated with the ferromagnetic elements.     

News on sputter theory: Molecular targets, nanoparticle desorption, rough surfaces

Sputtering theory has existed as a mature and well-understood field of physics since the theory of collision-cascade sputtering has been developed in the late 1960s. In this presentation we outline several directions, in which the basic understanding of sputter phenomena has been challenged and new insight has been obtained recently.Sputtering of molecular solids: after ion impact on a molecular solid, not all of the impact energy is available for inducing sputtering. Part of the energy is converted into internal (rotational and vibrational) excitation of the target molecules, and part is used for molecule dissociation. Furthermore, exothermic or endothermic chemical reactions may further change the energy balance in the irradiated target.Nanoparticle desorption: usually, the flux of sputtered particles is dominated by monatomics; in the case of a pronounced spike contribution to sputtering, the contribution of clusters in the sputtered flux may become considerable. Here, we discuss the situation that nanoparticles were present on the surface, and outline mechanisms of how these may be desorbed (more or less intact) by ion or cluster impact.Rough surfaces: real surfaces are rough and contain surface defects (adatoms, surface steps, etc.). For grazing ion incidence, these influence the energy input into the surface dramatically. For such incidence angles sputtering vanishes for a flat terrace; however, ion impact close to a defect may lead to sputter yields comparable to those at normal incidence. In such cases sputtering also exhibits a pronounced azimuth and temperature dependence.     

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.     

A molecular dynamics investigation of kinetic electron emission from silver surfaces under varying angle of projectile impact

We present a computer simulation study on the influence of the impact angle of the projectile on kinetic electron emission yields for 5-keV Ag → Ag bombardment. By means of a hybrid computer simulation model incorporating (i) the particle dynamics following the primary particle impact, (ii) the kinetically induced electronic substrate excitations via electronic friction and electron promotion and (iii) the transport of excitation energy away from the spot of generation, a full three-dimensional electron temperature profile within the volume affected by the atomic collision cascade is calculated. This profile is evaluated at the very surface of the target and taken as input for a thermionic model (‘hot-spot-model’) for kinetic electron emission. Averaging the results for different choices of the polar angle of incidence Θ over a large set of impact points, the obtained kinetic electron emission yields can be compared with experimental data and predictions from simple geometrical calculations. The presented simulation results appear to be reasonable in comparison with experimental data as well as with simple geometrical considerations of kinetic electron emission under oblique incidence.     

Electron beam induced charging of Cu/MgO surfaces

The Auger beam surface charge build-up during deposition of thin layers of Cu upon MgO(100) and MgO(111) crystal surfaces was investigated. A three step model is proposed: 1) copper induced electron trapping, 2) formation of local conduction band in a thin metal film, and 3) formation of continuous isolated metal.     

Thermal desorption of helium from reactor steel

Thermal desorption was used to study the behavioral particulars of the ion-implanted at 20 and 650°C helium in reactor ferrite-martensite EP-450, Eurofer-97, dispersion-hardened EP-450, Eurofer-97, and EK-164 austenitic steel. It is established that compared with the thermal desorption spectrum of EP-450 and Euroffer-97 steel the temperature interval for helium release from dispersion-hardened EP-450 and Eurofer 97 steels is wider for helium implantation at room temperature and especially at 650°C. In steel hardened by disperse oxides Y₂O₃, a substantial amount of helium is released at high temperatures after the main maximum in the thermal desorption spectrum as a result of the formation at the incoherent particle–matrix boundary of bubbles with high bonding energy with particles. In contrast to the ferrite-martensite steel irradiated in the temper state, the thermal desorption spectrum of EK-164 austenitic steel is more complex because of the multiple stage helium release due to the structural defects introduced by cold deformation.     

An inelastic thermal spike model to calculate ion induced desorption yields

Ion induced desorption is a severe luminosity limitation for low charge state heavy ion accelerators. Therefore, it was intensively investigated in dedicated experiments during recent years. Several experimental results were obtained providing numerous desorption yields for different ion beam parameters and different materials as well as surface treatments. The heavy ion induced desorption was identified as a pure surface cleaning effect. Nevertheless it was shown that the yields have a strong link to the irradiated material. The initial desorption yield decreases during the irradiation reaching a dynamic equilibrium. Desorption yields of several hundred molecules per incident ion from one monolayer adsorbed gas can not be explained with the geometrical cross section of the projectile. Therefore we have expanded the inelastic thermal spike model to describe the process as thermal desorption from a microscopic heated region. The obtained results of this extended model represent very well the numbers from many experimental studies.     

大型X射线医疗设备的环评分析

通过一个环评实例阐述大型X射线医疗设备销售项目环境影响评价的基本分析要点,给出了正常工况下和事故工况下环境影响分析过程和剂量估算方法。大型X射线医疗设备销售的环境影响评价要点是:根据项目实际情况,确定污染因子和管理限值,采用类比、现场实测加计算的评价、预测方法,区分正常工况与事故工况,对污染进行分析、监测、计算和预测。