Simulation of radiation damage in materials under high fluence ion bombardment

The results of analytical estimations and computer simulation of a radiation damage level dpa are presented and discussed taking into account the material sputtering under ion bombardment. It is shown that the calculations of a stationary level of radiation damage are necessary for the interpretation of regularities of a radiation damage in materials under high fluence ion bombardment.     

Ion implantation induced by Cu ablation at high laser fluence

High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 10⁶ J/cm², the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis.     

Anisotropic energy distribution of sputtered atoms induced by low energy heavy ion bombardment

The theory of anisotropic sputtering published in Phys. Rev. B 71(2), 026101 (2005) and Radiat. Effects Defects Solids 159(5), 301 (2004) has been modified and used to calculate the sputtering yield energy distributions for copper, tungsten, and aluminum targets bombarded by low-energy argon ion. As usual, the electronic stopping is ignored in the analysis. The present theory (modified Sigmund’s theory) has been shown to fit the corresponding experimental results of sputtering yield energy distributions well, except for the cases where the larger ion incident angle and larger sputtering emission angles were considered. The larger discrepancy between the present theory and the experimental result in the latter cases is probably due to the influence of direct recoil atoms on the energy spectrum. Compared with Falcone’s analytical theory, the present theory can reproduce much better experimental results of sputtering phenomena. The fact clearly demonstrates the intrinsic relation between the ion–energy dependence of the total sputtering yield and the sputtering yield energy distribution and suggests the great importance of momentum deposited on the target surface in the physical sputtering     

Resistance changes in thin metallic films under ion bombardment

Resistance changes in thin films of copper, aluminium and bismuth have been studied under the bombardment of nitrogen, carbon and argon ions. Variations in resistance with implantation dose have been observed upto doses of ∼ 3 × 10¹⁷ ions/cm² for ion energies in the range 40 to 120 keV. The results are discussed in terms of desorption of gases from the film and a composite action of sputter removal of the film and its structural changes upon ion bombardment. A simple theoretical model is discussed which can qualitatively explain the experimental observations.     

A reversion of atomic segregation under ion bombardment of Nix Pdy alloys

The angular distribution of atoms sputtered from Ni_xPd_y alloys (x, y=1, 5) under 3 and 10 keV Ar⁺ ion bombardment has been studied experimentally and using computer simulations. A collector technique combined with RBS to analyze the distribution of collected material was used. It was found that the Pd/Ni yield ratio increases with the polar ejection angle θ for all targets excluding NiPd₅. This peculiarity of sputtering was explained by a reversion of atomic segregation at high initial concentrations of Pd atoms in the target.     

Simulation of energy and fluence dependence of heavy ion induced displacement damage factor in bipolar junction transistor

This article presents the theoretical calculation of the variation of displacement damage factors as a function of energy and rad equivalent fluence in bipolar junction transistor for various particulate radiation viz., He, Si, Cl, Ti, Ni, Br, Ag, I, and Au. The calculation is based on the experimental data on γ-ray induced gain degradation in a commercial space borne BJT (2N3019). The method involves the calculation of γ-ray dose (rad(Si)) equivalent of effective particle fluence. The linear energy transfer (LET) in silicon for different particle radiation obtained from TRIM calculation has been used for the conversion of γ-dose into fluence of various particles. The estimation predicts a smooth increase in the displacement damage factor as the mass of the ion increases. Further, the displacement damage factor reaches a maximum at the same value of energy, which corresponds to maximum LET for all heavy ions. The maximum value of damage factor marginally decreases with increasing ion fluence for an ion of given energy. The results are compared with the data available in the literature for proton, deuteron, and helium induced displacement damage.     

Ion bombardment of alkali halides,II.: Effects observed at high fluence

Color-center effects unique to crystals irradiated to high fluence : 10¹⁶ particles/cm²) are reported for KCl and other alkali halides. These include (1) a minimum in F-center concentration at the end of the proton range and (2) an n-center distribution profile which changes with time in the dark at room temperature after irradiation. The first effect is due to annihilation of F centers by hydrogen atom capture (U-center formation) and the second is due to F-center diffusion and aggregation in a heavily damaged region of the crystal.     

The variation of defect damage with depth in molybdenum irradiated with 2MeV nitrogen ions

Transmission electron microscopy has been used to examine the damage produced in specimens of high purity molybdenum following irradiation with 2 MeV nitrogen ions in the Van de Graaff accelerator. A simple electropolishing technique was developed to allow the defect damage to be examined at different depths on a single specimen. Some results are presented here for specimens irradiated in the range 600–800°C, together with theoretical predictions of the nitrogen range and the variation of displacement damage with range, The experimental results show that the defect damage to the lattice results in a structure of dislocation loops, interstitial in nature, whose size drops with distance from the bombarded surface and whose concentration increases. Nevertheless the retained defect damage, measured by integrated loop area, agrees reasonably with the theoretical variation of displacement damage with range while the increase in loop concentration with depth coincides with the variation in ratio of nitrogen concentration to displacement damage. This latter result agrees with the hypothesis that the nucleation of damage is controlled to a large extent by the ratio of impurity atoms to displaced atoms.     

Angular distribution of neutron spectral fluence around phantom irradiated with high energy protons

Extended Bonner Spheres spectrometer was used to measure the angular distribution of neutron spectral fluence around NYLON6 phantom irradiated with pencil beam of 100, 150 and 200 MeV protons at the Proton Therapy Center Praha. Measurements were supplemented by a calculation of neutron spectral fluences at different depths of the phantom. The calculation of neutron spectral fluence at different depth of the phantom demonstrated that the majority of high energy neutrons was generated at the beginning of the proton trajectory in the phantom and the neutron yield decreased with increasing depth, with a minimum at the depth corresponding to the Bragg peak. Therefore, attention should be paid not only to the tissue behind the irradiated volume, but also to the preceding tissue. However, the neutron spectral fluence in the vicinity of the treated tissue can only be determined by calculation, mainly due to the dimensions of the neutron spectroscopic instrumentation. This paper presents a suitable technique and experimental conditions to acquire reliable data necessary for the proper determination of neutron spectral fluence. From the measured spectral fluences, the neutron fluence in whole-range and partial energy intervals were determined together with the corresponding ambient dose equivalents at measurement positions. The obtained results indicate that high energy neutrons predominate at the direction of the proton beam and more neutrons are generated by higher energy protons.     

Implications of neutron spectrum and flux differences on fission-fusion correlations at high neutron fluence

Abstract

The application to fusion environments of materials data derived from fission reactors involves considerations related not only to neutron spectra but also the often dominant effect of displacement rate. It is shown in this paper that fission-fusion correlation experiments directed toward helium effects and PKA recoil spectra are frequently difficult to interpret due to the strong influences of displacement rate, low energy recoils from thermal neutron absorption and in some cases a large influence of solid transmutants. It is also shown that materials data published in earlier decades must be reevaluated in light of recent advances in defining irradiation parameters.     

Secondary ion emission from organic films under bombardment with a 32S beam at 3 MeV/A

Abstract

An experimental investigation of the secondary ion emission from organic films under heavy ion bombardment at high energies was carried out by using a time-of-flight mass spectrometer coupled to a Tandem heavy ion accelerator. Preliminary results obtained at 3 MeV/A are reported and discussed.     

密闭实验大厅内散射中子注量的模拟研究

为能够快速评判实验测试方案和预估实验结果,建立了中子体通量的快速估算模型。理论上,封闭空间中子平均体通量与特征长度的平方成反比,且中子体通量的大小能反映散射中子注量强弱。采用蒙特卡罗模拟方法,计算得到了密闭实验大厅内中子的体通量,以及不同位置处的散射中子注量,并将模拟得到的体通量和散射中子注量拟合成便于工程实践中应用的解析表达式,拟合结果与模拟结果的相对偏差小于10%。研究结果表明,球形空间内中子的体通量与球半径的1.905次方成反比;密闭实验大厅的中子体通量与大厅横截面宽度的1.948次方成反比,与长宽比的0.775次方成反比;球形空间结构内,每个源中子的平均径迹长度约为半径的5.4倍,而长方体密闭实验大厅内,单个源中子的平均径迹长度为大厅特征尺度的2~3倍。     

The loss of boron in ultra-shallow boron implanted Si under heavy ion irradiation

Heavy ion impact has been known to cause a loss of light elements from the near-surface region of the irradiated sample. One of the possible approaches to a better understanding of the processes responsible for the release of specific elements is to irradiate shallow-implanted samples, which exhibit a well-known depth distribution of the implanted species. In this work, the samples studied were produced by implantation of Si<1 0 0>wafers with ¹¹B at implantation energies of 250 and 500 eV and fluence of 1.0×10¹⁵ atoms/cm ². Elastic Recoil Detection Analysis was applied to monitor the remnant boron fluence in the sample. Irradiation of the samples by a 14.2 MeV ¹⁹F ⁴⁺ beam resulted in a slow decrease of boron remnant fluence with initial loss rates of the order of 0.05 B atom per impact ion. Under irradiation with 12 MeV ³²S ³⁺ ions, the remnant boron fluence in Si decreased exponentially with a much faster loss rate of boron and became constant after a certain heavy ion irradiation dose. A simple model, which assumes a finite desorption range and corresponding depletion of the near-surface region, was used to describe the observations. The depletion depths under the given irradiation conditions were calculated from the measured data.     

Energy,fluence and temperature dependence of MeV nitrogen implantation profiles in steel

Abstract

1 to 2 MeV nitrogen (N⁺ and N₂ ⁺) ions were implanted at high fluences in stainless steel, and their depth distributions were measured subsequently by Rutherford backscattering and thermal neutron depth profiling. The range profiles were broader than theoretically expected. With increasing fluence, deviations from ballistic computer codes increase. These deviations can well be described by the assumption of radiation enhanced diffusion for which a simple analytical model is presented. The thermal mobility shows a different behavior for low, and for high implanted fluences.     

基于FLUKA计算压力容器快中子注量的可行性研究

基于知识产权的考虑,通过与蒙特卡罗程序MCNP计算结果比对,研究使用FLUKA程序替代MCNP程序进行反应堆压力容器快中子注量计算的可行性。通过修改和调用子程序对次级粒子堆栈进行操作,解决了关闭裂变中子这一关键问题,FLUKA程序的计算结果与MCNP程序的计算结果相对偏差在5%以内,符合得较好,证明使用FLUKA程序替代MCNP程序用于计算反应堆压力容器快中子注量在技术上是可行的。     

Study of neutron generation from relativistic heavy ion interactions

Study has been carried out on neutron generation in the interactions of 44 and 18 GeV ¹²C with Cu-and Pb-targets. The properties studied include, (1) Relative yields of secondary neutrons generated in 44 GeV versus in 18 GeV ¹²C bombardment to Cu target and also to Pb target. The ratios of these are 2.12±0.19 and 2.04±0.15, respectively, which is nearly in consistence with but somewhat larger than the values of theoretical calculations for Cu target. (2) Relative yields of secondary neutrons produced in Pb target versus in Cu target. The ratios obtained are 2.54±0.20 and 2.56±0.20 for 44 GeV and 18 GeV, respectively. More neutrons are generated in Pb target than in Cu target.     

Absence of voids in fast neutron irradiated zirconium by small angle neutron scattering

Abstract

The aim of this research was to resolve a difference of opinion in the literature on the presence of voids in fast neutron irradiated zirconium. There is a great interest in the study of zirconium, since zirconium and its alloys are used extensively in modern power reactors, for example in the fuel rods as a containment material for enriched uranium. A polycrystalline sample of zirconium was irradiated in the HERALD reactor at 40°C with 10²⁰ fast neutrons per cm^?2. The neutron scattering from irradiated and unirradiated standard samples was studied over a wide Q range from 0.001 to 1.12 Å^?1 on a D11 Spectrometer at the ILL (France). The defect cross-section (the difference between the scattering of the standard zirconium crystal and irradiated crystal) was nearly flat as a function of Q (momentum transfer vector) with an average value of 8.5 mb/Str/atom. This indicates a point defect concentration of about 1.8%. Thus the absence of any small angle (Q dependent) defect scattering indicates that large damage regions (e.g. voids) are not produced in zirconium by fast neutron irradiation.     

快中子照相模拟分析与实验验证

首次系统地推导出快中子照相像素值形成解析式, 建立图像反差不等式, 并利用该不等式首次对图像对比度与源强、照射时间和散射之间的关系进行说明. 并在像素值解析式基础上编制快中子照相模拟程序, 利用该程序对空间分辨率和图像对比度进行模拟, 并与实验对照, 研究结果表明空间分辨率模拟效果好于实验, 图像对比度模拟效果与实验相当. 最后通过对狭缝、方孔以及多材质组成的复杂样品模拟并与实验对照, 结果显示模拟效果与实验照片在反差灵敏度效果上非常一致, 该模拟计算方法可为实验设计和工程应用提供参考.     

The effect of incidence angle on disorder production in ion implanted Si

Abstract

Ne, Ar, Sb, and Xe ions have been implanted, at 30 keV or 80 keV and at various incidence angles, into Si substrates maintained at room temperature during implantation. Implantation-induced Si disorder was measured using RBS-channelling. The effects upon disorder of various incidence angles were studied over a fluence range of 10¹²-10¹⁶ ions·cm^?2.

The results show that, at low fluences the lighter (Ne) and slightly heavier (Ar) ion implantations generate a bimodal disorder-depth profile, whilst at higher fluences measurements of amorphised layer thickness as a function of ion incidence angle allow values of the standard deviation of the disorder profile parallel and transverse to the ion beam direction for each ion to be obtained with good agreement to theoretical predictions.     

Peculiarities of heavy ion channeling

Abstract

Depth distributions of implanted Mg⁺- and Ca⁺-ions and the corresponding radiation damage were studied for different channeling orientations of silicon crystals. The shape of the implantation profiles is discussed by using simple models for dechanneling and energy loss processes. A correlation between dechanneling, damage production and depth distributions of the channeled ions could be observed. This correlation is seen by the maxima shifts in damage and implanted ion distributions between channel and random incidence.