慢正电子束研究薄膜、界面和近表面微观结构

慢正电子束技术十几年来发展起来的探测材料近表面微结构的新手段,文章介绍了其在薄膜、界面和近表面测量的基本方法和部分应用结果。     

一种新的灵敏核探针——慢正电子束流装置

简要介绍了广泛应用于表面科学的灵敏核探针——慢正电子束设备的原理、构造和应用 ,阐述了北京慢正电子束流装置的设计原理和性能 ,讨论了北京慢正电子束流装置今后的研究前景. A novel and compact slow positron beam line——Beijing Slow Positron Beam Line was described. The basic physical concepts of the slow positron beam and its applications were reviewed. Recent experimental results showed the converting effcicency for the total positrons from the radioisotope to slow positrons was 8×10 -5 and the moderation efficiency was 1.7×10 -4 . The future development of Beijing Intense Slow Positron Beam, based on the BEPC LINAC, was also briefly discussed.     

慢正电子束技术的应用与发展

本文介绍了慢正电子束技术和最新发展,慢正电子束作为探针的基本特性,以及在多种学科中的应用等。     

慢正电子束技术的应用与发展

本文介绍了慢正电子束技术和最新发展,慢正电子束作为探针的基本特性,以及在多种学科中的应用等。     

慢正电子技术在固体表面与界面研究中的应用

综述了慢正电子技术的发展以及从放射源β衰变发射出的慢正电子的慢化原理，概述了利用慢正电子技术研究固体薄膜表面和界面的基本原理和方法，并讨论了慢正电子束技术在固体薄膜表面和界面研究中的应用。     

He离子注入ZnO中缺陷形成的慢正电子束研究

在ZnO单晶样品中注入了能量为20—100keV、总剂量为4.4×10¹⁵cm^-2的He离子.利用基于慢正电子束的多普勒展宽测量研究了离子注入产生的缺陷.结果表明，He离子注入ZnO产生了双空位或更大的空位团.在400℃以下退火后，He开始填充到这些空位团里面，造成空位团的有效体积减少.经过400℃以上升温退火后，这些空位团的尺寸开始增大，但由于有少量的He仍然占据在空位团内，因此直到800℃这些空位团仍保持稳定.高于800℃退火后，由于He的脱附，留下的空位团 关键词： 慢正电子束 ZnO 离子注入 缺陷     

脉冲慢正电子束流的直流化装置

在基于加速器的北京慢正电子强束流装置上, 用Penning-Trap技术实现了脉冲慢正电子束流的存储和直流化. 实验表明, 系统的真空度对正电子的存储效率有严重影响, 脉冲正电子的释放方式决定了直流化后束流的能量分散; 改变释放级信号波形以及释放方式可以得到能量分散小于1eV的准直流化慢正电子束流.     

锆离子注入锆-4合金缺陷的慢正电子研究

采用慢正电子束多普勒展宽谱研究了Zr离子注入Zr-4合金产生的缺陷及其退火回复行为，发现经过大于离子注入剂量为1×10¹⁶cm^-2的样品所产生的缺陷在注入过程中已经回复，而对剂量为1×10¹⁵cm^-2样品做300℃退火处理，其缺陷基本回复，得出合金缺陷回复能较低的结论. 考虑到材料的缺陷含量越高，其抗腐蚀性能越差，在辐照环境下通过给材料保持一定温度，即可使其缺陷得到较好回复，从而提高材料的抗腐蚀性能.     

22Na放射源慢正电子束流插入装置的研制

北京慢正电子强束流是利用北京正负电子对撞机电子直线加速器电子打靶产生的高强度低能单色正电子束流. 为了提高强束流的机时利用效率和节省强束流用于新建实验站的调试机时, 设计了一套基于²²Na放射源的慢正电子束流装置插入到强束流输运线上. ²²Na放射源慢正电子束流插入装置主要包括²²Na放射源及慢化体、E×B能量选择器、多级静电加速管、磁场输运系统、真空系统、高压绝缘和辐射防护措施等.     

外延生长La0.7Sr0.3MnO3薄膜厚度对缺陷影响的慢正电子分析

采用脉冲激光沉积方法在(LaAlO₃)_0:3(Sr₂AlTaO₆)_0:7衬底上外延生长了La_0.7Sr_0.3MnO₃薄膜,并采用慢正电子束方法分析了薄膜在不同厚度和不同退火气氛下参数S的变化. 分析表明,薄膜中包含两种机制引入的氧空位,分别是薄膜生长气氛中氧压偏低造成薄膜的氧缺乏和由于薄膜应变引入空位型缺陷. 当薄膜厚度较薄时,应变造成的晶格畸变化比较大,当薄膜的厚度大于11 nm时,薄膜的应变驰豫已经比较完全. 原位退火的样品中正电子主要是被氧缺乏引起的氧空位捕获. 在氧气中退火的样品,S随厚度的变化反映了应变对薄膜微结构的影响.     

利用正电子湮没技术研究钾掺杂钨合金中的缺陷

钨合金中钾的掺杂会引入大量的缺陷,如尺寸几十纳米的钾泡、高密度的位错以及微米量级的晶粒带来的晶界等,这些缺陷的浓度和分布直接影响合金的服役性能.本文运用正电子湮没谱学方法研究钾掺杂钨合金中的缺陷信息,首先模拟计算了合金中各种缺陷的正电子湮没寿命,发现钾的嵌入对空位团、位错、晶界等缺陷的寿命影响很小;然后测量了不同钾含量掺杂钨合金样品的正电子湮没寿命谱,建立三态捕获模型,发现样品中有高的位错密度和低的空位团簇浓度,验证了钾对位错的钉扎作用,阐述了在钾泡形成初期是钾元素与空位团簇结合并逐渐长大的过程;最后使用慢正电子多普勒展宽谱技术表征了样品中缺陷随深度的均匀分布和大量存在,通过扩散长度的比较肯定了钾泡、晶界等缺陷的存在.     

GaN/SiC异质结的慢正电子研究

通过对不同生长厚度GaN/SiC(n-n)的慢正电子研究,发现在GaN/SiC的界面中存在大量各种缺陷并在界面两端形成两个不同方向的电场. 这些缺陷的产生和SiC衬底表面制备以及GaN和SiC不同的热膨胀系数有关. 而缺陷中大量的带状缺陷在界面中形成一个费米能级钉扎(Fermi level pinning),它的存在使界面中存在一定高度的势垒,导致在界面两端的一定区域内形成两个不同方向的电场. 用VEPFIT模拟该电场的存在,分四层(GaN/Interface/SiC1/SiC2)进行拟合,得到了很好的拟 关键词： 正电子湮没 缺陷 半导体     

SIMION simulation of a slow pulsed positron beam

A new slow pulsed positron beam, including a positron source, a moderator, a chopper, a pre-buncher, a main-buncher and a sample chamber, etc, has been installed and tested. It is necessary to simulate the acceleration, transportation and space focusing of positrons to meet the needs of beam debugging and further positron annihilation experiments. The result from SIMION simulations shows that the radius of the focused positron beam is less than 5 mm, which is further confirmed in our practical debugging process.     

Positron annihilation studies of mesoporous silica films using a slow positron beam

Positron annihilation lifetime spectra were measured for mesoporous silica films, which were synthesized using triblock copolymer (EO₁₀₆PO₇₀EO₁₀₆) as a structure-directing agent. Different positron lifetime spectra for the deposited and calcined films indicated the formation of meso-structure after calcination, which was confirmed by Fourier transform infrared (FTIR) spectra and field emission-scanning electron microscopy (FE-SEM) observation. Open porosity or pore interconnectivity of a silica film might be evaluated by a two-dimensional positron annihilation lifetime spectrum of an uncapped film. Pore sizes and their distributions in the silica films were found to be affected by thermal treatments.     

Dislocation driven chromium precipitation in Fe-9Cr binary alloy: a positron lifetime study

The influence of initial heat treatment on anomalous Cr precipitation within high temperature solubility region of the Fe–9Cr alloy has been investigated using positron lifetime studies. Air-quenched samples with pre-existing dislocations exhibited a distinct annealing stage in positron lifetime between 800 and 1100?K corresponding to Cr-precipitation. During this stage, Transmission Electron Microscopy showed fine precipitates of average size 4 nm, dispersed throughout the sample and from Energy-dispersive X-ray spectroscopy (EDS) analysis they are found to be Cr-enriched. The presence of dislocations is found to be responsible for Cr precipitation.     

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ₂ = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ₁, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ₁ and the intensity I₂, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.     

Performance of the plugged-in 22Na based slow positron beam facility

The Beijing intense slow positron beam facility is based on the 1.3 GeV linac of Beijing ElectronPositron CoUider (BEPC) aiming to produce mono-energetic intense slow positron beam for material science investigation. The plugged-in 22Na based slow positron beam section has been newly constructed to supply continuous beam time for the debugging of positron annihilation measurement stations and improve the Beijing intense slow positron beam time using efficiency. Performance testing result of the plugged-in 22Na based slow positron beam facility are reviewed in this paper, with the measurement of the beam transport efficiency, the view of beam spot, the adjustment of beam position, the measurement of beam intensity and energy spread etc. included.     

Helium/hydrogen synergistic effect in reduced activation ferritic/martensitic steel investigated by slow positron beam

Here, we investigated the irradiation defect in reduced activation ferritic/martensitic steels by slow positron beam. Three ion-irradiation experiments were carried out: (i) He²⁺ irradiation, (ii) H⁺ irradiation and (iii) He²⁺ irradiation followed by H⁺ irradiation, at temperature 450?°C. The presences of vacancy defects, represented by ?S_He+H parameter, induced by sequential irradiations was larger than the sum of defects, ?S_He parameter + ?S_H parameter, caused by single He ions and single H ions. The synergistic effect of He and H was confirmed clearly from the perspective of positron annihilation spectroscopy.     

Annealing studies of Ti/Al multilayer film by slow positron beam

Single detector and coincidence Doppler broadening (CDB) spectroscopy measurements using slow positron beam were carried out to study as-deposited and annealed Ti/Al multilayer films. The changes of the film structure and defects in each layer by heat treatment have been investigated through the analysis of Doppler broadening lineshape variation. The coincidence Doppler broadening measurements revealed that Ti is the dominant diffusion species during the alloying process of Ti/Al by high temperature annealing. These results highlight the potential of slow positron beam in characterizing the vacancy-type defects evolution and mechanism of interlayer diffusion in Ti/Al multilayer film.     

Mesoporosity in silica films studied by a slow positron beam and ellipsometry

Porous silica films were synthesized via a sol–gel method using a nonionic amphiphilic triblock copolymer F127 as the structural template. Mesoporosities of the prepared silica films were investigated by Doppler broadening of positron annihilation radiation (DBAR) spectroscopy, positron annihilation gamma-ray energy spectroscopy based on a slow positron beam, and ellipsometry. For the mesoporous silica films, the variation of positron annihilation line shape parameter reveals that the porosity of the silica films increases with loading more F127, which is also confirmed by a decrease of refractive index n. Little variation in positron 3γ-annihilation fraction is found for the silica films prepared with F127 loading less than 15 wt%, whereas a remarkable increment is seen for the films with higher loading. This indicates the pore percolation in porous silica films occurs around a loading of F127 with 15 wt%.