Variable Energy Positron Annihilation Spectroscopy of GaN Grown on Sapphire Substrates with MOCVD

Depth profiled Doppler broadening of positron annihilation spectroscopy (DBPAS), which is also called the variable energy positron annihilation spectroscopy (VEPAS), is used in characterization of GaN grown on sapphire substrates with metal-organic chemical vapour deposition (MOCVD). The GaN film and the film/substrate interface are investigated. The VEPFIT (variable energy positron fit) software was used for analysing the data,and the positron diffusion length of the sapphire is obtained. The results suggest that there is a highly defected region near the GaN/sapphire interface. This thin dislocated region is generated at the film/substrate interface to relieve the strain. Effects of implantation dose on defect formation, for the GaN/Sapphire samples, which implanted by Al^ ions, are also investigated. Studies on AI implanted GaN films (not including the interface and sapphire) have revealed that there are two different regions of implantation damage. For the low Al^ implantation dose samples, in the region close to the surface, defects are mainly composed of vacancy pairs with small amount of vacancy clusters, and in the interior region of the film the positron traps are vacancy clusters without micro-voids. For the highest dose sample, however, some positron trap centres are in the form of micro-voids in the second region.     

A generalized method of converting CT image to PET linear attenuation coefficient distribution in PET/CT imaging

The accuracy of attenuation correction in positron emission tomography scanners depends mainly on deriving the reliable 511-keV linear attenuation coefficient distribution in the scanned objects. In the PET/CT system, the linear attenu- ation distribution is usually obtained from the intensities of the CT image. However, the intensities of the CT image relate to the attenuation of photons in an energy range of 40 keV-140 keV. Before implementing PET attenuation correction, the intensities of CT images must be transformed into the PET 511-keV linear attenuation coefficients. However, the CT scan parameters can affect the effective energy of CT X-ray photons and thus affect the intensities of the CT image. Therefore, for PET/CT attenuation correction, it is crucial to determine the conversion curve with a given set of CT scan parameters and convert the CT image into a PET linear attenuation coefficient distribution. A generalized method is proposed for con- verting a CT image into a PET linear attenuation coefficient distribution. Instead of some parameter-dependent phantom calibration experiments, the conversion curve is calculated directly by employing the consistency conditions to yield the most consistent attenuation map with the measured PET data. The method is evaluated with phantom experiments and small animal experiments. In phantom studies, the estimated conversion curve fits the true attenuation coefficients accurately, and accurate PET attenuation maps are obtained by the estimated conversion curves and provide nearly the same correction results as the true attenuation map. In small animal studies, a more complicated attenuation distribution of the mouse is obtained successfully to remove the attenuation artifact and improve the PET image contrast efficiently.     

Photoelectron Angular Distributions of Positronium in Intense Laser Field

When positronium is generated in super intense laser fields irradiated by an intense laser field, it breaks up into an electron and a positron. The momentum of the electron as well as that of the positron depends on the relative direction with respect to the laser propagation. Thus, the photoelectron angular distributions show inversion asymmetry. The inversion asymmetry becomes notable for higher kinetic energy of electrons.     

Optical Potential Calculations of Positron-H2 Elastic Collision

We report the calculations of the elastic scattering differential cross section for positron H2 collisions with the impact energy below the positronium formation threshold. Our calculation is based on the static-exchange-optical model which has obtained great success in the case of electron scattering. The effective potential used here includes the static and optical potentials. The optical method can completely include the second-order effect arising from real and virtual excitation of target states, which is important for the scattering. A comparison is made with the available theoretical calculations.     

Annealing Behaviour of Helium Bubbles in Titanium Films by Thermal Desorption Spectroscopy and Positron Beam Analysis

Helium-containing Ti Glms are prepared using magnetron sputtering in the helium-argon atmosphere. Isochronal annealing at different temperatures for an hour is employed to reveal the behaviour of helium bubble growth. Ion beam analysis is used to measure the retained helium content. Helium can release largely when annealing above 970K. A thermal helium desorption spectroscopy system is constructed for assessment of the evolution of helium bubbles in the annealed samples by linear heating （OAK/s） from room temperature to 1500K. Also, Doppler broadening measurements of positron annihilation radiation spectrum are performed by using changeable energy positron beam. Bubble coarsening evolves gradually below 680K, migration and coalescence of small bubbles dominates in the range of 68-970 K, and the Ostwald ripening mechanism enlarges the bubbles with a massive release above 970K.     

正弦平方势与沟道辐射的谱线展宽

利用曾经提出的正弦平方势解析地处理了非线性效应和偶极效应对沟道谱线宽度的影响, 解释了正电子能量在5 GeV附近的Si (110)面沟道辐射谱特征. The spectral properties of the planar channeling radiation of positron have been investigated by using Sine squared Potential. At positron energy of 5 GeV a sharp line has been explained for the planar channeling radiation in Si(110 ). It is that theoretic results are compatible to the experiments.     

Synergistic Effect of Triple Ion Beams on Radiation Damage in CLAM Steel

The synergistic effect of triple ion beams is investigated by simultaneous and scquential irradiations of gold, hydrogen and helium ions on the low activation martensitie steel （CLAM） developed in China. The depth profile measurements of the positron annihilation Doppler broadening S parameter are carried out as a function of slow- positron beam energy to examine the produced radiation damage. The synergistic effect of displacement damage and hydrogen and helium on the formation of radiation damage is clearly observed. In the preset case ,this effect suppresses the radiation damage in the CLAM steel due to the helium and/or hydrogen filling of vacancy clusters.     

Dephasing time of a positron accelerated by a laser pulse

The dephasing time of a positron in the total field associated with a laser pulse in a plasma J28is studied numerically. It is shown that the dynamics of the positron is quite different from that of an electron due to the electrostatic potential in the body of the pulse. The dephasing time of the positron increases with the pulse length and decreases with the pulse intensity nonlinearly. In the long pulse case (L》λ_p) the dephasing time is proportional to the pulse length. These results provide a scientific basis for experiments to observe the positron acceleration scheme, and may be important to the physics of laser-article interactions in multi-component plasmas.     

Semiconductor nanowires for photovoltaic and photoelectrochemical energy conversion

Semiconductor nanowires （NW） possess several beneficial properties for efficient conversion of solar energy into electricity and chemical energy. Due to their efficient absorption of light, short distances for minority carriers to travel, high surface-to-volume ratios, and the availability of scalable synthesis methods, they provide a pathway to address the low cost-to-power requirements for widescale adaptation of solar energy conversion technologies. Here we highlight recent progress in our group towards implementation of NW components as photovoltaic and photoelectrochemical energy conversion devices. An emphasis is placed on the unique properties of these one-dimensional （1D） structures, which enable the use of abundant, low-cost materials and improved energy conversion efficiency compared to bulk devices.     

Ion Beam Analysis of the Annealing Behavior of Helium in Ti Films

We present a theoretical calculation finding that a spectrum from ion beam analysis will change at different stopping cross sections. This is more visible at a deeper place in the sample. Helium-contained Ti films annealed at different temperatures are prepared to gain different stopping cross sections whereby the stopping cross section will change with the helium phase states and the pressure of helium bubbles. Then ion beam analysis is used to measure the concentration of helium. It is found that the concentration curve rises greatly after the sample is annealed at 673K which reflects the increasing size of the helium bubble. The results axe consistent with that of positron annihilation radiation spectra which are performed by using a changeable energy positron beam.     

Positron Characteristics in Cadmium and Zinc Chalcogenides

Electron energy levels and positron states have been calculated for cadmium and zinc chalcogenide compounds within the pseudo-potential approach and the independent particle model.Furthermore,the present contribution deals with the electron and positron chemical potentials allowing the calculation of the positron affinity to different materials of interest and hetero-structures formed by these materials.Besides,we here determine the positron diffusion constant by means of the positron deformation potential.An attempt has been made to scale positron affinity and diffusion constant with the lattice constant and the band gap energy,respectively.Such scaling is found to be not possible.The information gathered by the present study is of prime importance for a better understanding of positron trapping at interfaces and precipitates and should be useful in slow positron beam experiments.     

正电子慢化体的研究和进展

正电子湮没技术是一种研究材料的微观缺陷和相变的灵敏工具,在通常的正电子谱仪中,正电子能量为MeV量级,在样品中注入深度比较学（－100μm）,主要研究材料体内的平均缺陷密度,慢正电子束方法把正电子的能量降低为keV量级（而且可以调节）,注入比较浅（－μm）,所以是研究表面缺陷的探测手段,正电子慢化体是产生慢正电子的关键设备,对其研究有重要意义,文章综述了慢化体研究的历史和现状,从物理概念出发介绍使正电子慢化的四种可能方法和当今慢化体的五种几何排列方法,其中应用最广泛的是钨慢化体和百叶窗式的排列方式,效率最高的是惰性气体固体慢化体,而加电场慢化体是有待开发的高效慢化体。     

北京慢正电子强束流运行性能测试

慢正电子强束流采用高能脉冲电子束流轰击金属钽靶, 以产生正负电子对的方式提供正电子, 作为慢正电子束流方法学研究和薄膜材料缺陷研究的束流基础. 本文是在该装置实现运行后, 对慢正电子束流的强度、能散、形貌等运行性能的测试工作的介绍, 以及慢正电子湮没多普勒测量系统的调试和标准样品的测量结果.     

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.     

Early stage ageing effects and shallow positron traps in Al–Mg–Si alloys

Room temperature ageing, so-called natural ageing, of Al–Mg–Si alloys has a subtle but striking influence on the mechanical properties achievable by subsequent ageing at more elevated temperatures. Though strongly debated, different clustering processes are generally accepted to give rise to this effect. Using temperature-dependent positron lifetime measurements of naturally aged Al–Mg–Si alloys, it is shown that in the early stages of ageing, small clusters of alloying atoms without embedded vacancies take part in the decomposition process. These clusters serve as shallow positron traps with a binding energy of about 55(10) meV, grow in the course of natural ageing and transform to deep positron traps with binding energies well above thermal energies. Thus, results of positron annihilation spectroscopy techniques need to be interpreted carefully with respect to the microstructure of age-hardenable Al alloys. Moreover, it is shown that a simple approach to bind positron states using a three-dimensional potential well and (bulk) positron affinities cannot explain the present findings.     

Planned positron experiments at FRM-II

The new research reactor FRM-II near Munich has a strong positron source, which delivers an intense, nearly monoenergetic positron beam. Our positron systems, the pulsed low energy positron source (PLEPS) and the scanning positron microscope (SPM) will be operated at this beam. Some aspects of matching these systems to the new positron source will be discussed.Considerable improvements are expected, e.g. more than 10⁵ s⁻¹ recorded events at PLEPS and sub-micrometre resolution at SPM. They will enable investigations in so far inaccessible problems like the evaluation of annihilation characteristics and trapping constants of individual defects or studies of fast dynamical processes. In applied materials science complex defect structures will be studied which demand a resolution into many differing lifetimes, e.g. fractured specimens, wear, corrosion, etc. Also large series of measurements at small systematic modifications are planned. There is also the opportunity to analyse in addition the chemical microstructure of the specimens by means of a hydrogen microprobe and other ion beam techniques available close to FRM-II at the Technical University of Munich.     

超短超强激光产生正电子的蒙特卡罗模拟

通过理论分析,建立了超短超强激光与固体靶作用产生正电子的蒙特卡罗模拟模型及Geant4模拟程序。模拟研究了靶材料、靶厚度及超热电子温度等对正电子产额的影响,结果表明:对铝、铜、锡、钽、金、铅6种靶材料,金靶的正电子产额最高,是优秀的正电子产生靶;不同超热电子温度下存在不同的最佳靶厚度,在最佳靶厚度以下,正电子产额随靶厚度增长而增大,靶厚度取3 mm较为合适;超热电子温度越高,正电子产额也越高,提高激光强度是增加正电子产额的有效途径。模拟研究给出了正电子角分布及其能谱,结果显示,正电子发射明显前倾,从大于90方向范围发射的正电子数量极少,且超热电子温度越高前倾特点越明显,能量呈类麦克斯韦分布,靶背法线方向出射的正电子的温度随超热电子温度升高而升高。     

闪锌矿结构晶体的正电子体寿命计算

在局域密度近似理论(LDA)和广义梯度近似理论(GGA)的基础上,用中性原子叠加的方法(ATSUP),理论上对亚铜的卤化物(CuF除外,因为它不是闪锌矿结构)及某些硼化物进行计算,计算结果与实验符合得很好;其次,计算了其他具有闪锌矿结构的众多晶体的正电子体寿命,得到的结果与其他文献计算的结果符合得较好,文中以CuCl晶体为例,给出了正电子在其中的势能分布,概率密度分布与湮没率分布;最后,这些系统性的正电子体寿命结果被拟合成晶格常数的函数,拟合结果与其他一些文献的结果做了比较. 关键词： 正电子体寿命 闪锌矿结构     

Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique

A high-performance positron age-momentum correlation (AMOC) spectrometer was newly developed. The counting rate is increased up to 200 cps much larger than the value 20~cps reported by other international groups. And at the same time, the time resolution still keeps at the international level of 220 ps. Furthermore, positronium (Ps) annihilation in silica aerogel was investigated by AMOC, which indicates: (1) Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps; (2) Annealing below 400 ℃ changes the grain surface conditions, i. e. the desorption of hydrogen and the decrease of the defect centers concentration.     

低能单色正电子束装置的原理、研制及应用

简要介绍了一种新的灵敏核探针－－低能单色正电子（慢正电子）束流装置的原理及应用。同时介绍了北京慢正电子束流装置的建设进展情况。