正电子湮没参量及其所反映物质信息的探讨

本根据正电子在凝聚态物质中的湮没机制及捕获模型,讨论了正电子湮没参量——正电子寿命谱和多普勒线形等参量所反映的物质信息,给出了各正电子湮没参量与所反映的物质信息之间的定量关系。分析了在该体系中正电子寿命参量与局域电子密度、多普勒线形参量和角关联参量与电子动量密度分布和费米面之关联。     

Doppler broadening of in-flight positron annihilation radiation due to electron momentum.

We report the first observation of electron momentum contributions to the Doppler broadening of radiation produced by in-flight two-photon annihilation in solids. In these experiments an approximately 2.5 MeV positron beam impinged on thin polyethylene, aluminum, and gold targets. Since energetic positrons easily penetrate the nuclear Coulomb potential and do not cause a strong charge polarization, the experimental annihilation line shapes agree well with calculations based on a simple independent-particle model. Moreover, annihilations with the deepest core electrons are greatly enhanced.     

Behavior of structure defects and hydrogen in neutron-irradiated stainless steels studied by positron annihilation method

Measurements of angular distributions of annihilation photons are applied to investigating structural defects and hydrogen behavior in annealed, plastically deformed, and irradiated stainless steels. It is determined that the whole cycle of investigations performed by positron diagnostics helped in tracing the formation and evolution of the defect structure and hydrogen behavior in different kinds of steels being subjected to complex physical–mechanical influences such as plastic deformation, hydrogenation and irradiation by fission neutrons. The high sensitivity of the electron–positron annihilation method allowed understanding even of details of the changes of the crystalline structure of multi-component materials.     

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2 annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 Å. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.     

金刚石膜微结构的慢正电子束测量研究

用慢正电子束辅助以拉曼光谱方法对一批较高质量的PECVD金刚石膜的微结构进行测量研究．拉曼光谱实验结果显示,这批金刚石膜中金刚石相含量比较高,正电子湮没实验进一步从微观结构上指出各个金刚石膜之间存在很大差异,并且从缺陷角度发现各样品中缺陷尺寸和缺陷浓度不一样,造成膜质量不同．S-E曲线变化趋势反映出各样品金刚石晶体结构存在明显不同．这表明正电子湮没技术是测量金刚石膜微结构的有力手段．     

碳纳米管束中的正电子理论

采用中性原子叠加模型和有限差分方法(SNA-FD)计算了大范围内不同管径的单壁碳纳米管束中的正电子情况，发现对于单壁碳纳米管束，正电子的主要湮没区域，湮没对象和正电子寿命随碳纳米管管径的不同而发生规律性变化.计算得到管径范围在0.8—1.6nm的碳纳米管束的正电子寿命范围为332—470ps，与实验测得的394ps符合较好.     

The use of positron annihilation Doppler broadening spectroscopy in the characterization of radiochromic dosimetry films

The positron annihilation Doppler broadening (PADB) spectroscopy has been employed to probe the defects and structural changes of various types of materials. In this work, an investigation was carried out for the first time to use PADB spectroscopy in probing the γ irradiated radiochromic films. The GafChromic MD–55 and HD–V2 radiochromic films irradiated with absorbed doses ranges of 0–80 and 0–2000 Gy, respectively, were subjected to Doppler broadening measurements employing a HPGe γ–ray spectrometer. The Doppler broadening line–shape parameters (S and W) of 511 keV annihilation radiations were discussed in terms of the polymerization degree of the active components of these dosimetry films. The S- and W-parameters were found to be dependent on the active component structures of both films. A reasonable correlation was also found between the values of the S–parameter and the optical density of these radiochromic films. In addition, the Doppler broadening line–shape parameters were successfully used to provide explanation of the observed nonlinearity of these films at the end of their dynamic dose ranges. The results demonstrate the applicability of the PADB technique to probe the physical and chemical changes occurred in the active layer of the studied radiochromic films during the solid–state polymerization reaction caused by γ irradiation of different absorbed doses.     

Electrical properties of MOS structures on nitrogen-doped Czochralski-grown silicon: A positron annihilation study

Measurements of interface trap density, effective generation lifetime (GL) and effective surface generation velocity have been performed using different methods on selected MOS structures prepared on nitrogen-doped Czochralski-grown (NCz) silicon. The application of the positron annihilation technique using a pulsed low energy positron system (PLEPS) focused on the detection of nitrogen-related defects in NCz silicon in the near surface region. In the case of p-type Cz silicon, all the results could be used for the testing of homogeneity. In n-type Cz silicon, positron annihilation was found insensitive to nitrogen doping.     

Tunable monoenergy positron annihilation spectroscopy of polyethylene glycol thin films

Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol(PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.     

Positron implantation profile in nickel

A method for measurement of the positron implantation profile in the geometry employed in most positron annihilation experiments is described. The method is applied to the case of Ni as absorber and²²Na as positron emitter. The experimental accuracy is discussed and a proposal for its improvement is outlined. Since absorption studies of positrons is usually performed in geometries quite different from the present, we give a short discussion on the impact of these differences on the transmission curves obtained.     

Positron annihilation in alkali-halide crystals after electron irradiation

The effect of electron beam irradiation on the positron annihilation in KCl, KBr, NaCl single crystals was investigated. The narrowing of the angular correlation curves increases up to the saturation value with the absorbed radiation dose. The shape of the received narrow components differs from the narrow component measured in the additively colored samples. This phenomenon shows the considerable influence of the hole centres upon the positron annihilation in ionic crystals. Paper presented at 3rd Internat. Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

POSITRON STUDY OF DEFECTS IN TiNi ALLOYS

A number of TiNi samples with different compositions and states were measured by positron annihilation technique. And microdefects and the influence of them on shape memory effect were analyzed, giving some valuable information for the study of technological processes of preparing TiNi functional shape memory devices.     

Positron trapping in the liquid and solid copper alloys

The peak counting rates of γ?γ angular correlation curves of positron annihilation (PCAC) in the liquid state fitted well to the straight line which had the same slope of the temperature dependence of the PCAC's attributed to the free annihilation in the solid state, for the Cu-8Ge and Cu-10.5Ge alloys and other copper solid solutions. The results of the positron trapping model fitting suggest the possibility that the positron in the liquid state repels the surrounding ions.     

Line shape analysis of two-colour photoassociation spectra on the example of the Cs ground state

Two-photon photoassociation spectra in a Λ-type excitation scheme are analysed under the systematically varied experimental conditions of frequency detunings and laser intensities. Line shape fits are presented as well as the investigation of intensity and detuning dependent line shifts. From both we determine the attained spectroscopic precision, that is corrected for a systematic line shift due to the thermal distribution of atoms in the trap. An energy correction for this effect is given. Information about the feasibility of generating translationally cold molecules in a well defined rotational and vibrational level by the photoassociation process is derived from the analysis. Received 21 May 2002 Published online 15 October 2002 RID="a" ID="a"Current address e-mail: christian.lisdat@ptb.de RID="b" ID="b"e-mail: pierre.pillet@lac.u-psud.fr     

Silicide formation in Co/Si system investigated by depth-resolved positron annihilation and X-ray diffraction

The transformation of Co/Si to CoSi₂/Si in the temperature range of 300-1170 K has been investigated using depth-resolved positron annihilation and Glancing incidence X-ray diffraction (GIXRD). The different silicide phases formed are identified from the experimental positron annihilation characteristics, which are consistent with the GIXRD results. The present study clearly indicates the absence of vacancy defects in the silicide overlayer.     

Spatial distribution of cold antihydrogen formation

Antihydrogen is formed when antiprotons are mixed with cold positrons in a nested Penning trap. We present experimental evidence, obtained using our antihydrogen annihilation detector, that the spatial distribution of the emerging antihydrogen atoms is independent of the positron temperature and axially enhanced. This indicates that antihydrogen is formed before the antiprotons are in thermal equilibrium with the positron plasma. This result has important implications for the trapping and spectroscopy of antihydrogen.     

Study of the effect of annealing on defects in Fe-Mn-Si-Cr-Ni-C alloy by slow positron beam

FeMnSi shape memory alloys (SMAs) have received much attention as one-way SMAs due to their cost-effectiveness. Variable-energy (0-30 keV) positron beam studies have been carried out on a Fe-Mn-Si-Cr-Ni-C alloy with different degrees of deformation. Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by deformation. The variation of the nature and the concentration of defects are studied as a function of isochronal annealing temperature. These results are correlated with the data measured with the positron annihilation lifetime spectroscopy (PALS). The positron annihilation results are compared to XRD and optical microscopy (OM).     

介孔SiO2薄膜孔结构的慢正电子技术表征

通过蒸发诱导自组装技术制备了具有不同有序结构的介孔SiO₂薄膜,并采用同步辐射X射线反射率以及慢正电子束流技术对其进行表征.实验结果表明,随着旋涂速率的增加,介孔结构由三维立方向二维六角结构转变,同时平均孔隙率随转速增加而减小.利用各向同性无机孔收缩模型和Fourier变换红外光谱,探讨了薄膜结构和正电子湮没参数的内在联系. 关键词： 同步辐射 X射线反射率 Doppler展宽 正电子素飞行时间谱     

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%.     

Effects of positron phonon interaction in metals

We investigate the importance of positron phonon interaction for the many body effects observed in positron annihilation in metals. The interaction is formulated as the screened Coulomb interaction of the positron with the polarization charge of the metal produced by lattice vibrations. The phonon contribution to the effective mass is found negligibly small. Due to the interaction with phonons the equilibrium positron momentum distribution deviates from a Boltzmann distribution leading to a larger thermal smearing of the annihilation curves than expected from the rather small effective mass. The positron energy loss in the thermalization process due to phonon excitation is discussed and found comparable to the energy loss due to the excitation of electron hole pairs in the experimentally interesting temperature range.