基于LINAC慢正电子源的EGS4模拟

介绍基于ＢＥＰＣ ＬＩＮＡＣ和ＢＦＥＬ ＬＩＮＡＣＥ慢正电子束设备中正电子源部分的Ｍｏｎｔｅ－Ｃａｒｌｏ模拟过程。结果表明,从Ｔａ靶出射的快正电子产额在ＢＥＰＣ ＬＩＮＡＣ和ＢＦＥＬ ＬＩＮＡＣ运行兼容模式时分别为１．８４＊１０＾１０／ｓ和３．９＊１０＾１１／Ｓ。     

用单能慢正电子束测量Si中的正电子迁移率

用单能慢正电子束流作为探针,测量了Ｐ型Ｓｉ中不同Ｂ杂质浓度的正电子湮没Ｓ参数和正电子能量的函数关系。在实验上系统地研究了正电子扩散长度和迁移率随半导体中杂质浓度的变化规律,观察到杂质的掺入不影响缺陷的开体积,但正电子在硅中的迁移率随杂质浓度的增加而减小。     

A scanning positron microscope for defect analysis in materials science

The realisation of a scanning positron microscope will be presented and discussed. A positron beam with a variable energy from 0.5 to 30 keV, with a spot diameter of 1 μm or below, can be scanned over an area of 0.6 × 0.6 mm². This beam is formed after a double stage stochastic cooling (moderation) of positrons emitted from a radioactive isotope. In addition the positron beam will be pulsed in order to have a well-defined time base for positron lifetime measurements. In the system included is a conventional scanning electron microprobe for surface analysis. The design of the scanning positron microscope is dominated by the special demands of positron physics.     

Development of an electrostatic high-brightness slow positron beam apparatus for positron scattering experiments

A new apparatus for the study of positron scattering on gaseous target is presented. The apparatus uses electrostatic lens elements and a remoderator in a reflection geometry for the brightness enhancement. The system will provide a high quality positron beam with a small diameter and a small angular divergence, even at low energies.     

正电子湮没率的计算

用有效位势方法计算了金属中正电子的平均寿命和温度的关系,结果显示计算值与文献实验值相当吻合。这种方法还可用于计算其它固体材料中的正电子平均寿命和温度的关系。     

正电子辐照多孔硅的研究

多孔硅经来自＾２３Ｎａ的正电子束辐照后，其光荧光谱出现两个新特点，一是主峰峰位显著蓝移，由辐照臆的红色光谱区蓝移到辐照后的绿以光谱区；二是出现一个较高能量的附加发光峰，红外吸收谱表明，正电子辐照有助于增强多孔硅的表面氧化对多孔硅经正电子辐照后光荧光结构改变的机理进行了讨论。     

On the stabilization of positron annihilation Doppler-broadening data

The present investigation compares three different methods for stabilizing positron annihilation Doppler-broadening data. The three methods have in common the use of a reference gamma ray line, which is simultaneously measured with the positron annihilation line at 511 keV. It is shown that a regressional method should be used whenever the dominating instability in the data has a random nature. A practical example of an application of the regressional method is given.     

Molecular model for annihilation rates in positron complexes

The molecular approach for positron interaction with atoms is developed further. Potential energy curves for positron motion are obtained. Two procedures accounting for the nonadiabatic effective positron mass are introduced for calculating annihilation rate constants. The first one takes the bound-state energy eigenvalue as an input parameter. The second is a self-contained and self-consistent procedure. The methods are tested with quite different states of the small complexes HPs, e⁺He (electronic triplet) and e⁺Be (electronic singlet and triplet). For states yielding the positronium cluster, the annihilation rates are quite stable, irrespective of the accuracy in binding energies. For the e⁺Be states, annihilation rates are larger and more consistent with qualitative predictions than previously reported ones.     

短半衰期正电子核素放射性药物

介绍了目前临床上应用较广的＾１１Ｃ、＾１３Ｎ、＾１５Ｏ、＾１８Ｆ等短半衰期正电子核素标记的放射性药物,特别是＾１８Ｆ和＾１１Ｃ标记的放射性药物在临床上的应用及制备途径。     

Differential ionisation measurements for positron impact

An overview of methods being used at the University of Missouri-Rolla to measure differential ionisation for positron impact is presented. Examples of triply differential electron emission and doubly differential positron scattering and energy loss data are given and contributions to each due to binary and recoil events are discussed. In addition, angular distribution data for electron emission due to double and triple ionisation are presented.     

Application of the zero-range potential model to positron annihilation on molecules

In this paper we use a zero-range potential (ZRP) method to model positron interaction with molecules. This allows us to investigate the effect of molecular vibrations on positron–molecule annihilation using the van der Waals dimer Kr₂ as an example. We also use the ZRP to explore positron binding to polyatomics and examine the dependence of the binding energy on the size of the molecule for alkanes. We find that a second bound state appears for a molecule with ten carbons, similar to recent experimental evidence for such a state emerging in alkanes with twelve carbons.     

Doppler-broadening measurements of positron annihilation with high-momentum electrons in pure elements

Doppler-broadening measurements of the electron–positron annihilation line in twenty-seven single-element samples are presented. A coincidence technique has been used to suppress the background and to evidence the contribution of positron annihilation with core electrons. Systematic dependences on the atomic number of the target material are found in ratio curves obtained dividing the measured spectra by the spectrum of a reference material. The positron lifetime technique has been used to detect the presence of positron traps in all the samples. The change in the high-momentum part of the annihilation line due to positron trapping is illustrated. The measured data are in a good qualitative agreement with recent theoretical calculation and constitute the most complete measurement series, up to now, to establish a future data-base for positron annihilation spectroscopy.     

4.5 tesla magnetic field reduces range of high-energypositrons-potential implications for positron emission tomography

We have theoretically and experimentally investigated the extent to which homogeneous magnetic fields up to 7 Tesla reduce the spatial distance positrons travel before annihilation (positron range). Computer simulations of a noncoincident detector design using a Monte Carlo algorithm calculated the positron range as a function of positron energy and magnetic field strength. The simulation predicted improvements in resolution, defined as full-width at half-maximum (FWBM) of the line-spread function (LSP) for a magnetic field strength up to 7 Tesla: negligible for F-18, from 3.35 mm to 2.73 mm for Ga-68 and from 3.66 mm to 2.68 mm for Rb-82. Also a substantial noise suppression was observed, described by the full-width at tenth-maximum (FWTM) for higher positron energies. The experimental approach confirmed an improvement in resolution for Ga-68 from 3.54 mm at 0 Tesla to 2.99 mm FWHM at 4.5 Tesla and practically no improvement for F-18 (2.97 mm at 0 Tesla and 2.95 mm at 4.5 Tesla). It is concluded that the simulation model is appropriate and that a homogeneous static magnetic field of 4.5 Tesla reduces the range of high-energy positrons to an extent that may improve spatial resolution in positron emission tomography     

Low energy positron scattering from helium

A new experiment has been developed for high resolution studies of positron scattering from atoms and molecules. Based on the Surko trap technology, a pulsed positron beam has been used to obtain preliminary measurements of low energy, differential elastic scattering cross sections from helium. The operation of the beamline is described and preliminary absolute cross section values for scattering energies of 5, 10 and 15 eV are presented and compared with contemporary theoretical calculations.     

ECC study in positron impact ionization of helium

A 3C approximation which was employed in the study of the ECC phenomenon in positron impact ionization of H₂ is now applied to positron impact ionization of helium. Our absolute triple differential cross sections are used to calibrate the existing experimental data.     

PET分子影像学研究进展

正电子发射断层(PET)分子影像学是分子影像学的重要内容。本文主要阐述PET分子影像学技术的理论基础及其在蛋白质功能、基因表达与基因治疗、受体显像及血流代谢显像方面应用的最新进展。     

Interlaboratory comparison of positron and positronium lifetimes in polymers

A comparison of the results of a series of positron annihilation lifetime measurements performed in 12 laboratories is presented. The measurements were conducted on three different polymer samples, all prepared in one laboratory under standard conditions. The objective of the work was to gain insight into the variation in derived positron and positronium lifetimes and intensities measured in the different laboratories on identical specimens. Lifetime data were collected at room temperature by each laboratory following their own standard measurement and data evaluation procedures. The polymers used were a semi-crystalline linear polyethylene, an amorphous poly(methyl methacrylate) and an amorphous copolymer of styrene-acrylonitrile. The results show that the ortho-positronium lifetimes and intensities agree fairly well between different laboratories, despite large differences in experimental conditions, while the lifetimes and intensities of short-lived components show appreciable variations between laboratories.     

A variable-energy positron beam for low to medium energy research

A positron beam facility is described which provides a monoenergetic beam (± 1 eV) which is variable in energy from a few eV up to a maximum of ∼ 80 keV. The positron moderation process, beam transport, and design of the target chamber are discussed in detail. Some of the research being done with the facility is summarized, including scattering and energy-loss measurements, near-surface defect profiling studies, and low energy positron channeling studies. An ultrahigh-vacuum 2-axis goniometer is described, which is used in the channeling work.     

高温超导中多个正电子捕获位及其对正电子湮没参数的影响

从正电子陷阱物理图象出发，引入正电子在陷阱中湮没的竞争机制，提出了一个新的陷阱湮没模型，并采用此模型分析了高温超导材料中正电子在陷阱中湮没的特征。