明胶大分子中的正电子湮没行为

利用正电子湮没寿命谱 ,从分子尺度上对两种分别从深海鱼皮和牛骨提取的明胶大分子的微观结构性能进行了研究 .正电子湮没寿命谱的长寿命组分给出了关于明胶大分子中自由体积空穴的信息 .结果表明 ,鱼明胶大分子中自由体积空穴的尺度与数量均低于骨明胶大分子 .同时 ,用鱼明胶和骨明胶作为成核分散介质的乳剂试验表明 ,鱼明胶可以改善卤化银颗粒的单分散性并抑制晶核的生长和聚结 .由此认为 ,明胶作为保护性胶体的功能与其微结构特征相关 .鱼明胶在控制卤化银颗粒成核与生长中的功能强于骨明胶 .     

金属元素的正电子鉴别技术

测量了20种金属元素的符合多普勒展宽谱,并与理论计算的结果比较．CDB谱的高动量部分反映了正电子与核心电子的湮灭,由于核心电子在原子组成材料时仍然保留了元素特性,CDB谱的高动量部分可以用来鉴别元素．     

Applications of slow positrons to cancer research: Search for selectivity of positron annihilation to skin cancer

Slow positrons and positron annihilation spectroscopy (PAS) have been applied to medical research in searching for positron annihilation selectivity to cancer cells. We report the results of positron lifetime and Doppler broadening energy spectroscopies in human skin samples with and without cancer as a function of positron incident energy (up to 8 μm depth) and found that the positronium annihilates at a significantly lower rate and forms at a lower probability in the samples having either basal cell carcinoma (BCC) or squamous cell carcinoma (SCC) than in the normal skin. The significant selectivity of positron annihilation to skin cancer may open a new research area of developing positron annihilation spectroscopy as a novel medical tool to detect cancer formation externally and non-invasively at the early stages.     

The effect of target representation in positron-impact ionization of molecular hydrogen

Ionization cross sections for positron collisions with hydrogen molecules have been calculated using a two-center molecular representation. The results are compared with our previous work which used sphericaly averaged H₂ wavefunctions as well as with existing experimental results. The use of a two-centre wave function for the molecular target, either of the Heitler–London type or a Gaussian representation, produces results which lie between the two most recent sets of experimental data. We find that our CPE4 model produces results in better agreement with experiment over the whole energy range than our CPE model.     

正电子湮没技术（PAT）在高分子材料中的应用（下）

正电子湮没技术（ＰＡＴ）在高分子材料中的应用（下）李桂芝，施良和，叶美玲（中国科学院化学研究所高分子物理开放实验室，北京，１０００８０）３．４高分子导体高分子导体有两种：（１）靠共轭分子链导电的高分子；（２）因掺入小分子而具有导电作用的高分子。现在已...     

Positron spin relaxation in naphthalene

Summary The remaining spin polarizationP of positrons was measured in naphthalene against longitudinal magnetic-field strength. The measuring method was based on the relative yield of Ps in triplet and singlet state. In a zero fieldP is reduced to 50% of the initial polarization and approaches 100% upon increasing the applied field to larger values. This effect, which in teflon is quite absent, appears to be determined by the specific behaviour of positrons in naphthalene. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Positron scattering by atoms

We describe the present status of coupled-state calculations for positron scattering by ‘one-electron’ atoms. We show how pseudostates are used to represent the continuum channels. Illustrative results from positron scattering by atomic hydrogen and the alkali metals are presented.     

Positron scattering from alkaline-earth elements

The total (elastic + inelastic) cross sections fore ⁺ impact on alkaline-earth elements from Be to Ra are calculated by employing a complex spherical optical potential. This potential has static, polarization and absorption components. The positron energy range is from a few eV to several thousand eV. We have compared our elastic cross sections for Mg and Ca with the other available results and the agreement is good for energies above 100eV. We have also compared our absorption cross sections withe ⁻ ionization cross sections at high energies where our absorption cross sections are in good accord. We have made Bethe plots fore ⁺ scattering on these elements.     

Studying the recovery of as-received industrial Al alloys by positron annihilation spectroscopy

Positron annihilation lifetime (PAL) spectroscopy, Doppler broadening of annihilation radiation (DBAR) spectroscopy and Vickers microhardness (Hv) measurements were performed to study the micro- and macro-structure variations during isochronal annealing from room temperature (RT) to 500 °C of commercial pure Al (1 1 0 0), Al-Mn-Mg (3 0 0 4) and Al-Mg-Si (6 2 0 1) alloys. Three annealing stages of microstructures have been identified as recovery, partial recrystallization and complete recrystallization followed by grain growth. A positive correlation between the macroscopic mechanical properties (Hv) and positron annihilation parameters has been achieved for the three samples under investigation.     

Positronium decay and formation in paraffin wax from polarized and unpolarized positrons

Summary Ps formation and decay in heterogeneousn-alkane samples (paraffin waxes) have been studied both in the solid and in the liquid phase; then, in the solid phase, the positron's residual degree of polarization was measured at the instant of Ps formation. Differently from what is already known in homogeneousn-alkane samples, Ps shows, many degrees below the melting point, a mean lifetime longer than that typical of the liquid phase; furthermore, the mean lifetime's values pertaining to the transition between solid and liquid do not show a sharp variation across the melting temperature but gradually decrease over a range of temperatures of several degrees. Positronium decay in static magnetic fields indicates that o-Ps magnetic quenching in liquid phase is regular, and corresponds to a contact density value α=|ψ(0)|²/|ψ(0)|_vac ²=0.79±0.07; instead, in the solid phase, o-Ps magnetic quenching shows anomalous behaviour for fields weaker than 7kG. Positrons' residual polarization measurements do not reveal the presence of depolarization effects during the whole slowing-down process until Ps is formed.