Positron annihilation techniques

Table 5. CMC values as obtained by Positron annihilation techniques and literature data. [J. Am. Chem. Soc. 100, 984 (1978)1...

Abstract. Free-volume structure in the lanthanum salt of laurinic acid in crystalline and liquid-crystalline states and an effect of dissolved Cgo molecules on the mean nanovoid radius and concentration were studied by means of the positron annihilation technique. La(Ci2H25COO)3 clathrate compound with dissolved C6o molecules was obtained, which is thermodynamically more stable than a simple mixture of components. Increased mean nanovoid radius (from 0.28 to 0.39 nm) after the inclusion of C6o molecules and concomitant decrease of the positronium annihilation rate by a factor of 2.7 indicate the decrease of the smallest nanovoid concentration. 

Lanthanum laurates in crystalline (powder) and liquid-crystalline states, as well as after dissolution of Cgo fullerenes were studied by means of the positron annihilation technique which is extremely sensitive to the free-volume defects. 

Rivera, A., Montilla, I., Alba Garcia, A., Escobar Calindo, R. et al. (2001) "Native and irradiation-induced defects in Si02 Structures studied by positron annihilation techniques , Mater. Sci. Forum 363-365, 64. 

The aim of this chapter is to introduce the reader to the application of positron annihilation techniques to polymers. An extensive review of the large volume of publications related to positron studies in polymers will not be presented. Rather it is intented to introduce the reader to the theory and techniques used in polymer studies and indicate the types of information that can be obtained about different polymer systems. The main focus of this chapter will be on the use of positron annihilation lifetime spectroscopy (PAL) in polymer studies. Chapter 11 discusses the use of monoenergetic slow positron beams used to study polymers surfaces. One of the interesting new developments in the application of positron annihilation techniques in polymers is the positron age-momentum correlation technique (AMOC). This technique promises to shed new light on the mechanisms of positronium formation and annihilation in polymer systems. A more detailed discussion of this technique can be found elswhere in this text. 

Djermouni, B., Ache, H.J. (1980) Effect of casting solvent on the properties of styrene-butadiene-styrene block copolymers studied by positron annihilation techniques . Macromolecules, 13,168. 

El-Samahy, A.E., Abdel-Rehim, N., El-Sayed, A.M.A. (1996) Temperature dependence of free-volume holes in poly(vinyl alcohol) studied by positron annihilation technique . Polymer 37(19), 4413. 

Positron annihilation techniques [78] can be used to obtain information about the momentum density of the annihilating positron-electron pair. In solids, particularly metals, the distortion of the electron momentum density by the Coulomb interaction between the positron and electrons is relatively small, and this technique then gives us the electron momentum density. 

Hamielec, A. E., Eldrup, M., Mogensen, O., and Jansen, R, Positron annihilation techniques in polymer science and engineering, J. Macromol. Chem. Rev. Macromol. Chem. C, 9,305-337 (1973). 

More advanced techniques are also available [38]. Microwave, nuclear magnetic resonance (NMR) and positron annihilation techniques may be useful, but they tend to be complex and expensive techniques for the measurement of specimens which can be transported to the laboratory. 

STUDY OF STRUCTURAL CHANGES IN MICELLAR SOLUTIONS AND MICROEMULSIONS BY POSITRON ANNIHILATION TECHNIQUES THEIR RELEVANCE TO LIQUID SCINTILLATION COUNTING PHENOMENA ... 

Levey, B., Lavoric, M., and Ache, H.J. 1989. Solid-solid phase transitions and molecular motions in long-chain paraffins studied by positron annihilation techniques. J. Chem. Phys. 90 3282-3291. 

PA studies on RPV steels are important because they have the potential to provide information on matrix defects. However, since interpretation of the data from complex commercial steels is difficult, many studies have focused on model alloys. In Section 9.11.1 we include a brief review of a selection of PA data from the literature, focusing first on model alloys and then on steels. It is shown that, in combination with post-irradiation annealing and other microstructural techniques, positron annihilation techniques can help elucidate the nature of the positron traps. 

Positrons can be used as particle probes, suitable to detect low concentrations of defects in materials. Positron physicists generally are in need of intense positron beams for applying positron annihilation techniques such as two dimensional (2D) Angular Correlation of Annihilation Radiation (ACAR) for investigating surfaces and interfaces of materials. The 2D-ACAR technique allows high resolution measurements of the electron momentum distribution for depth, localized defects, thin layer systems, and interfaces. In addition, a submicrometer size positron beam can be created for defect depth profiling on a lateral scale smaller than a micrometer. Vacancy type defects can be mapped in a three dimensional fashion. 

As the study of the electron momentum density and the Fermi surface by the positron annihilation techniques is rather indirect, it is important to perform the experimental investigations in close relation with a strong computational approach. The first valuable quantity to estimate is the positron density distribution in the lattice unit cell it will indicate which electronic states might be observed experimentally. One has then to evaluate the electron momentum density as seen by the positron. From this, the... 

The positron annihilation technique is quite promising for studying free-volume holes (FVH) in polymer structure. Information on free-volume holes is useful for imder-standing many physical and mechanical properties of polymeric materials. However, some problems of rationalizing the annihilation characteristics obtained by measiuing positron lifetimes still remain unsolved. 

Early experiments with positrons were dedicated to the study of electronic structure, for example Fermi surfaces in metals and alloys [78,79], Various experimental positron annihilation techniques based upon the equipment used for nuclear spectroscopy underwent intense development in the two decades following the end of the Second World War. In addition to angular correlation of the annihilation of y quanta, Doppler broadening of the annihilation line and positron lifetime spectroscopy were established as independent methods. By the end of the 1960s, it was realised that the annihilation parameters are sensitive to lattice imperfections. It was discovered that positrons can be trapped in crystal defects i.e., the wavefunction of the positron is localised at the defect site until annihilation. This behaviour of positrons was clearly demonstrated by several authors (e.g., MacKenzie et al. [80] for thermal vacancies in metals, Brandt et al. [81] in ionic crystals, and Dekhtyar et al. [82] after the plastic deformation of semiconductors). The investigation of crystal defects has since become the main focus of positron annihilation studies. 

The study of vacancies is now mainly performed by the positron annihilation technique (see for example Corbel, 1987). Because the electron density is weak in a vacancy, the lifetime r of a positron trapped in it is... 

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