Positron methods

Table 11.1 Commercial coatings and paints studied by positron method [34],...

The aim of this paper is to demonstrate the usefulness of positron method in the estimation of average pore sizes. 

In the case of metals containing defects of various types, the positron tends to localize in these defects and, if trapped in a defect, it exhibits a characteristic lifetime that is different from annihilation with a free electron. Positron annihilation has been used extensively in the study of metals, metal oxides, alloys and semiconductors [73-81], for the reason that it is possible to determine such important characteristics of metals as the electron momentum distribution, the Fermi energy ep, the number of free electrons Zc per metal atom, and the concentration (in cm ) of such electrons in the conduction band, together with the concentration of defects and their type. These characteristics, as is well known, largely determine the mechanical, electrical, magnetic and optical properties of materials [71]. In semiconductors, movement of the Fermi level and defect concentrations can be accurately measured. These studies have established that positron methods are defect-sensitive and nondestructive in nature, as the information is carried away by the aimihilation photons and the material can be reused after the test. 

Positrons diffusing through matter can be captured in special trapping sites. As shown in early studies, these trapping centres are crystal imperfections, such as vacancies and dislocations. The wavefunction of a positron captured in such a defect is localised until it annihilates with an electron of its immediate surroundings into y-rays. Since the local electron density and the electron momentum distribution are modified with respect to the defect-free crystal, the annihilation radiation can be utilised to obtain information on the localisation site. The different positron techniques are based on analysing the annihilation radiation. The principles of the basic positron methods are illustrated in Figure 4.27 [84]. 

One can immediately conclude that thermalisation times are very short compared to positron bulk lifetimes tb. The fact that positrons reach thermal energies very quickly is important for the application of the positron method. Clearly only thermalised positrons annihilate. The momentum of positrons is thus very small compared to the momentum of the electrons with which they annihilate. It is evident from Table 4.14 that positron scattering off phonons occupies more than 50% of the thermalisation time. Therefore, the positron spends most of the slowing-down process with an energy slightly above the thermal energy. This fact is important for the possible trapping of non-thermalised positrons. 

Positron Imaging. Creating images of distributions of positron emitters requires a somewhat different type of apparatus. Positron cameras use many of the same technologies as do cameras for other isotopes, but there is a broader array of methods and physical arrangements. AH of these systems take advantage of the physical characteristics of positrons. 

The other method for quality assurance inspection of pellet wt in the primer eliminates the need for a comparator oxygen-containing standard. Here, the Cu in the cup-anvil combination in the primer is used as an internal standard by comparing the 0.511 MeV positron annihilation radiation from 62Cu produced by the 63Cu(n,2n)62Cu reaction to the 6.1 MeV 7 from l6N produced by oxygen activation. In this case the actual determination of pellet wt is not required the ratio of Cu to O, which should be fixed for a pro-... 

In this work positron annihilation lifetime spectroscopy (PALS) was used to investigate structural diversity inside zeolite precursor matrix caused by the presence of alkali cations Na, K, Rb and Cs. PALS is an established and well-proven method for structural investigations of various materials, extensively used for metals and alloys, semiconductors and porous materials [3, 4]. In the investigations of zeolites PALS has been mostly used for their void structure and size study [5, 6, 7, 8], also in correlation to... 

Breier, A., Su, T. P., Saunders, R. et al. Schizophrenia is associated with elevated amphetamine-induced synaptic dopamine concentrations evidence from a novel positron emission tomography method. Proc. Natl Acad. Sci. USA 94 2569-2574,1997. 

Xenon difluoride labelled with positron-emitting F has been prepared by reaction of cyclotron produced [ F]p2 with xenon [88]. This low-yielding method requires high pressure. [ F]Xep2 was also obtained by treating sulfuryl chloride fluoride solutions of Xep2 in fluorinated ethylene propylene vessels with Bronst-... 

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