Positron Annihilation Spectroscopy PAS

PA techniques can therefore be used to assess the amount of damage in a material, if that damage gives rise to vacancies or other positron trap sites. 

When positrons enter a material they annihilate with electrons in the material, giving rise coincidentally to two annihilation y-rays of -511 keV (equal to the electron or positron mass) at nearly 180° apart (Fig. 9.21). The annihilation occurs in a time (lifetime) after their emission from the positron source which depends on the density of electrons in the metal around the positron at the time of annihilation. The energy spread of the y-rays and the angle between them depend on the momentum of the annihilated electron. There are several experimental methods that can be used to measure these quantities including positron lifetime, positron annihilation 

Irradiation Embrittlement of Reactor Pressure Vessels (RPVs) 

Diffusion of thermalised positron and subsequent trapping at an open volume defect ( 10 s) 

The presence of vacancy defects, or other positron traps, disturbs the balance of positron annihilation statistics between annihilations with core and conduction electrons. At positron traps, annihilation is most likely to be with unlocalised conduction electrons which have a smaller momentum distribution than the core electrons. Thus as, for instance, the vacancy defect concentration is increased so also is the probability of the positron annihilating with conduction electrons rather than core electrons. This results in a slight reduction in the width of a histogram of the energies of the annihilation y-rays and also in the angular range between the coincidentally emitted y-rays. 

This method has also been called PASCA (Positron aimihilation spectroscopy for 

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Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Spectroscopy, 490. See also 13C NMR spectroscopy FT Raman spectroscopy Fourier transform infrared (FTIR) spectrometry H NMR spectroscopy Infrared (IR) spectroscopy Nuclear magnetic resonance (NMR) spectroscopy Positron annihilation lifetime spectroscopy (PALS) Positron annihilation spectroscopy (PAS) Raman spectroscopy Small-angle x-ray spectroscopy (SAXS) Ultraviolet spectroscopy Wide-angle x-ray spectroscopy (WAXS)... 

Positron annihilation spectroscopy (PAS) was first applied to investigate [Fe(phen)2(NCS)2] [77]. The most important chemical information provided by the technique relates to the ortho-positronium lifetime as determined by the electron density in the medium. It has been demonstrated that PAS can be used to detect changes in electron density accompanying ST or a thermally induced lattice deformation, which could actually trigger a ST [78]. 

Numerous applications of positron annihilation spectroscopy (PAS) in investigations of the physico-chemical properties of matter require a precise understanding of the process of Ps formation. Usually it proceeds on a picosecond time scale and is strongly influenced by early (pico- and femtosecond) processes initiated by ionizations in the track of a fast positron. These early intratrack processes initiate all subsequent chemical transformations and, consequently, take a key position in radiation chemistry. 

Attempts to verify the above volume diffusion mechanism experimentally included X-ray and electron diffraction experiments with electrodes that were corroded at > Ec, as well as investigations by positron annihilation spectroscopy (PAS). In the former case, the occurrence of broadened diffraction lines at Bragg angles between those of the bulk alloy and the pure, noble component was taken as a confirmation of the volume diffusion mechanism [54, 120, 131]. More direct evidence was obtained from the PAS experiments with dezincified brass, where experimental positron Kfetimes correlated well with calculated values in vacancies or vacancy aggregates [78-80]. On the other hand, it has been objected that Eq. (20) predicts a dependence of the current density, which is in contradiction to many experimental results. It has been shown, however, that this particular problem may... 

Positron annihilation lifetime spectroscopy (PALS) can be used to measure the free volume in various materials. Jean et al. discussed the application of positron annihilation spectroscopy (PAS) in the detailed study of polymers and polymers with fillers. The primary experimental PAS technique used in this research is PALS, one of the three techniques in the PAS family and a powerful tool for measuring the free volume in various materials. The free volume has a great role in polymer research and is widely used to explain the behaviour of physical properties such as glass transition temperature, viscosity and physical ageing. Free volume is affected by the blending of polymers, ageing and addition... 

In the case of extremely short half-lives, the starting point of decay-constant determination is still something like Eq. (7.8), but special tricks/equipment (coincidence circuit, time-to-amplitude converter, multichannel analyzer) as well as special conditions (available start and stop signals informing of the birth and death, respectively, of individual nuclei/particles) are also needed. The waiting-time distribution shown in Fig. 9.13 of Chap. 9 in Vol. 1 has also been obtained with such an apparatus, which is the standard equipment of Positron Annihilation Spectroscopy (PAS) measurements (see Chap. 27 in Vol. 3). 

Positron annihilation spectroscopy (PAS) is used in research on free volume (95) or on aging effects in polymers (96), and muon spin resonance with muon beams (97) 5delds information on d5mamic processes in polymers (98), but specimen size may be limited. 

One of the new techniques to scan membranes and determine pore size distributions is positron, annihilation spectroscopy (PAS). With this method also the free spaces in nanofiltration membranes can be determined. For instance in a study by Boussu et al. [9] it was found that some much-studied NF membranes (like Desal-5 DL, NTR7450) have two sizes of spaces, one size about 0.12-0.15 nm and the other between 3nm and 4nm. It could be speculated that diffusive transport would happen through the smaller spaces (the size of a water molecule), depending on the hydrophilicity properties of the membrane, and convective transport through the larger spaces, depending on size and charge conditions. 

One of the main tasks of nuclear-reactor safety research is assessing the integrity of the reactor pressure vessel (RPV). The properties of RPV steels and the influences of thermal and neutron treatments on them are routinely investigated by macroscopic methods such as Charpy V-notch and tensile tests. It turns out that the embrittlement of steel is a very complex process that depends on many factors (thermal and radiation treatment, chemical compositions, conditions during preparation, ageing, etc.). A number of semi-empirical laws based on macroscopic data have been established, but unfortunately these laws are never completely consistent with all data and do not yield the required accuracy. Therefore, many additional test methods are needed to unravel the complex microscopic mechanisms responsible for RPV steel embrittlement. Our study is based on experimental data obtained when positron annihilation spectroscopy (PAS) and Mdssbauer spectroscopy (MS) were applied to different RPV steel specimens, which are supported by results from transmission electron microscopy (TEM) and appropriate computer simulations. 

In particular, the most powerful method for studing lattice defects, due to the high sensitivity of positrons to open volume defects such as vacancies, vacancy clusters, voids, dislocations, grain boundaries, etc., is positron annihilation spectroscopy (PAS). A diagram illustrating the applicability of PAS and other techniques as a function of defect size and density versus depth in material is shown in Figure 4.25. Thus, PAS represents a non-local experimental technique that is sensitive to microstructural defects at the atomic scale. A well-established theory of positron annihilation phenomena is currently available. Especially for metallic materials, it is possible to perform ab initio calculations of positron parameters for various defects and atomic arrangements [72,73]. 

Positron annihilation spectroscopy (PAS) is an excellent technique for investigating vacancy clusters and vacancy—solute complexes behavior during irradiation since positrons are very sensitive to these types of defects. These defects are important for the formation of the feamres responsible for hardening. In this technique, positrons are applied as a probe and positrons are trapped by defects with electron densities different from the bulk materials. These defects can be vacancies, vacancy clusters, interfaces, second-phase particles, dislocations, etc. [69]. Positrons annihilate with a different probability in the defects as compared to the bulk material because of the difference in positron affinity to different atomic species [69]. The advantage of the technique lies in its nondestructiveness, self-seeking nature, and ability to find small defects (>0.1 nm) even in low concentrations (>1 ppm) [69]. PAS can provide information... 

Positron annihilation spectroscopy (PAS) is a special nondestructive evaluation (NDE) technique for materials... 

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