Positron lifetime spectroscopy

When a positron (generated e.g. by a Na source) enters a condensed medium, it may be annihilated directly with an electron, or it may capture an 

Positronium lifetime dependence on pore diameter. Solid line is the theoretical relation found from the Schrddinger equation for spherical pores [118]. 

The experimental set-up consists of a positron source ( Na), a scintillation counter, to detect the y radiation from the positronium decay, and electronic peripheral equipment to analyse the time spectrum of the positron annihilation. 

Positronium lifetime spectroscopy is particularly well suited for stud)hng defects in crystals and structural fluctuations in amorphous materials and can give an estimate of free volumes in condensed matter [116]. It is a useful technique to estimate the free volume of polymeric membranes [117]. In a study on silica gels, the decay lifetime has been found (Fig. 4.16) to be proportional to the pore diameters (measured by N2 adsorption) between 30 and 100 A [118]. Information on pore size distribution and surface area may also be obtained by means of calibration curves. 

---

The influence of alkali cations on the structure of zeolite precursor gels investigated by positron lifetime spectroscopy... 

The applicability of positron lifetime spectroscopy for the characterization of the partly charged nickel hydroxide was investigated [90]. The positron lifetime spectra of 8-Ni(OH)2/j8-NiOOH systems were presented. Three different parts of the annihilation curves were observed and identified. 

Positron lifetime spectroscopy radiochemical (nuclear) methods in electrochemistry... 

The most important techniques are -> tracer methods (i), Mossbauer spectroscopy (ii), neutron activation (iii), thin layer activation (TLA) (iii), ultrathin layer activation (UTLA) (iii), and positron lifetime spectroscopy (iv). 

Shaefer, H.-E., Wurschum, R. Schwarz, R., Slobodin, D. et al. (1986) Amorphous hydrogenated silicon studied by positron lifetime spectroscopy , Appl. Phys. A 40, 145. 

Suzuki, R., Kobayashi, Y., Mikado, T., Ohgaki, H. et al. (1992) Investigation of near surface defects by variable-energy positron lifetime spectroscopy", Mater. Sci. Forum 105-110,1459. 

A new spectroscopic method for the characterization of surface vacancy clusters is a combination of positron lifetime spectroscopy, which determines the size of vacancy clusters, and coincidence Doppler broadening of annihilation radiation, which gives information on where vacancy clusters are located [5, 6]. If these clusters are located on the surface of gold nanoparticles, namely the interface between the particle and host matrix, the surroundings of the clusters should include both particle atoms and the matrix atoms. Doppler broadening of annihilation radiation (DBAR) with two-detector coincidence should be able to reveal these atomic constituents, and therefore elucidate the location of vacancy clusters. 

Kluin, J.E., Vleeshouwers, S., McGervey, J.D., Jamieson, A.M., Simha, R. (1992) Temperature and time dependence of free volume in bisphenol-A polycarbonate studied by positron lifetime spectroscopy . Macromolecules. 25, 5089. 

Bohlen, J., Wolff, J., Kirchhelm, R. (1999) Determination of free-volume and hole number density in polycarbonates by positron lifetime spectroscopy . Marcromolecules, 32,3766. 

H.E. Shaefer, R. Wurschum, R. Birringer and H. Gleiter, Structure of nanometer-sized poly crystalline iron investigated by positron lifetime spectroscopy. Phys. Rev. B, 38 (14) (1988) 9545. 

To understand the importance of nanostructures in microsieving membranes, the basic structure of nanophased ceramics must be briefly described. Because the particles are extremely small, one to a few tens of nanometers, an important fraction of the atoms is found in or very near the interface between grains, as reported in Table 2 [32]. Figure 11 is a schematic representation of a nanophase material. One can see that individual grains in the 5 nm range induce a biphasic material with an interfacial phase between the grains and a residual nanoporosity, evidenced by positron lifetime spectroscopy [33]. Transmission electron microscopy is also a well-adapted technique for nanoscale structure characterization, as illustrated later. 

Positron lifetime spectroscopy has been shown to be a good means of investigating the structural levels of nanocrystalline materials [48]. Different annihilation sites (dislocations, micropores, and mesopores) have been attributed to the different measured positron lifetimes. 

Becvar, R, Methodology of positron lifetime spectroscopy present status and perspectives, Nud. Instrum. Methods Phys. Res. B, 261, 871-874 (2007). 

Monge, M. A., Diaz, J. A., and Pareja, R., Slrain-mduced changes of free volume measured by positron lifetime spectroscopy in ultrahigh molecular weight polyethylene. Macromolecules, 37, 7223-7230 (2004). 

The second study (Meghala and Ranganathaiah 2012) was dedicated to the evaluation of interfaces in poly(styrene-co-acrylonitrile) (SAN)-based ternary polymer blends using also positron lifetime spectroscopy. The method successfully applied for binary blends (single interface), mentioned above, was theoretically modified for ternary blends and experimentally verified by measuring free volume content in blends and their constituents. They tested the efficacy of this method in two ternary blends S AN/PVC/PMMA and SAN/EVA/PVC at different compositions. The effective hydrodynamic parameter evaluated using individual values turned out to be handy in predicting the overall miscibility level of a ternary blend. 

Figure 27.2 outlines a general scheme of fast-fast coincidence systems. For positron lifetime measurements, several special requirements should be fulfilled. In positron lifetime spectroscopy, the whole system should be very fast because of the expected very short lifetimes (from lOO ps to 100 ns). Thus, the response of detectors (Leo 1987a) should be as fast as possible (rise time 2 ns). To ensure the sharp response signal, fast plastic scintillators... 

With more and more data/experiments having been collected, positron lifetime spectroscopy might become a unique characterization method for polymers, that can even give qualitative results on the free-volume hole distribution of polymers. 

Radiochemical methods, such as tracer methods [1-3], Mossbauer spectroscopy [4], neutron activation [5], thin layer activation (TLA) [5], ultrathin layer activation (UTLA) [5], and positron lifetime spectroscopy [6], are applied for the study of a wide range of electrochemical surface processes. The most important areas are as follows adsorption and electrosorption occurring on the surface of electrodes the role of electrosorption in electrocatalysis deposition and dissolution of metals corrosion processes the formation of surface layers, films on electrodes (e.g., polymer films), and investigation of migration processes... 

Siivegh K, Horanyi TS, Votes A (1988) Characteriza-tirai of the P-Ni(OH)2/P-NiOOH system by positron lifetime spectroscopy. Electrochim Acta 33 1061-1066... 

Ishibashi, S., Y. Suzuki, H. Maruyama, R. Yamamoto and M. Doyama, 1989, Positron lifetime spectroscopy on high-Tj superconductor ABajCujO,... 

The most commonly used experimental methods for observing positron annihilation in matter are the three methods mentioned above (i) measurement of the positron mean lifetime, using positron lifetime spectroscopy (PLS) (ii) measuring... 

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. 

Positron annihilation spectroscopy can provide essential information about the deterioration in the mechanical properties of RPV steels (microstructural defects and precipitates) during their irradiation, which is known as neutron embrittlement. Currently, there are three main techniques based on annihilation phenomena positron lifetime spectroscopy, Doppler-broadening spectroscopy and angular correlation measurements. 

---