Positron lifetime

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

These traps, (Fig. 6) and similar effects in the motion of holes and other charges through polymers, would eventually be correlated also with such structural probes as positron lifetimes in macromolecular solids. Extensive recent studies of positron lifetime are based on positronium decay. In this, the lifetime of o-positronium (bound positron-electron pair with total spin one) is reduced from about 140 nanoseconds to a few nanoseconds by "pick-off annihilation" in which some unpaired electron spins in the medium cause conversion quenching of orthopositronium to para-positronium. The speed of the t2 effect is supposed, among other things, to represent by pick-off annihilation the presence of defects in the crystalline lattice. In any case, what amounts to empty space between molecules can then be occupied by orthopositronium.(14,15,16) It is now found in linear polyethylene, by T. T. Wang and his co-workers of Bell Laboratories(17) that there is marked shift in positron lifetimes over the temperature range of 80°K to 300°K. For... 

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. 

Influence of filler content on the positron lifetime parameters... 

Figure 2. The relative intensities of the positron lifetimes as a function of the weight fraction of glass beads. The straight lines are least-square fitted through the data points.

Heat of fusion (AH) of the composites together with the intensities and lifetimes of the two longest positron lifetime components... 

The positron lifetime spectra of polyethylene and glass-filled polyethylene were resolved in four exponentials, representing different annihilation processes. The... 

Current theories of particle physics predict that, in a vacuum, the positron is a stable particle, and laboratory evidence in support of this comes from experiments in which a single positron has been trapped for periods of the order of three months (Van Dyck, Schwinberg and Dehmelt, 1987). If the CPT theorem is invoked then the intrinsic positron lifetime must be > 4 x 1023 yr, the experimental limit on the stability of the electron (Aharonov et al., 1995). 

Fig. 1.4. (a) Schematic of a positron lifetime spectrometer the star indi-... 

In solids the free positron lifetime r lies in the approximate range 100-500 ps and is dependent upon the electron density. Following implantation, the positrons are able to diffuse in the solid by an average distance L+ = (D+t)1//2, where D+ is the diffusion coefficient. This quantity is usually expressed in cm2 s-1 and is of order unity for defect-free metallic moderators at 300 K (Schultz and Lynn, 1988). The requirement of very low defect concentration arises because the value of D+ is otherwise dramatically reduced owing to positron trapping at such sites. 

Additional, but rather less direct, evidence for the accuracy of the variational results for models H5 and H14 is provided by the excellent agreement between the theoretical and experimental lifetime spectra for positrons diffusing in helium gas, where calculation of the theoretical spectrum requires a knowledge of the momentum transfer and annihilation cross sections, both of which are derived from the wave functions generated in the calculations of the elastic scattering phase shifts. A detailed discussion of positron lifetime spectra is given in Chapter 6. 

In the section on excitation we shall treat only electronic transitions thus rotational and vibrational processes in molecules are excluded. As will be described in Chapter 6, information on these latter processes has been derived from positron lifetime and other experiments. Our theoretical discussion will mainly concern excitation of the lower levels of... 

Fig. 6.5. Examples of positron lifetime spectra for (a) argon and (b) xenon gases. The argon data are for a density of 6.3 amagat at 297 K. The channel width is 1.92 ns. In (a), (i) shows the raw data, (ii) shows the signal with background removed, (iii) shows the free-positron component and (iv) shows the fitted ortho-positronium component. In (b), the spectrum for xenon is for room temperature and 9.64 amagat and has a channel width of 0.109 ns. The inset shows the fast components as extracted and discussed by Wright et al. (1985).

Fig. 6.13. Superimposed zero field and pulsed field (81 V cm-1 peak amplitude) positron lifetime spectra. The pulsed field spectrum has been decomposed into heated components (broken line) and unheated components (crosses) to illustrate how the electric field splits up the positron ensemble. This is also illustrated by the inset, which shows, schematically, the energy distribution p(E,t) of the positron ensemble in the two-threshold model (see text). Reprinted from Physical Review Letters 56, Tawel and Canter, Observation of a positron mobility threshold in gaseous helium, 2322-2325, copyright 1986 by the American Physical Society.

Positron lifetime spectra for the noble gases. J. Phys. B At. Mol. Phys. 8 1734-1743. 

Osmon, P.E. (1965). Positron lifetime spectra in molecular gases. Phys. Rev. 140 A8-A11. 

Poster 31. Masataka Mizuno, Teruo Kihara, Hideki Araki, Yasuharu Shirai and Takashi Onishi (Osaka University, Kobe Steel Ltd.) Theoretical Calculation of Positron Lifetimes of Vacancy Clusters in Cu... 

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). 

Ps02 1959 > 2.3 Positron lifetime spectra in liquid oxygen [62] gas-phase quenching data with a Born cycle interpretation [63]. 

OPs 1969 2.2(5) Positron lifetime data and thermodynamic argument based on a proposed mechanism involving the oxidation of Ps by H+ [64]. 

PsF 1969 2.9(5) Interpretation of positron lifetime spectra in liquid C6H6 vs C6H5F [64]. 

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