Positron lifetime spectrum

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. 

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

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. 

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. 

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

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

Figure 3.4 Positron lifetime spectra for Ar gas (left) and In (right). Note time scales.

Figure 9.1 Positron lifetime spectra of the PECVD grown a-Si H films prepared at power densities of 0.03 W/cm2, 0.13 W/cm2,0.51 W/cm2, and 0.76 W/cm2. (Suzuki et a ., 1991)...

Figure 9.3(a) shows positron lifetime spectra of a porous Si thin film at the sample temperatures of 25°C (initial), 300°C, and 500°C, and Figure 9.3(b) shows positron lifetime spectra measured at 500°C and at 200°C after 500°C annealing. Strong temperature dependence was observed in the long-lived component. 

Figure 9.4 Positron lifetime spectra of amorphous Si02 (500 nm) on Si(100) at the positron incident energy of 2 keV and 15 keV.

Figure 9.8 (a) Positron lifetime spectra and (b) annihilation rate probability function for low-k films grown by a double-frequency PECVD method with different LF powers. (Suzuki et al., 2001)... 

Figure 9.9. (a) Positron lifetime spectra of porous Si02 with and without a Si02 cap layer (511 keV photo peak), (b) Pulse height spectra of the y-ray detector for the long-lived component in the annihilation time range between 14.4 ns and 220 ns from the peak. 

Insert of Figure 13.2 shows the positron lifetime spectra for MgO (open circles), Au-implanted MgO (crosses) and Au nanoparticles embedded in MgO (solid circles). These spectra were deconvoluted using Laplace inversion [CONTIN, 7] into the probability density functions (pdf) as a function of vacancy size. Figure 13.2 shows the pdf spectra for the MgO samples accordingly. The positron lifetime components obtained for the MgO layer are 0.22 0.04 ns with 89 3% contribution and 0.59 0.07 ns with 11 3% contribution. For the Au-implanted sample without annealing, the major lifetime component is at 0.32 ns. For the Au nanoparticle-embedded MgO, lifetime components are 0.41 0.08 ns at 90% and 1.8 0.3 ns at 7%. 

The positron source, 120 kBq of Na, was deposited onto a Kapton foil covered with identical foil and sealed. The foil 8 pm thick absorbed 10% of positrons in polyimides Ps does not form and annihilation in the source envelope gave one component only = 374 ps, which must be taken into account. The source was sandwiched between two samples of the material studied and placed into a container in a vacuum chamber. The source-sample sandwich was viewed by two Pilot U scintillators coupled to XP2020Q photomultipliers. The resolution of our spectrometer with a stop window broadened to 80% (in order to register the greatest number of three-quantum decays) was 300 ps FWHM. The finite resolution had no influence on the results of our experiment as FWHM was still comparable to the channel definition At = 260 ps.The positron lifetime spectra were stored in 8000 channels of the Tennelec Multiport E analyser. 

Kirkegaard, P, and Eldrup, M., Positronfit a versatile program for analysing positron lifetime spectra, Comput. Phys. CommurL,3, 240-255 (1972). 

Shukla, A., Peter, M., and Hoffmann, L., Analysis of positron lifetime spectra using quantified maximum entropy and a general filter, Nucl. Instrum. Methods Phys. Res. A, 335, 310-317 (1993). 

Data analysis with routine LT9.0 and interpretation of positron lifetime spectra... 

DATA ANALYSIS WITH ROUTINE LT9.0 AND INTERPRETATION OF POSITRON LIFETIME SPECTRA... 

When using routine LT9.0, a partial improvement occurs by assuming that the o-Ps lifetime xs shows a distribution. The artifacts are removed more or less completely when the distribution in the e+ lifetime X2 is also taken into account. As mentioned, we have confirmed these conclusions by LT9.0 analysis of simulated spectra. The allowance of a distribution in x uncouples the t2 analyzed from T3, and the allowance of a distribution in X2 uncouples ri from t2. The reason that LT9.0 avoids artifacts also observed in continuous Melt and Contin analysis lies in reduction in the degree of freedom by assuming a number of different lifetime channels in LT9.0. Moreover, because the lifetime analysis is less sensitive to the particular shape of the distributions, the assumed lognormal k function usually seems to describe the real situation sufficiently well. These are the reasons that we prefer to use the routine LT9.0 for the analysis of positron lifetime spectra. 

Nowadays positron lifetime spectra are almost exclusively measured by fast-fast coincidence systems (O Fig. 27.2). 

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