Positron Lifetime Experiments

Positron lifetime measurements can be used to investigate the type and the density of lattice defects in crystals [293]. In solid materials positrons have a typical lifetime of 300 to 500 ps until they are annihilated by an electron. When positrons diffuse through a crystal they may be trapped in crystal imperfections. The electron density in these locations is different from the density in a defect-free crystal. Therefore, the positron lifetime depends on the type and the density of the crystal defects. When a positron annihilates with an electron two y quanta of 511 keV are emitted. The y quanta can easily be detected by a scintillator and a PMT. 

The positrons for the lifetime measurement are conveniently obtained from the deeay of Na. In Na a 1.27 MeV y quantum is emitted simultaneously with the positron. This 1.27 MeV quantum is used as the timing reference for the positron lifetime measurement. The general experimental setup is shown in the Fig. 5.139. 

The Na source is placed between two identical samples. Two XP 2020 photomultipliers equipped with scintillators are attached directly to the two samples. The pulses from the photomultipliers are used as start and stop pulses for the TCSPC module. The pulses from PMT 2 are delayed by a few nanoseconds so that a stop pulse arrives after the corresponding start pulse. Eaeh y quantum generates a large number of photons in the scintillator. Therefore, the PMT pulses are multiphoton signals, and the time resolution can be better than the transit time spread of the PMTs. Moreover, the amplitudes of the photomultiplier pulses are proportional to the energy of the particle that caused the scintillation. Therefore the amplitudes can be used to distinguish between the 511 keV events of the positron decay and the 1.27 MeV events from the Na. The discriminator thresholds for start and stop are adjusted in a way that the stop channel sees all, the start channel only the larger Na events. The rate of the Na events is of the order of a few kHz or below. 

Therefore it is unlikely that a time measurement is started and stopped by two successive 1.27 MeV quanta of the Na decay. The by far most likely start-stop event is the detection of a 1.27 MeV quantum in the start PMT followed by the detection of a positron in the stop PMT. The histogram of these events gives the desired positron lifetime distribution. A typical result is shown in Fig. 5.140. 

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Fig. 5.140 Result of a positron lifetime experiment. SPC-630 TCSPC Module, XP2020 PMTs. Acquisition time 20 minutes...

Dlubek, G., Shaikh, M. Q., Raetzke, K., Faupel, R, and Paluch, M., The temperature dependency of the free volume from positron lifetime experiments and its relation to structural dynamics phenylphthalein-dimethylether, Phys. Rev. E, 78, 051505-1 to 051501-10 (2008b). 

Another frequently used quantity to analyze positron lifetime experiments is the mean lifetime Xm -... 

The positron lifetime experiments were carried out with a fast-slow coincidence ORTEC system with a time resolution of about 230 ps full width at half maximum. A 5mCi source of Na was sandwiched between two identical samples, and the total count was one million. The temperature-dependent Doppler broadening energy spectroscopic (DBES) spectra were measured using an HP Ge detector at a counting rate of approximately 800 cps. The energy resolution of the solid-state detector was 1.5 keV at 0.511 MeV (corresponding to positron 2y annihilation peak). The total... 

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). 

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... 

In this section we review the results from positron annihilation experiments, predominantly those performed using the lifetime and positron trap techniques described in section 6.2. Comparisons are made with theory where possible. The discussion includes positron thermalization phenomena and equilibrium annihilation rates, and the associated values of (Zeff), over a wide range of gas densities and temperatures. Some studies of positron behaviour in gases under the influence of applied electric fields are also summarized, though the extraction of drift parameters (e.g. mobilities) is treated separately in section 6.4. Positronium formation fractions in dense media were described in section 4.8. 

We now briefly review experimental evidence for the existence of some simple positronium compounds more detailed accounts have been given for early lifetime experiments by Goldanskii (1968) and for the liquid phase by Mogensen (1995). In the case of PsCl we shall see how traditional positron experiments using lifetime and ACAR techniques have provided strong evidence for the stability of this compound, in accord with theory. The first direct experimental evidence of the existence of PsH came from a positron-beam experiment (Schrader et al, 1992). 

This chapter will begin by looking at some of the hardware requirements for positron-based experiments and then move on to their application in the measurement of angular correlation, positron lifetimes and Doppler broadening parameters. We shall then look at the generation and application of beams of mono-energetic positrons. 

Further examples of positron study of defects in HTSC are studies carried out to understand the nature of flux-pinning defects that lead to an increase in critical-current density on neutron-irradiated Y 1 2 3. Experiments [59] on positron lifetime and critical-current density measurements on various neutron-irradiated samples of Y 1 2 3 indicate that the critical current density is correlated with the micro-void density, as obtained from the analysis of positron lifetime measurements. Investigation of defects in other HTSC superconductors, such as La-Sr-Ca-Cu-0 [60], Bi-Sr-Ca-Cu-0 [49], and Nd-Ce-Cu-O [52], have also been carried out. 

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. 

Dlubek, G., Pointeck, J., Shaikh, M. Q., Hassan, E. M., and Krause-Rehberg, R. (2007) Free volume of an oligomeric epoxy resin and its relation to structural relaxation Evidence from positron lifetime and pressure-volume-temperature experiments,/ /ryx Rev. E 75, 021802. 

DLU Dlubek, G., Gupta, A.S., Piontek, J., Krause-Rehberg, R., Kaspar, H., and Lochhaas, K.H., Temperature dependence of the free volume in fluoroelastomers from positron lifetime and PVT experiments (experimental data by G. Dlubek), Macromolecules, 37, 6606, 2004,... 

Dlubek, G., Pionteck, J., and Kilburn, D., The structure of the free volume in poly (styrene-co-acrylonitrile) from positron lifetime and pressure-volume-temperature PVT) experiments I. Free volume from the Simha-Somcynsky analysis of PVT experiments, Macromol. Chem. Phys.,205, 500-511 (2004). 

Dlubek, G., Bondarenko, V., Al-Qaradawi, I. Y., Kilburn, D., and Krause-Rehberg, R., Structure of free volume in SAN copolymers from positron lifetime and PVT experiments II. Local free volume from positron annihilation lifetime spectroscopy (PALS), Macromol. Chem. Phys., 205, 512-522 (2004c). 

These models have been quite useful as a means of explaining some of the phenomena associated with the rate of positron annihilation. Other experiments, however, seemed to indicate that the "free volume" model includes far too few properties apart from the factor of density as to satisfactorily explain variations in the positron lifetimes which occur as a result of phase transitions. It would appear that in this case an important part in the positron annihilation process is played by the nature of the intermolecular Interaction and by the internal order of the structures of the molecular substance. 

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. 

Perkins and Woll (1969) have investigated the possibility of observing the effects of superconductivity on positron thermalisation. If the positron lifetime in the superconducting compound is small, they conclude that a lack of thermalisation of the positron may be observed. This would be reflected, for example, in a loss of resolution in an ACAR experiment. But the effect is small and has never been reported in low-Tc superconductors. It is doubtful that it might be observed in high-To superconductors because positron lifetimes are rather long in cuprates. 

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