Positron experimental methods

In this chapter we consider the physics of the positronium atom and what is known, both theoretically and experimentally, of its interactions with other atomic and molecular species. The basic properties of positronium have been briefly mentioned in subsection 1.2.2 and will not be repeated here. Similarly, positronium production in the collisions of positrons with gases, and within and at the surface of solids, has been reviewed in section 1.5 and in Chapter 4. Some of the experimental methods, e.g. lifetime spectroscopy and angular correlation studies of the annihilation radiation, which are used to derive information on positronium interactions, have also been described previously. These will be of most relevance to the discussion in sections 7.3-7.5 on annihilation, slowing down and bound states. Techniques for the production of beams of positronium atoms were introduced in section 1.5. We describe here in more detail the method which has allowed measurements of positronium scattering cross sections to be made over a range of kinetic energies, typically from a few eV up to 100-200 eV, and the first such studies are summarized in section 7.6. 

MacKenzie, I.K. (1983). Experimental methods of annihilation time and energy spectrometry. In Positron Solid-State Physics, Proc. Int. School of Physics Enrico Fermi , Course 83, eds. W. Brandt and A. Dupasquier (North-Holland) pp. 196-264. 

The values of Df and dh for studied compositions HDPE+Z in table 1 are cited. The value dh can be compared with corresponding experimental data obtained by positrons annihilation method. For HDPE at 7=323 K experimental value is (4 6,8 A [14], This value dh corresponds well enough with corresponding calculated values dh cited in table 1. 

During the past two decades, positron annihilation lifetime spectroscopy (PALS) has developed to be the most important experimental method for studying the free volume... 

Recent years have seen a considerable extension of the experimental methods used in quantum chemistry and in investigations of the nature of chemical bonds in crystals. It is worth mentioning methods based on the studies of the elastic and the inelastic scattering (by crystals) of X rays, electrons, neutrons, protons, mesons, a and other particles, as well as the x ray spectroscopic methods. Methods based on the use of positron annihilation are also of considerable interest. 

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... 

Let us consider, which processes result to necessary for yielding realization fluctuation free volume increasing. Theoretically (within the frameworks of polymers plasticity fractal concept [35] and experimentally (by positrons annihilation method [22]) it has been shown, that the yielding pro-... 

The values and dj for compositions HDPE + Z are adduced in Table 8.1. The values d can be compared with the corresponding experimental data, obtained by the positrons armihilation method. For HDPE at T= 323 K the experimental value 6.8A [31]. This value d corresponds well enough to corresponding calculated values dj, adduced in Table 8.1. 

Apart from the theoretical approaches, electronic energy spectra of carbides and nitrides have been studied using a variety of experimental techniques X-ray emission and photoelectron spectrosopy, optical and Auger spectroscopy, electron energy loss and positron annihilation spectroscopy, etc. However, interpretation of the results obtained requires, as a rule, use of the computational methods of the band theory of solids and quantum chemistry. Moreover, the data provided by theoretical methods are important by themselves, because they give much more detailed information on the electron states and chemical bonding than any of the experimental methods. They also allow us to model theoretically... 

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... 

As noted above, the lower curve, L, is in good agreement with experiment above 100 eV but falls significantly below the experimental values at lower energies. This is attributed by Campeanu et al. (1987) to a failure of Parcell s polarized-orbital method. The experimental work of Coleman et al. (1982) and of Sueoka (1982) is cited in support of this, and the later results of Mori and Sueoka (1994), which supersede the earlier data of Sueoka (1982), do not alter this conclusion. Further discussion of elastic scattering can be found in Chapter 3 positron impact excitation is treated in Chapter 5. 

Until quite recently, helium was the simplest atomic target used in experimental studies of positron collisions. It is also the simplest atom for which the wave function is not known exactly. Accordingly, positron-helium scattering has attracted considerable theoretical attention, and detailed comparisons have been made between the experimental measurements of the scattering parameters and the corresponding theoretical results obtained using a wide variety of approximation methods. 

Most other calculations of positronium formation in positron-helium scattering have employed much simpler methods of approximation, but results have usually been obtained over energy ranges extending well beyond the Ore gap. It must therefore be borne in mind that the experimental results include contributions from positronium formation into excited states as well as into the ground state. The Born approximation, used first by Massey and Moussa (1961) and subsequently by Mandal, Ghosh... 

The positron-trap technique has been used by Surko and coworkers to measure the Doppler broadening of the 511 keV line for positrons in helium gas. This method does not have the drawback of the experiment described above, in which both positronium and free-positron events overlap on the angular distribution curves here the positrons are thermalized prior to the introduction of the gas and therefore cannot form positronium. A comparison of the theoretically predicted and experimentally measured Doppler spectra (Van Reeth et al., 1996) is shown in Figure 6.16. The theoretical results were obtained from the variational wave functions for low energy positron-helium scattering calculated by Van Reeth and Humberston (1995b) see equations (3.75) and (3.77). 

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