Angular correlation studies

Fig. 6.15. Cylindrically averaged angular correlation of annihilation radiation (ACAR) distributions for positron annihilation in the noble gases, (a) helium, (b) neon, (c) argon, (d) krypton and (e) xenon, from the work of Coleman et al. (1994). Reprinted from Journal of Physics B27, Coleman et al, Angular correlation studies of positron annihilation in the noble gases, 981-991, copyright 1994, with permission from IOP Publishing.

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. 

Baba K. et al. 11 ICd Time Dilferential Perturbed Angular Correlation Study of Deformed Ni. Materials Transactions, 43, No. 8, 2125-2129 (2002)... 

L. J. Rouse and P. G. Varlashkin, Angular correlation studies of positron annihilation in copper-nickel alloys, Phys. Rev. 4, 2377-97 (1971). 

Several experimental techniques such as Compton scattering, positron annihilation, angular correlation, etc., are used for measuring momentum densities. One of the most popular techniques involved in measuring momentum densities is termed as electron momentum spectroscopy (EMS) [29]. This involves directing an electron beam at the surface of the metal under study. Hence EMS techniques fall under what is classified as coincidence spectroscopy. 

Notice that we want to use 2 d-type functions in the basis. This is because for correlated wave functions the d-type functions serve two purposes, which cannot easily be accomplished by one basis function First they serve as polarization functions, polarizing the electron density in the bonding region, a feature that is especially important in 7t-bonded systems. Secondly, they give important contributions to the angular correlation effects, which is very important in linear molecules. Studies of the ANO s shows that the two properties of the d-type functions cannot be incorporated into one function. Hie polarization d is rather diffuse, while the correlating d is a much more contracted function. We therefore prefer to include two d-type functions in the basis set... 

The long-slit, i.e. one-dimensional, angular correlation (ACAR) apparatus used for these studies is similar to that shown in Figure 1.6. Silica aerogel (see subsection 1.5.2) was employed as a convenient source of positronium, emitted into the space between the pores. The chamber could be evacuated and the high purity gases under investigation then... 

An inexpensive target system for the production of the NCA radioisotope, 77Br323,324, has been described recently325 and applied for time differential perturbed angular correlation (TDPAC) studies which provide information of physical interest. TDPAC has also found application in the field of chemistry326 and biology31. 

Besides NQR spectroscopy and the study of nuclear quadrupole interaction effects in broad-line NMR spectroscopy, paramagnetic electron resonance 6°1, Mossbauer spectroscopy, and the study of perturbed angular correlation of y-rays, are suitable methods for studying nuclear quadrupole interactions in solids. Indirect methods are also available for acquiring information about the nuclear quadrupole couplinjg constant from the liquid state (particularly NMR spectroscopy in liquids and in liquid crystals in some cases gives information about this constant). By microwave spectroscopy, the nuclear quadrupole interaction may be studied in the gaseous phase (see the paper by Zeil). We shall deal here only with the aspect of NQR spectroscopy in solids since this method has the broadest applicability to chemical problems in comparison with the other methods mentioned. 

An intrafullerene dynamics of Ce atom in the Cs2 cage was also studied by time-differential perturbed angular correlation measurements (Sato et al., 1998). The observed angular correlation shows the presence of two different chemical species of Ce Cs2- The data at low temperatures reveal that Ce stays at a certain site for one of the species, whereas for the other the atom has an intramolecular dynamic motion. 

R. Bauer, Carbonic anhydrase studied by perturbed angular correlation of gamma rays Thesis/Dissertation, University of Copenhagen, Niels Bohr Institute, 1976. 

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