Positron annihilation compounds

Abstract. Free-volume structure in the lanthanum salt of laurinic acid in crystalline and liquid-crystalline states and an effect of dissolved Cgo molecules on the mean nanovoid radius and concentration were studied by means of the positron annihilation technique. La(Ci2H25COO)3 clathrate compound with dissolved C6o molecules was obtained, which is thermodynamically more stable than a simple mixture of components. Increased mean nanovoid radius (from 0.28 to 0.39 nm) after the inclusion of C6o molecules and concomitant decrease of the positronium annihilation rate by a factor of 2.7 indicate the decrease of the smallest nanovoid concentration. 

National Science Foundation Optical density Postapply bake Photoactive compound Photochemical acid generator Positron annihilation spectroscopy Poly(fert-butoxycarbonyloxystyrene)... 

For most of the intermetallic compounds, deviation from the stoichiometric composition is accommodated by antisite atoms CuZn, CusAu, NiTi, NisAl. Paired antisite defects are also formed thermally in high concentration for those alloys which present an order-disorder transition in the solid state (CU3AU, CuZn) their concentration is directly related to the LRO parameter value. The absence of structural vacancies has been checked by positron annihilation in the case of CuZn (Kim and Buyers, 1980), CujAu (Doyama et fl/.,1985a,b), FejAl (Schaefer et al., 1990), and TiAl (Shirai and Yamaguchi, 1992). In all cases, thermal vacancies form at high temperature. 

Bradley and Taylor (1937) observed that the density of Al-rich NiAl was smaller than calculated from the lattice parameter, assuming two atoms per unit cell. They concluded that the excess Al atoms do not substitute on the Ni sublattice, but that vacancies must be created on that sublattice. At room temperature, in Ni45Al5s, lO yo of the Ni sites are vacant. These vacancies, determined by composition and not temperature, are called structural (or constitutional). On the other hand, on the Ni-rich side, the extra nickel atoms substitute normally for Al atoms on the Al sublattice. The composition dependence of the vacancy content at room temperature in the NiAl intermetallic compound was checked by positron annihilation (Doyama et al., 1987) and detailed X-ray diffraction studies (Kogachi et al., 1992). 

Positron annihilation has been proven to be an effective tool to probe the interfacial characteristics of polymer nanocomposites. Zhang et al. used positron annihilation to study structures of melt-compounded HDPE/CaCOj nano-compositites in which nano-CaCOs fillers were treated with a coupling agent. A rapid decrease of free-volume concentration with increasing nano-CaCOs weight fraction was observed. An interfacial layer between the nano-CaCOs fillers and HDPE was also detected, which verified that die interactions between the nano-CaCOs and HDPE matrix indeed restricted the chain segmental motion and reduced the mobilization of polyethylene chains. 

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

Fig. 7.21. Angular correlation curves for mixtures of O2 and CI2 gases with an overall pressure of 120 atmospheres, (a) Pure O2, (b) O2 with 0.02 atmospheres of Cl2, (c) O2 with 0.05 atmospheres of CI2, (d) 02 with 0.2 atmospheres of CI2 and (e) O2 with 1 atmosphere of CI2. Goldanskii and Mokrushin (1968) attributed the components labelled Wi, W2 and W3 to the annihilation of thermalized para-positronium atoms (Wi, the narrow component), the annihilation of free positrons in O2 (W2) and the annihilation of positrons in the PsCl compound (W3). The intensity of the last, i.e. W3, grows progressively with the addition of CI2 to the O2 buffer.

A different method is PET (positron emission tomography) [4,5]. It is al.so an in-vivo tracer technique, but uses the annihilation of positions. Actually, one is confined to positron emitters for in-vivo studies of the distribution of elements in biologically active compounds, viz. carbon, nitrogen, oxygen. Metabolic processes can be studied in this way (see also Table 9.2). 

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