Annihilation detection techniques

The emitted gamma photons produced by positron-electron annihilation can be detected using scintillation crystal detectors such as sodiiun iodide (Nal), bismuth germaniiun oxide (BGO) and cerium doped lutetium oxyorthosili-cate (LSO). The short half-life of most positron emitters leads to high specific activity. Only a very small quantity of radio-labeled molecules is thus required, making positron annihilation detection techniques very non-invasive. In fact, practical catalyst studies can be carried out using less than 37 kBq... 

The PET technique relies on radioactive unstable atoms that disintegrate spontaneously, giving off particles called positrons. As soon as an atom emits a positron, the positron combines with an electron. Both particles are annihilated, producing a brief flash of gamma-ray radiation that is easily detected by radiation monitors. 

Positron annihilation spectroscopy (PAS) was first applied to investigate [Fe(phen)2(NCS)2] [77]. The most important chemical information provided by the technique relates to the ortho-positronium lifetime as determined by the electron density in the medium. It has been demonstrated that PAS can be used to detect changes in electron density accompanying ST or a thermally induced lattice deformation, which could actually trigger a ST [78]. 

All of the molecules in this study have triplet states which are easily detectable by the technique of nanosecond transmission laser flash photolysis. (11) The triplet state of acetoveratrone has a lifetime in excess of 15 ps in ethanol (Figure 2) under conditions of laser excitation the decay involves a mixture of first and second order kinetics, with the latter dominating at high laser powers. This second order decay demonstrates that the triplet state is decaying at least partly by triplet-triplet annihilation. 

The techniques used in the three measurements of the 23S —23Pj, J = 0,1,2 intervals are summarized in Figure 8. In all of these experiments the initial state is the 23S i state formed from positrons striking a metal target with about 100 eV kinetic energy. The first two measurements [15] [16] detected the transition as a 243 nm Lyman-a photon in delayed coincidence with a detected 7 ray from the annihilation of orthopositronium. The most recent and most precise experiment [17], which we detail below, uses only the Lyman-a detection. 

A broad overview of traditional methods and recent developments in experimental positron spectroscopy is presented. A discussion of the generation and detection of positrons and their annihilation radiation is followed by a survey of techniques used for positron lifetime measurement, Doppler broadening spectroscopy and angular correlation of annihilation radiation, and the opportunities presented by combining these methods (e.g. in age-momentum correlation) and/or extending their capabilities by the use of monoenergetic positron beams. Novel spectroscopic and microscopic techniques using positron beams are also described. 

The 3-to-2 photon ratio technique observes the ratio of 3 versus 2 photon annihilations of positronium (and positrons). In vacuum positronium will annihilate via 3-photon decays only. Trapped inside closed pores, both annihilation paths are possible. This change can be used to detect the onset of open porosity as shown in Figure 7.2. A change in slope (the slope is shown as a solid line) occurs at about 23% porogen load, indicative of an increased likelihood for positronium to escape from the sample. 

The specific fluorescent probe technique used in the present research is excimer formation between pyrene groups terminally attached to both ends of poly(ethylene glycol) (PEG) chains. Excimer formation in such a labeled polymer may result from both intramolecular and intermolecular interactions. A large body of literature over the past decade deals with intramolecular cyclization as detected by fluorescence techniques. This work has come largely from three groups. Cuniberti and Perico (24, 25) were the first to explore the possibility that excimer fluorescence could be used as a probe of end-to-end cyclization dynamics. Later, Horie and co-workers (26, 27) used triplet annihilation as a probe of cyclization in long chains. By... 

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