Positron energetics

Similar to beta decay is positron emission, where tlie parent emits a positively cliargcd electron. Positron emission is commonly called betapositive decay. Tliis decay scheme occurs when tlie neutron to proton ratio is too low and alpha emission is not energetically possible. Tlie positively charged electron, or positron, will travel at higli speeds until it interacts with an electron. Upon contact, each of tlie particles will disappear and two gamma rays will... 

Fig. 1, The cncigy regions for wliich negatron emission, positron emission, and orbital eleclron capture are energetically possible...

Our straightforward bookkeeping has shown us that for (3+ decay, the difference between the initial and final nuclear masses, must be at least 2m0c2 (1.02 MeV) for the decay to be energetically possible. This energy represents the cost of creating the positron. 

Perhaps of more general applicability for the study of the properties of positronium is its production by the desorption of surface-trapped positrons and by the interaction of positrons with powder samples. According to equation (1.15) it is energetically feasible for positrons which have diffused to, and become trapped at, the surface of a metal to be thermally desorbed as positronium. The probability that this will occur can be deduced (Lynn, 1980 Mills, 1979) from an Arrhenius plot of the positronium fraction versus the sample temperature, which can approach unity at sufficiently high temperatures. The fraction of thermally desorbed positronium has been found to vary as... 

Just like (3 decay, the mass numbers of the parent and daughter in positron decay are identical but the atomic numbers are different. However, in positron decay, the daughter has a Z that is one less than the parent nucleus. Positron decay becomes energetically possible only when the decay energy exceeds... 

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. 

It is of great interest to note that after Cherry s observation of positron emission from a solid surface the first beam system—developed in the late 60 s—was based at a LINAC facility [15]. Bremsstrahlung gamma radiation from the energetic (50MeV) LINAC electrons create electron-positron pairs in a Ta target the fast positrons thus created are then moderated (see section 8) to form the slow positron beam. The efficiency of this process clearly depends on the LINAC electron energy and the thickness of the converter. 

As expressed by reaction (VIII), all positron scavengers characterized in polar solvents lead to partial inhibition and therefore are supposed to react specifically with the localized particles. The reasons for this are not well established but, in the same way as for those Solutes that are very poor quasi-ffee electron scavengers although reacting effectively with the solvated electron (e.g., H+), the explanation may lie on thermodynamics. Too much energy may be released upon reaction with the quasi-free particles, either e or e+, so that the bound-state is unstable localization or solvation would reduce the energetics of the process, allowing the reaction to occur. Note that most of the partial inhibitors, whether electron (e.g. H+, Tl+) or positron (Cf,... 

In this chapter we will briefly discuss mechanisms of the positron slowing down, the spatial structure of the end part of the fast positron track, and Ps formation in a liquid phase. Our discussion of the energetics of Ps formation will lead us to conclude that (1) the Ore mechanism is inefficient in the condensed phase, and (2) intratrack electrons created in ionization acts are precursors of Ps. This model, known as the recombination mechanism of Ps formation, is formulated in the framework of the blob model. Finally, as a particular example we consider Ps formation in aqueous solutions containing different types of scavengers. 

The preceding discussion reserves the unique possibility of Ps formation in molecular media. It is the recombination mechanism, which postulates that Ps is formed via combination of the deenergized positron with one of the electrons formed at the end part of its track. This process is not energetically forbidden in the sense discussed above. 

Further developments in this field would probably be forthcoming with more precise studies of the energetics of Ps formation, and measurements of the work functions for e+ and Ps using low-energy positron beams. Better understanding may come from studies of Ps formation at different temperatures and external electric fields (determination of e+ mobility, investigation of the positron-blob interaction, e+ thermalization parameters and its spatial distribution). 

The energetic positron slows down on its track to it s implantation depth, it ionizes the sample and leaves a spur of free electrons behind [27, 28]. The number of electrons at the terminal of the spur and their mobility determine the formation likelihood for positronium. The cross section for positronium formation becomes constant independent of incident energy. The second path to positronium formation is the 0re process [29]. When the potential energy needed to ionize an electron from a molecule is less than the binding... 

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