Ionization slowing down

Usually positrons are born in nuclear / +-decay with typical initial energies about several hundreds of keV. Upon implantation into a medium they 

While positron energy W is greater than Wcyi 3 keV, the mean distance li between adjacent ionizations produced by the positron is greater 

Typical dependencies of h W) and hr W) versus the energy of the positron are shown in Fig. 5.2. The calculation of k(W) = Wiep/LET(W) is based on LET (Linear Energy Transfer) data of e. The estimation of the transport path has been done in the framework of the Born approximation (the wavelength of the positron with energy 100 eV is small in comparison with the size of molecules), where the Born amplitude was calculated simulating a molecule of the liquid by an iso-electronic atom. 

Now we can define Wu and a , . These quantities are obtained from the following equations  

Here Ry stands for a typical ionization potential. Equation (2) indicates that the terminal positron blob is a spherical nanovolume, which confines the end part of its trajectory. This is where ionization slowing down is the most efficient (the thermalization stage of the subionizing positron is not included here). The mathematical formulation of this statement is twofold. Just after the first blob formation step , which is ltr Wu) (the thick arrow in Fig. 5.1), the positron reaches the center of the blob. After that, the end part of the ionization slowing-down trajectory is embraced by the blob i.e., the slowing-down displacement of the positron, Rion Wbi, Ry) — au is equal to the radius of the blob, au- 

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The solution of these equations in the case of liquid water is shown in Fig. 5.2. It gives Wbi 500 eV and au 40 A. One may assume that the values of au and Wu do not differ significantly from one liquid to another because the ionization slowing-down parameters depend mostly... 

In negative-ion chemical ionization (NCI), a buffer gas such as methane is used to slow down the electrons in the electron beam until some of the electrons have just the right energy to be captured by the analyte molecules. The buffer gas can also help in stabilizing the energetic anions and reduce fragmentation. This is really a physical process and not a true CI process. 

Regarding the formation of subexcitation electrons, many authors consider only two possibilities they assume that each subexcitation electron either has been ejected during ionization (and happened to have the energy below fcco01), or it is one of the fast electrons that has slowed down to energies below hw(n. However, there may be other possible ways for subexcitation electrons to be formed. One of them is via a decay of a superexcitation state according to the ionization channel. Such a process may occur both in gaseous and in condensed media, and the majority of electrons produced in this case are the subexcitation ones. 

However, the spatial inhomogeneity in the distribution of reagents is not the only reason why the radiolysis of substances in the condensed state is different from that of gases. As we have already mentioned in Section VIII, as we pass from the gaseous state to the condensed one, at the primary stage of radiolysis we already observe a redistribution of yields of primary active particles (resulting in the increase of the yield of ionized states). Also different are the subsequent relaxation processes, as well as the processes of decay of excited and ionized states.354 Another specific feature of processes in a condensed medium is the cage effect, which slows down the decay of a molecule into radicals.355 Finally, the formation of solvated electrons is also a characteristic feature of radiation-chemical processes in liquids.356... 

When diffusion slows down, starting to control the ionization, the shape of the distribution gradually transforms from kinetic to static. This transformation is especially dramatic in the normal Marcus region. This is illustrated by the example of the exponential transfer rate (Fig. 3.31). Being exponential at the fastest diffusion, the distribution /o(r, c) shifts to higher distances, acquiring a bell shape at slower one. Finally it acquires a static shape monotonously decreasing with r. 

Formation When an p slows down in He, its kinetic energy eventually falls below the He ionization threshold (Jo = 24.6 eV), at which point it replaces one of the electrons in a He atom to form pHe+. The antiprotonic helium atom thus formed with an initial kinetic energy around 5 eV reaches thermal equilibrium within nanosecond without suffering destruction. 

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. 

At the end of the slowing-down by ionization and electronic excitation, the spatial distribution of e+ coincides with the distribution of the blob species (i.e., exp(-r2/o )). Such a subionizing positron having some eV of excess kinetic energy may easily escape from its blob because there is no Coulombic interaction between the blob and the e+ (the blob is electrically neutral). It is expected that by the end of thermalization, the e+ distribution becomes broader with the dispersion ... 

Despite the inefficiency of the Ore mechanism in the condensed phase discussed above, reactions of sub-ionizing electrons and the positron during the slowing-down process cannot be ignored because of the possibility of... 

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... 

When a positron with a well-defined energy is injected from a vacuum into a polymer, it is either reflected back to the surface or it penetrates into the polymer. The fraction of positrons that enter the polymers increases rapidly as a function of the positron energy. As the positrons enter the polymer, inelastic collisions between the positron and molecules slow down the positrons by ionization, excitation and phonon processes. The implantation—stopping profile P(z,E) of the positrons varies as a function of depth as [1, 2] ... 

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