Positronium range---depth profiles

For a fixed temperature the diffusion length is constant, while the introduction of pores permits the motion of positronium to the surface from deeper in the sample. For isolated and closed pores this change in range is determined by the size of the pores. When the pores connect, the length of the connected chain becomes the dominant value. As percolation is reached, this length rises sharply. 

For atoms with a finite lifetime the diffusion length and the diffusion coefficient are linked via the lifetime of the particle. Here, we refer to the mean depth from which positronium can leave the sample as the escape depth. 

In addition to the effective diffusion length, which determine the curvature in the depth dependent data, the amount of positronium that is formed plays a role. Two effects contribute, one of which is energy dependent [26]. 

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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Principles and Applications of Positron and Positronium Chemistry 7.3.2 Positronium range—depth profiles... 

Positrons and positronium (Ps), the bound state of a positron with an electron, are highly sensitive to open volume in a material. Their sensitivity ranges from monovacancies (positrons) to voids to pores of several 10 nm in cross section (positronium). With a variable energy monoenergetic beam, positrons can be implanted in a sample at a chosen depth (or range of depths for a profile). Dependent on the porosity properties of the sample, positronium may be formed there. When positrons and positronium... 

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