Positron diffusion length

The contribution of positron diffusion length (L+ = 10 nm [22]) was removed from the escape depth values. The diffusion constant in a material is a function of diffusion length and annihilation rate D = L2X. Here, the rates for positrons and positronium are similar (X 2 ns 1). Thus the measured combined effective diffusion length of positrons L+ and positronium escape Lesc is l eff = L2+ + L2esc [30],... 

Positrons may be trapped by grain boundaries (GBs) in metals. Nevertheless, trapping in GBs is likely only when the mean linear dimension of the grains does not exceed a few pm. This means that the grain size is comparable to (or smaller than) the positron diffusion length L+, and some of the positrons have a chance of reaching a GB by diffusive motion. The movement of a positron to a GB substantially limits the positron trapping rate in GBs [91]. 

Figure 7.8 Escape depth from which pores are connected to the surface versus porogen load. The contribution of positron diffusion was removed from the fitted diffusion length to obtain the positronium escape depth. At 50% porogen load the escape depth exceeds the thickness of the samples. The present model does not include this and becomes...

The mean diffusion length of the positron or Ps undergoing onedimensional Brownian motion can be expressed as [44] ... 

Mean diffusion length at room temperature for the defect-free material. For the free positron lifetimes, we used the bulk Ufetimes calculated by GGA and the experimental ones shown in Table 4.17. If the non-negbgible defect concentration is included, the parameter L+ will be smaller. 

The mean diffusion length L+ of the positron is defined as the mean distance from the point of thermalisation that the positron reaches by diffusion movement. This quantity is related to the diffusion coefficient by the relation [72]... 

The values of the diffusion length for some metals are listed in Table 4.19. Clearly, L+ determines the region in the medium that is probed by a single positron during... 

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