Skin layers and buried open porosity

In some processes the porous sample may show depth dependent variations in porosity. An extreme case is when the porosity is large in the sample, but 

In the first case, open symbols, a thin skin layer was created by an etch treatment. The second case shows a sample capped with a 200 nm thick cap. Either treatment seals the surface and prevents the escape of positronium. Below the surface layer, significant porosity can be observed. However, because the implantation profile is spread across larger depth regions in the second case, a careful data analysis must be undertaken to extract the actual porosity. 

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