Depth-profiled positronium annihilation lifetime spectroscopy on porous films

Positronium annihilation lifetime spectroscopy (PALS) is a unique porosimetry technique with broad applicability in characterizing nanoporous materials, especially insulators. In beam-based PALS a focused beam of several keV positrons forms positronium (Ps, the electron–positron bound state) with a depth distribution (typically 5 nm–5 μm) that depends on the selected positron beam energy. Ps localizes in the pores where its natural annihilation lifetime of 142 ns is strongly reduced by collisions with pore surfaces. The collisionally reduced Ps lifetime is directly correlated with pore size, the key feature in transforming a Ps lifetime distribution into a pore size distribution over the 0.3–30 nm diameter range. Depth-profiling with PALS has proven to be an ideal way to non-destructively search for depth-dependent changes in the pore structure and to explore porosity hidden beneath dense layers or diffusion barriers whereby the positrons are injected through the barrier into the porous film. Profiling also determines the pore interconnection length, a unique measure of the degree of pore interconnection prior to film percolation. The capability of PALS is rapidly maturing as new intense positron beams around the globe spawn more accessible PALS facilities.