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PALS traditionally is the acronym for positron annihilation lifetime spectroscopy, but we generalize its use to include Ps lifetime spectroscopy. Positron is identical to the electron in all respects except charge.  In an insulator, a positron can bind to an electron and form a hydrogen-like atom called positronium (Ps).  As a neutral atom, Ps is very sensitive to the presence of open-volume and the Ps annihilation lifetime spectroscopy has long been used to study the void/pore structure and related properties in insulating materials such as polymers and other molecular solids.  

Beam-PALS uses slow positron beams (i.e., as distinct from the positron beams used in high-energy physics) allows the study of surfaces and thin films.  It has recently been used to study mesoporous (pore diameter in the range of 2 nm-50 nm) structure.  In this section, the fundamental positron physics and related techniques will be reviewed.  We will focus on PALS basics and our facilities.  A calibration model that relates positronium annihilation lifetime with pore size will be introduced.  Since there is no standard of nanopore size calibration for sub-micron thin films, a round-robin comparison has been performed focusing on comparing results from independent methods and searching for consistency.