Vacancy Spectroscopy Using Positronium

Abstract

Positron annihilation technique as a probe of subatomic vacancies is described for non-positron polymer scientists.Although positronium (Ps) is an unparalleled unique probe of inter-molecular vacancies in insulating materials, with both its intensity and mean lifetime carrying information about the free volume, the processes that lead to these physical quantities are not simple. The state-of-the-art understandings of how Ps is created and how its mean lifetime is determined in vacancies are described, and examples of the unique feature as well as the problems and precautions are illustrated.

Impact of Heating on the Structure of Perfluorinated Polymer Electrolyte Membranes: A Positron Annihilation Study

“…Following a number of successes in the past, the positron annihilation lifetime (PAL) technique has been used for sensitively probing the nanostructure of various polymers. [6][7][8][9][10] Positrons emitted from a radioactive atom (most commonly 22 Na) slow down quickly in a polymer and survive until annihilation either in the form of free positrons or as hydrogen-like positronium atoms (Ps). Depending on the spins of the electron and the positron, Ps can form as para-Ps ( p-Ps, singlet spin of the positron and the electron) or ortho-Ps (o-Ps, spin triplet).…”

Electrical conductivity and mechanical properties, free volume, and γ‐ray transmission of ethylene propylene diene monomer/butadiene rubber composites

“…The o ‐Ps and p‐Ps will annihilate into 3 γ‐ray and 2 γ‐ray, respectively, with the total energy of 1.022 MeV. In consolidated media, 75% of the Ps framed will be o ‐Ps and 25% will be p ‐Ps, and its reality relies upon their being locales of low electron density . The o ‐Ps demolition might be improved on the off chance that it can communicate with different electrons and a spin exchange or pick‐off annihilation happens to lessen the lifetime to around 0.5 to 5 ns.…”

Positron annihilation study of structural effect on photocatalytic activity of mesoporous TiO2 thin films

“…Probably, there is no positronium formation [9][10][11] in the studied titania films, and positron annihilation results here only reflects the structural properties of film backbone. Otherwise, characteristic S for films annealed at median temperatures within 200 and 800°C must be changed systematically, due to the reduced contribution from para-positronium (singlet state of positronium) annihilation in a film annealed at a higher temperature with smaller specific surface area and film porosity [12,13]. The smallest characteristic S value for 800°C calcined sample can be ascribed to the rutile phase structure, which maintains a smaller lattice constant in comparison to that in anatase.…”