Positron annihilation : A probe for nanoscale voids and free volume ?

It has been known for many years that an electron and its antiparticle, the positron, may together form a metastable hydrogen-like atom, known as positronium or Ps (ref. 1). In 1946, Wheeler speculated that two Ps atoms may combine to form the di-positronium molecule (Ps2), with a binding energy of 0.4 eV. More recently, this molecule has been studied theoretically; however, because Ps has a short lifetime and it is difficult to obtain low-energy positrons in large numbers, Ps2 has not previously been observed unambiguously. Here we show that when intense positron bursts are implanted into a thin film of porous silica, Ps2 is created on the internal pore surfaces. We found that molecule formation occurs much more efficiently than the competing process of spin exchange quenching, which appears to be suppressed in the confined pore geometry. This result experimentally confirms the existence of the Ps2 molecule and paves the way for further multi-positronium work. Using similar techniques, but with a more intense positron source, we expect to increase the Ps density to the point where many thousands of atoms interact and can undergo a phase transition to form a Bose-Einstein condensate. As a purely leptonic, macroscopic quantum matter-antimatter system this would be of interest in its own right, but it would also represent a milestone on the path to produce an annihilation gamma-ray laser.

The production of molecular positronium

Positron annihilation spectroscopy (PAS) is a novel method that provides molecular-level information about complex macromolecular structure in a manner different from, but complementary to, conventional physical and chemical methodology. This paper presents a perspective of PAS in polymeric systems covering 12 aspects: historical, spacial, spherical quantum model, anisotropic structure, voids, positronium chemistry, time, positron annihilation lifetime spectroscopy and data analysis, variable monoenergetic slow positron beam techniques and depth profiling, elemental analysis, multidimensional instrumentation advances in PAS, and free volume and free-volume theories.

Perspective of Positron Annihilation Spectroscopy in Polymers

Carbohydrate polymers in amorphous states: an integrated thermodynamic and nanostructural investigation

The current status of certain aspects of positron scattering in gases is reviewed. A brief resume of the experimental techniques used in this field is also given. Results for total scattering cross sections in a number of gases are presented along with a detailed discussion of potential systematic errors which can affect such measurements. Important features of the cross sections are pointed out and comparisons are made with electron scattering data. Results from experiments which go beyond total cross section determinations are discussed. Emphasis is placed upon recent measurements of positronium formation cross sections. Recent positron lifetime studies in dense gases are reviewed. The subjects covered include positron annihilation in clusters, ortho-positronium annihilation in bubbles and positronium formation in spurs.

http://iopscience.iop.org/article/10.1088/0034-4885/48/6/001/pdf

Experimental studies of positrons scattering in gases

A new model of positronium (Ps) formation is proposed. Positronium is assumed to be formed by a reaction between a positron and an electron in the positron spur. Ps formation must compete with electron‐ion recombination and electron or positron reactions with solvent molecules and scavenger impurities. It is also influenced by electron and positron solvation. The model correlates the measured Ps formation probabilities with the spur electron properties determined in radiation chemistry. The predictions of the model are shown to be in good agreement with experimental results for liquids and solids.

O. E. Mogensen

Papers

Spur reaction model of positronium formation