Positron annihilation in simple condensed gases

Abstract

The angular-correlation technique of positron annihilation has been used to detect and measure the localized bubble state of positronium (Ps) in liquid Ne, Ar, Kr, H2, and N2 and in liquid and solid He at various pressures and temperatures. No bubble state was seen in liquid O2 or in solid Ne and Ar. The dynamics of bubble formation is not yet understood. In the cases where theoretical calculations, and adequate data, exist, viz. He, Ar, and H2, there is reasonable agreement for the momentum of the photons from the annihilation of positrons with the outer electrons of these atoms. The Ps annihilations from the self-trapped bubble state are reasonably well described in terms of a simple finite potential-well model.

Microcavities in Semiconductor Materials

“…A simple model for positronium in a microcavity of radius r can be obtained under the assumption that the interaction of the positronium atom with its surroundings can be described as a quantum mechanical particle in a Q three-dimensional rectangular potential well [26]. Since positronium cannot exist in the bulk of metals and silicon (the binding energy equals zero), the potential well has a depth Vo which is larger than the positronium work function, i.e.…”

Positronium Nanocavities in Liquids

“…Using the wave function of positronium (5), we can relate R ∞ with width Θ of the narrow component of angular distribution of the spectrum of photons formed as a result of positron annihilation [3] (7)…”

“…For example, in the model of infinitely deep well [6], correct lifetimes are obtained, but the nanocavity radius for all studied substances appeared to be smaller than the experimental value; moreover, the difference between these values is noticeably larger than the experimental error. One can obtain good agreement with experiment for the nanocavity radius by varying the depth of the well; however, the lifetime differs from experimental value by the order of magnitude [3]. The combination of models considered, i.e., the description of nanocavity using two parameters (finite depth U and ∆ > 0) was also unsuccessful.…”

“…The autolocalization of positronium was first observed in experiments reported in [1,2]; in subsequent studies (for the detailed bibliography see [3,4]), the bubble radii and the lifetime of positronium in the bubble were determined. Bubble radii vary from the unity to several tens of a distance between molecules in a liquid.…”

“…The same physical mechanism of annihilation with the electrons that are the nearest neighbors of nanocavity is taken into account also in the model proposed in [3,8,9] where it is assumed that ∆ = 0; however, the depth U of the well is considered to be finite and depends on the nature of a liquid. In this model, the exponential "tail" of positronium wave function is overlapped by wave functions of electrons of liquid that ensures the finite lifetime of positronium.…”