Microstructural studies of silica gel powder were carried out using positron annihilation lifetime spectroscopy (PALS) in conjunction with transmission electron microscopy (TEM). It is argued that the two distinct longlived components found (labeled by τ 3 and τ 4 ) may be ascribed to ortho positronium annihilation in microcavities within the grains and intergranular mesoscopic pores, respectively. In the latter type of void, a significant fraction decays via the three-photon mode. A simple physical picture of positronium annihilation in the larger pores is put forward, while the situation vis-a `-vis the smaller cavities is shown to be well described by a modification of the currently prevailing model for the pick-off process. The simple parametrization finally arrived at provides a sharpening of the use of the positron as a useful probe for microstructural study of porous substances. It is emphasized that two different positron annihilation mechanisms prevail in the microcavities and mesopores.

The characteristics of positronium (Ps) annihilation in molecular substances (ranging from organic liquids to molecular solids), manifested through the observed lifetimes (τp) for the ‘pick-off’ process and values of angular correlation (θ1/2) of decay gammas, can be shown to be simply related to the size (radius R) of the cavity which the Ps creates in a liquid or finds in a molecular solid. The measured τp and θ1/2 are in turn calculable from the wavefunction describing the Ps centre-of-mass motion, which is determined from the average potential experienced by it in the confining cavity. Thus the height of this repulsive barrier (U0) at the wall of this vacant region corresponding to different materials (with varying R) can be obtained by fitting experimental observations, namely τp and θ1/2. The model we use is an improved version of the usual spherical well description of the cavity (where we take the walls to be diffuse). It is found that the values of U0 and R, taking into account all available and relevant data for different molecular substances, fall on a universal curve. We attempt to explain the reason behind this ‘universality’ by relating the potential to the Ps work-function in materials. Finally, the fit provides us with a very convenient linear relationship between the size (R) of the cavity and the pick-off lifetime (τp).

The bubble model conventionally used to fit the observed characteristics of the pick-off component of ortho-positronium decay in liquids is subjected, in the present study, to a critical assessment. It is demonstrated that in its usual form (namely that of a bubble with a sharp boundary) the model is untenable, when confronted conjointly with experimental data on the lifetime and angular correlation of the decay gammas. A modified version of the model that is relatively free from such shortcomings is presented.

The prevalent bubble model to account for the medium dependent pick-off process for positronium ͑Ps͒ annihilation in liquids is based on the notion of a bubble ͑or cavity͒ in which Ps gets self-trapped. This description, however, suffers from several rather unrealistic features. The Ps atom is treated as a structureless point particle, the potential responsible for its entrapment ͑as well as the molecular density profile of the cavity͒ is taken to have a sharp and discontinuous boundary, and the expected change in the surface tension from its bulk value ͑due to the curvature effects in such microbubbles͒ is neglected. We demonstrate that all these ad hoc assumptions can be corrected for in a rather simple manner, without the introduction of any new free parameter. The finite size of the positronium atom taken in conjunction with the diffusivity of the bubble boundary plays a crucial role. As a consequence, the discrepancies in the prediction of the annihilation characteristics are removed and satisfactory agreement with observations is achieved.

The bubble model is widely used for the analysis of the decay characteristics of the positronium atom in liquids. However, according to some authors this description is inappropriate in the case of polar liquids with high surface tension. It has been advocated that for such media, rather than employing the value of the bulk surface tension (which appears as a parameter in the model), one should incorporate the notion of a transition layer between the liquid phase and the cavity that encloses the positronium. Accordingly, the usual bubble model with sharp boundaries is modified in the present work through the introduction of a diffusivity in the bubble surface, thus developing the proper setting (without involving any extra free parameters) such that liquids with high surface tension may also be meaningfully discussed in this context and confronted with experimental data.

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