An existing model that relates the annihilation lifetime of positronium trapped in subnanometer pores to the average size of the pores is extended to account for positronium in any size pore and at...

Determination of Pore Size in Mesoporous Thin Films from the Annihilation Lifetime of Positronium

Evaluations of cavity concentrations and their size distributions in industrially important materials by positron annihilation lifetime methods require a well calibrated ortho-positronium (o-Ps) lifetime standard curve as a function of pore size. A semiempirical equation with a fitting parameter obtained by Nakanishi and Jean has been successfully utilized in various porous and polymeric materials to estimate pore sizes up to approximately 1 nm in radius. However, as experimental data accumulate recently, it has become clear that the equation no longer yields a good correlation between o-Ps lifetimes and the pore size for the pore radius larger than 1 or o-Ps lifetime longer than ∼20 ns. Therefore, we assumed for the larger pore that o-Ps behaves more like a quantum particle, bouncing back and forth between the energy barriers as the potential well becomes large. An equation derived from this hypothesis gives excellent fitting with experimental data despite a crude approximation used in our model. More im...

Extension of the equation for the annihilation lifetime of ortho-positronium at a cavity larger than 1 nm in radius

Pick-off models in the studies of mesoporous silica MCM-41. Comparison of various methods of the PAL spectra analysis

We have recently introduced the idea of making use of three-photon positron annihilations in positron emission tomography. In this paper, the basic characteristics of the three-gamma imaging in PET are studied by means of Monte Carlo simulations and analytical computations. Two typical configurations of human and small animal scanners are considered. Three-photon imaging requires high-energy resolution detectors. Parameters currently attainable by CdZnTe semiconductor detectors, the technology of choice for the future development of radiation imaging, are assumed. Spatial resolution is calculated as a function of detector energy resolution and size, position in the field of view, scanner size and the energies of the three-gamma annihilation photons. Possible ways to improve the spatial resolution obtained for nominal parameters, 1.5 cm and 3.2 mm FWHM for human and small animal scanners, respectively, are indicated. Counting rates of true and random three-photon events for typical human and small animal scanning configurations are assessed. A simple formula for minimum size of lesions detectable in the three-gamma based images is derived. Depending on the contrast and total number of registered counts, lesions of a few mm size for human and sub mm for small animal scanners can be detected.

https://iopscience.iop.org/article/10.1088/0031-9155/50/23/019/pdf

Performance of three-photon PET imaging: Monte Carlo simulations.

Structural characterization of poly(N-isopropylacrylamide) gels and some of their copolymers with acrylamide through positron annihilation lifetime spectroscopy

Brian Hopkins

Papers

Oxygen quenching of positronium in silica gels