Positronium Trapping in Porous Solids: Means and Limitations for Structural Studies

Abstract

Positron annihilation techniques are used for the structural investigation of solids but the interpretation of results in grainy and porous media is still unclear. A unique picture can be obtained assuming that the dominant process is Ps trapping in competing "extended free volume" sites. In samples with a large amount of free volumes near-saturation Ps trapping will rule the lifetime pattern, and very long lifetimes of over 100 ns might arise from o-Ps trapped in mesopores. It is shown that lifetime parameters must be corrected for the 3γ/2γ counting efficiency ratio. The results demonstrate the high sensitivity of Ps to mesopores in zeolites but also that Ps-trapping poses limitations on the applicability of lifetime to structural investigation in porous systems. The evolution of the lifetime spectra upon changes in the sample and measuring conditions should be considered in a complex way, observing not only changes in some selected components but in the whole lifetime pattern simultaneously.

Competitive positron and positronium trapping in porous media

“…It has been also shown already that even a higher number of lifetime components could be significantly resolved: for silicalite-1 MFI samples five components were extracted. Furthermore, in case of LT measurements performed on the MFI samples in closed N 2 gas atmosphere, at low temperatures due to the capillary condensation into the channels and cages of the zeolite skeleton a newer component-ascribed to a Ps-bubble-like state in the condensed gas-became evident (Kajcsos et al, , 2005, as will be discussed further.…”

“…A satisfactory fit of the LT spectra usually requires at least four lifetime components: 0.2-0.5 ns combining the p-Ps and e + -annihilations, 1-4 ns for o-Ps probably trapped in microvoids of the lattice, 10-90 ns for o-Ps decaying in internal channels of the zeolites and 100-135 ns for o-Ps decaying outside of the crystallites, in the intergrain space , whereas mesopores may also contribute to this latter two components (Kajcsos et al, 2005). It has been also shown already that even a higher number of lifetime components could be significantly resolved: for silicalite-1 MFI samples five components were extracted.…”

“…The intensities of the long-lived components are subject to corrections due distortions by the unequal efficiencies for the detection of 2g-and 3g-events (Kajcsos et al, 2005), and the registered spectra mirror the ''survival proportions'' of the various components and not those of the ''birth fractions'' Liszkay et al, 2001). The smaller the trapping site in which the o-Ps is confined, the higher the 2g-decay fraction, whereas the larger the site, the higher the 3g-decay fraction will be.…”

“…For performing temperature-dependent measurements in the range of 110-300 K, we had a sample-holder cryostat operating with gas cooling from a liquid nitrogen reservoir. Earlier results (Kajcsos et al, 2000(Kajcsos et al, , 2005 have shown that the capillary gas condensation at low temperatures practically eliminates the 3g-decaying long-living o-Ps components and could allow so to determine the ratio r of the 3g/2g counting efficiencies which we want to apply for our data.…”

“…As, however, the overall counting efficiencies may drastically differ for the 3g-and 2g-decay modes leading to a distortion in the measured intensities of the various positron and Ps states, serious doubts are cast on the relevance of the measured o-Ps intensity with respect to the true, physical intensity. Thus, the determination of the true LT intensities requires that the ratio r of the 3g/ 2g counting efficiencies be determined (e.g., Dauwe, 1992;Kajcsos et al, 1995;Kajcsos et al, 2005), which might more easily be achieved when combining LT and Doppler broadening (DB) measurements.…”