Positron Annihilation in Synthetic Zeolites 4A and 13X

Publisher Website
Google Scholar
Abstract
We have measured the rates of positron annihilation in synthetic zeolites 4A and 13X under a variety of conditions. For zeolites 4A and 13X, respectively, long lifetime values (τ2 in nanoseconds) and intensities I2 (listed in parentheses) were found as follows: hydrated, 2.26 ± 0.18(20.5%) and 1.95 ± 0.20(21.6%); evacuated, 3.76 ± 0.10(25.4%) and 4.42 ± 0.25(21.8%); filled with O2, 3.34 ± 0.25(4.2%) and 4.15 ± 0.35(4.6%); filled with He, 2.90 ± 0.11(23.8)% and 3.28 ± 0.12(20.0%); filled with N2, 2.97 ± 0.12(23.7%) and 3.39 ± 0.10(25.3%); filled with methane, 4.10 ± 0.06(28.2%) and 3.78 ± 0.11(24.8%). Very long components (τ3) with lifetimes of 42.2 ± 11.2 nsec and 41.4 ± 4.0 nsec for 4A and 13X, respectively, were observed in evacuated zeolites. Because the cavity sizes are well known from x-ray diffraction studies, the slightly larger lifetime observed in 13X relative to 4A for evacuated zeolites is thought to reflect the larger free volume present in the 13X configuration. The reduced lifetimes observed with He or N2 inclusion are postulated to result from reduced free volumes, whereas the reduced intensities observed with O2 inclusion may result from low O2 and positronium mobility coupled with nonuniform O2 distribution. Values for the short-lifetime component (τ1) were also observed and found to be relatively constant for each type of zeolite.

This publication has 17 references indexed in Scilit: