Positron Annihilation Spectroscopy of High Performance Polymer Films under CO2 Pressure

Positron annihilation Lifetime and Doppler broadening measurements are reported for six polymer films as a function of carbon dioxide absolute pressure ranging from 0 to 45 psi. Since the polymer films were thin and did not absorb all positrons, corrections were made in the lifetime analysis for the absorption of positrons in the positron source and sample holder using the Monte Carlo transport code MCNP. Different polymers are found to behave differently. Some polymers studied form positronium and some, such as the polyimide structures, do not. For those samples that form positronium an interpretation in terms of free volume is possible; for those that don’t form positronium, further work is needed to determine how best to describe the behavior in terms of the bulk positron annihilation parameters. Some polymers exhibit changes in positron lifetime and intensity under CO2 pressure which may be described by the Henry or Langmuir sorption models, while the positron response of other polymers is rather insensitive to the CO2 pressure. The results demonstrate the usefulness of positron annihilation spectroscopy in investigating the sorption of CO2 into various polymers at pressures up to about 3 atm

Measurements of L-shell x-ray production cross sections of W, Pt, and Au by 10-30-keV electrons

We present results from measurements of Lα x-ray production cross sections of the elements W, Pt, and Au by impact of electrons with energies in the range 10–30 keV. The cross sections were obtained by measuring Lα x-ray intensities emitted from very thin films of the studied elements deposited on thick carbon substrates. The directional and energy spreading of the electron beam within the active film and the x-ray enhancement due to electron backscattering from the substrate were accounted for by means of Monte Carlo simulation. Recorded x-ray intensities were converted to absolute x-ray production cross sections by using two different methods; the first employs measured values of the sample thickness and the number of incident electrons and estimated detector efficiencies; the second is based on a comparison between measured and calculated bremsstrahlung intensities. Experimental data are compared with the results of simple analytical formulas of common use in practical electron probe microanalysis, with calculated cross sections obtained from the distorted-wave Born approximation and with other experimental data available in the literature