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Positron annihilation lifetime spectroscopy (PALS) has been widely used to determine the free volume and void properties in polymeric materials. Recently, a two dimensional positron annihilation lifetime spectroscopy (2DPALS) system has been developed for membrane applications. The system measures the coincident signals between the lifetime and the energy which could separate the 2γ and 3γ annihilations and improve the accuracy in the determination of the free volume and void properties. When 2D-PALS is used in coupling with a variable mono-energy slow positron beam, it could be applied to a variety of material characterization. Results of free volumes and voids properties in a multi-layer polymer membrane characterized using 2D-PALS are presented.

The free volume properties of pristine and UV irradiated Lexan polycarbonate have been investigated using Positron Lifetime Spectroscopy (PLS). The decrease in o-Ps life time and free volume size of irradiated sample is attributed to free volume modification and formation of more stable free radicals. These free radicals are formed due to the breakage of C-O bonds in Lexan polycarbonate after irradiation. This is also supported by the decrease in the intensity of C-O bond after exposure to UV-radiation as studied from Fourier Transform Infrared (FTIR) spectroscopy and it also shows that benzene ring does not undergo any changes after irradiation.

Pulsed‐slow‐positron‐beam‐based positron lifetime spectroscopy was used to investigate the nature of vacancy defects induced by 20 MeV Au implantation in single crystals 6H‐SiC. Preliminary analysis of the data shows that at lower fluence, below 1014 cm−2, a positron lifetime of 220 ps has been obtained: it could be associated with the divacancy VSi−VC in comparison with the literature. At higher fluence, above 1015 cm−2, a positron lifetime of 260–270 ps, increasing with the incident positron energy, has been observed after decomposition of the lifetime spectra. By comparison with lifetime calculations, open‐volumes such as quadrivacancy (VSi−VC)2 clusters could be associated with this value.

This article reviews some recent materials analysis results using high‐energy positron beams at Lawrence Livermore National Laboratory. We are combining positron lifetime and orbital electron momentum spectroscopic methods to provide electron number densities and electron momentum distributions around positron annihilation sites. Topics covered include the correlation of positron annihilation characteristics with structural and mechanical properties of bulk metallic glasses and compositional studies of embrittling features in nuclear reactor pressure vessel steel.

We applied three positron annihilation techniques, positron 3γ‐annihilation spectroscopy, positron annihilation lifetime spectroscopy, and angular correlation of annihilation radiation, to the characterization of nano‐scale pores in thin films by combining them with variable‐energy positron beams. Characterization of pores in thin films is an important part of the research on various thin films of industrial importance. The results of our recent studies on pore characterization of thin films by positron beams will be reported here.

The Idaho Accelerator Center (IAC) has developed new techniques for Positron Annihilation Spectroscopy (PAS) using highly penetrating γ‐rays to create positrons inside the material via pair production. γ‐Ray induced positron annihilation spectroscopy can provide highly penetrating probes for material characterization and defect analysis. Bremsstrahlung beams from small, pulsed electron Linacs (6 MeV) have been used to bombard the materials to generate positrons, which annihilate with the material electrons emitting 511 keV radiation. We have also synchronized bremsstrahlung pulses with laser irradiation pulses to study dynamic structural changes in material. In addition, we have developed another method using (p,γ) reactions from a 2 MeV proton beam, which induce coincident γ‐rays to perform positron life‐time spectroscopy. We have showed the feasibility of extending PAS into thick samples and a wide variety of materials and industrial applications.

A program using a positron beam to probe defects in bulk materials has been developed at Lawrence Livermore National Laboratory. Positron annihilation lifetime spectroscopy (PALS) provides non-destructive analysis of average defect size and concentration. A 3 MeV positron beam is supplied by Sodium-22 at the terminal of a Pelletron accelerator. The high-energy beam allows large (⩾1 cm2) engineering samples to be measured in air or even sealed in an independent environment. A description of the beam-PALS system will be presented along with a summary of recent measurements.

Natural opal samples have been studied with positron annihilation spectroscopy methods: Lifetime spectroscopy (LT) and Doppler Broadening (DB) spectroscopy. Additionally, samples have been examined by the Scanning Electron Microscope (SEM) and the BET method. The mean lifetimes and DB spectra shape of opals have been compared to other rocks by constructing the quotient spectra. Unusual properties of opals have been explained by positronium formation in the three-dimensional tektosilicate framework structure (SiO2*nH2O) of the opals.

An intense slow positron beam has been produced for the first time by using a compact proton cyclotron. A slow positron beam intensity of about 2×106 e+/s has been achieved using a proton current of 30 μA. In the near future beam intensities of the order of 107−109 e+/s are expected from this system. In order to apply the slow positron beam from the compact cyclotron to the analyses for material surfaces, two applications are under development. One is variable-energy positron lifetime spectroscopy and the other is polarized positron beam application.

Defect analysis is needed for samples ranging in thickness from thin films to large engineering parts. We are meeting that need with two positron beam lifetime spectrometers: on on a 3 MeV electrostatic accelerator and the second on our high current linac beam. The high energy beam spectrometer performs positron lifetime analysis on thick samples which can be encapsulated for containment or for in situ measurements in controlled environments. At our high current beam, we are developing a low energy, microscopically focused, pulsed positron beam to enable positron annihilation lifetime spectroscopy for defect specific, 3-D maps with sub-micron location resolution. The data from these instruments with the aid of first principles calculations of defect specific positron lifetimes.

The positron annihilation lifetime spectra and positronium energy spectra of an epoxy polymer with a 5:2:3 equivalent ratio of DGEBA/DDA/DAB epoxy (Tg=52 °C) are measured as a function of temperature and of positron incident energies (0–5 keV). Preliminary results from these experiments show: (1) a 1–9% of long‐lived o‐Ps are emited from the energy surface, (2) the lifetime of o‐Ps near the surface is about three times longer than that in the bulk, (3) the positron lifetime spectra obtained from the incident positron energy exceeding 2 keV are essentially the same as those in the bulk, and (4) the energies of emitted Ps are distributed from thermal to about 100 eV. These results are discussed in terms of the potential uses of slow positrons in probing the free‐volume hole properties of thin films and near surfaces of polymeric materials.

Correlated measurements of the lifetime and of the Doppler broadening of the 511 keV annihilation radiation of positrons (Age‐Momentum Correlation, AMOC) using an MeV positron beam have become a powerful tool for investigating reactions of positrons or positronium as a function of time. The room‐temperature reaction rate of the spin conversion of positronium in methanol induced by the presence of a paramagnetic solute (HTEMPO) has been found to be proportional to the HTEMPO concentration with a reaction‐rate constant of (22.5±0.5)⋅109 l mol−1 s−1 up to the highest concentration investigated (C≤0.1 mol/l). By means of the AMOC technique it was shown that a large Doppler broadening in natural and synthetic diamonds is correlated with a very short lifetime component. At higher positron ages a second positron state with a narrower momentum distribution of the positron‐electron pair indicates positron trapping at defects with concentrations ranging from 10−7 to 10−6 in different types of diamonds.

Positron lifetime spectroscopy with an intense pulsed positron beam has been used to characterize defects in hydrogenated amorphous silicon films, porous silicon, diamond films, ion implanted SiO2, and metal‐oxide‐semiconductor samples. Both long‐lived component and short‐lived component of the positron lifetime spectra strongly depend on the deposition condition, annealing temperature, and other conditions. The relationship between positron lifetime spectra and microscopic structures is discussed.

Angular correlation of annihilation radiation (ACAR) spectra for positrons trapped in the surface state are calculated within an approach developed by the authors for simple metals. Positron annihilation parameters (momentum‐dependent enhancement factors and lifetimes) are presented. Characteristics of host material (electron and positron work functions) are discussed. The role of electron‐positron correlations in theoretical studies of surface properties by positron annihilation is emphasized.

Samples of silicon dioxide on silicon and hydrogen‐implanted silicon were studied using a low‐energy positron beam. The Doppler S parameter vs. positron implantation energy curves were analysed using a positron diffusion model. Results reveal positron trapping at an interface 950 A beneath the surface on the SiO2/Si sampler and the defect distribution resulting from implantation in the H+ implanted specimen.

A review is given on the results of PAES (positron annihilation induced Auger Electron Spectroscopy) studies to data, with a concentration on those results obtained at the University of Texas at Arlington. Low energy positions, trapped in a surface localized state annihilate with core electrons resulting in the emission of Auger electrons. The advantages of PEAS include: (i) the elimination of the very large secondary electron background, and (ii) increased surface selectivity. (AIP)

Variable‐energy positron annihilation provides a sensitive technique for the measurement of point defects in semiconductors. Quantitative interpretation of experimental results is however difficult due to the need to deconvolute the contributions from the positron implantation profile and positron diffusion. The present study aims to enhance understanding of positron annihilation spectra by measuring samples with known defect distributions. Silicon wafers were implanted at room temperature with 540 keV silicon ions in doses ranging from 1011 to 1015 ions/cm2. The lattice damage was measured by ion channeling and by positron annihilation. We find that the slow positron technique is sensitive to levels of damage approximately 2 orders of magnitude below the minimum detectable by ion channeling.