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Rare and exotic annihilations of free positrons have been investigated extensively since the first discovery of positrons. The annihilation into three photons, however, has been neglected following theoretical work by Ore and Powell and several experimental studies using radioactive sources. Ore and Powell predicted that only one in 372 annihilations would result in the emission of three photons due to the parallel spin alignment of the electron-positron pair. A number of approximations were made to make the calculation tractable most notably the assumption of low momentum. No material dependence was expected. Subsequent experiments showed some dependence on material properties, notably on Z and density. We have set up an experiment to measure the annihilation into two and three photons with a five detector coincidence technique. Our results for low Z targets of Kapton, silicon and carbon, indicate no material dependence. The simplicity of the experiment and the high sensitivity for three photon annihilations render this method suitable for the characterization of porosity particularly when the concentration of pores is small. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The pore distribution of the commercial nanofiltration membranes Desal 5DL, Desal 51HL, N30F, NTR 7450 and NF-PES-10 were analyzed by positron annihilation spectroscopy using slow positron beam analysis. From the depth selective 3 gamma/2 gamma analysis a dense nanoporous filtration layer could be determined. From the depth selective lifetime analysis the size and distribution of the nanopores could be extracted. The measurements indicate the presence of 2 pore sizes in the samples with a radius of 1.25-1.55 angstrom and 3.2-3.95 angstrom. These experimental values are compared with the molecular weight cut-off values of these membranes.

(Ti,Al)-type ordered γ’ phase in the commercial nickel-based superalloy Inconel X-750 was investigated using Coincidence Doppler Broadening (CDB) tech-nique. The results obtained are discussed in terms of positron annihilation in two well-defined states: one corresponding to the matrix (γ phase) and a second related to the γ’ precipitates. Between these two aging stages, CDB distributions corresponding to selected intermediate aging treatments could be presented ex-actly, within the experimental scatter, as a linear combination of the γ and γ’ signatures.

In order to progress positron and antimatter related research, it is highly desirable to develop options for intense sources of positrons capable of fueling positron beams with >107 positrons/sec. At the Center for Materials Research at Washington State University, we are investigating 13N as a positron source based on the nuclear reaction 12C(d,n)13N. Based on cross section data and earlier tests a source activity of about 11 mCi per μA of 3 MeV deuterons can be achieved. At 300 μA a 3 (4) MeV Van de Graaff accelerator will generate 1.2 (2.0) × 1011 positrons/sec in a graphite target. Several methods of utilizing this production rate for moderated positron beams have been tried. These include single crystal CVD diamond, high and low density graphite, frozen source gases, tungsten foil moderation and solid gas moderation. To date, the most successful approach yielded ∼105 moderated positrons per second – efficient moderation of the source flux has not been accomplished. We will present the current status and discuss possible reasons for the poor performance. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cadmium Telluride (CdTe) is the most well established II–VI compound largely due to its use as a photonic material. Existing applications, as well as those under consideration, are demanding increasingly stringent control of the material properties. The deposition of high quality thin films is of the utmost importance to such applications. In this regard, we present a report detailing the role of lattice mismatch in determining the film quality. Thin films were deposited on a wide variety of substrate materials using the pulsed laser deposition technique. Common to all substrates was the strong tendency towards the preferential alignment of CdTe's (111) planes parallel to the substrate's surface. X-ray diffraction analysis, however, revealed that the crystalline quality varied dramatically depending upon the substrate used with the best results yielding a single crystal film. This tendency also manifested itself in the surface morphology with higher structural perfection yielding smoother surfaces. Slow positron beam techniques revealed a strong correlation between the defect concentration and the degree of structural perfection. Simulations of the data using the POSTRAP 5 program were used to calculate the defect concentration in relative (atom-1) and absolute units and to determine the diffusion lengths of the positrons in the film. All of these characterization techniques point towards lattice mismatch as being the dominant mechanism in determining the quality of CdTe films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.

Our investigations into the ZnO–TeO2 system have produced a new phase, zinc(II) hexa­tellurium(IV) trideca­oxide, ZnTe6O13, with trigonal (R\overline{3}) symmetry, synthesized by repeated heating and cooling to a maximum temperature of 1053 K. The asymmetric unit consists of a Zn atom coordinated in a distorted octa­hedral fashion by two unique tellurium(IV) oxide units that form trigonal–bipyramidal TeO4 and TeO3+1 corner- and edge-shared polyhedra. Except for the Zn and an O atom, which occupy 6c positions, all atoms occupy 18f general positions.

Periodic measurements of positron lifetime and X-ray line broadening were made at various times during the plastic fatigue cycling of pure Ni and Ni–66.5% Co alloy. Observations of the sub-structure at fracture were made by transmission electron microscopy. The positron lifetime increases rapidly during the early part of the fatigue life and remains almost constant thereafter. The mean positron lifetime changes from about 140 to about 230 ps during fatigue. This early saturation of positron lifetime far precedes the saturation of the change in X-ray particle size. Wahrend der Ermudungsprufung von reinem Nickel und einer Nickel–Kobalt-Legierung (66,5% Ni) im plastischen Beanspruchungsbereich wurden bei unterschiedlichen Zeiten periodische Messungen der Positronenlebensdauer und der Rontgenlinienverbreiterung durchgefuhrt. Mit Hilfe der Transmissionselektronenmikroskopie wurden Substrukturen beim Bruch beobachtet. Die Lebensdauer der Positronen nimmt zu Beginn der Schwingbeanspruchung sehr rasch zu und bleibt danach konstant. Die mittlere Positronenlebensdauer andert sich wahrend der Ermudungsprufung von 140 bis zu 230 ps. Dieser fruhzeitig erreichte Endzustand hinsichtlich der Positronenlebensdauer tritt wesentlich vor dem Grenzzustand der rontgenographisch ermittelten Korngrosenanderung ein.

The systematic variation among the elemental solids of mean positron lifetime and the fraction of the annihilation spectrum which is approximated as a Gaussian are discussed in the context of a simple model where the ion cores are treated as repulsive square wells within which the electron density is constant. The calculated results exhibit the observed periodic behavior and are in reasonable quantitative agreement with experimental data. La variation periodique, pour les solides elementaires, de la durbe de vie moyenne du position et la fraction du spectre d'annihilation qui est assimilbe a une gaussienne sont discutees dans le contexte d'un modele simple ou les ions sont traites comme des puits carries potentiels repulsifs dans lesquels la densite electronique est constante. Les resultats du calcul suivent le comportement periodique des donnees experimentales et sont en accord raisonnable avec celles-ci.