Positron annihilation in fine powders and fine-grained materials

Using as examples fine tungsten powders (powder particle sizes between 0.5 and 4.0 mu m) and fine-grained copper and nickel powder compacts (effective powder particle size 5 - 10 mu m with grains in the micron size), we investigated the annihilation parameters (lifetimes and intensities) for positrons becoming trapped at grain boundaries and at inner surfaces (pores). Furthermore, we can monitor grain growth and sintering (volume shrinkage) during successive heat treatment of powder compacts. To reach this aim, we correlated the annihilation parameters with results of a Monte-Carlo simulation and analytical solutions of the positron diffusion. We find that it is possible to determine an effective average powder particle size as well as grain sizes by lifetime spectroscopy.