Toward a European Network of Positron Laboratories

Abstract

Positron defectoscopyPositron annihilation is one of the few really nondestructive techniques in material science, giving unique information on types and concentrations of defects, even at a single parts per million (ppm) level, see, for example, [1]. Applications of positron annihilation techniques span from semiconductors [2,3] and metals [4] to polymers [5] and porous media [6]. In each case, answers given by positron methods are unique and complementary to other techniques.Two most common techniques, the analysis of Doppler broadening (DB) of the 511-keV annihilation line and the positron lifetime measurements, are to some extent complementary. The broadening brings information on momenta of electrons that positrons annihilate with. The main contribution to the annihilation comes from valence electrons, that is, possessing a few electron-volts energy. In the relativistic transformation, these energies translate to broadening of a few kiloelectron-volts, i.e. similar to the intrinsic resolution of gamma detectors used in experiments. Therefore, the information on relative contributions from valence and core (i.e., from defect sites) electrons is not straightforward. Somewhat phenomenological line-broadening parameters (central area S parameter, W lateral area, V 3 annihilation) are used, and some normalization is needed to make comparison between different experiments and theories. Evaluation of DB requires not only information of the overlap between a positron and electrons in the material but also a detailed