PRECISION STUDY OF POSITRONIUM: TESTING BOUND STATE QED THEORY

As an unstable light pure leptonic system, positronium is a very specific probe atom to test bound state QED. In contrast to ordinary QED for free leptons, the bound state QED theory is not so well understood and bound state approaches deserve highly accurate tests. We present a brief overview of precision studies of positronium paying special attention to uncertainties of theory as well as comparison of theory and experiment. We also consider in detail advantages and disadvantages of positronium tests compared to other QED experiments.

Characterization of Defect Structures by Perturbed Angular Correlation Technique

ABSTRACTIn applications of the perturbed angular correlation (PAC) technique to the characterization of defect structures in materials conclusions are drawn from the local electric field gradient measured at the site of a radioactive probe atom. In metals, PAC has been very successful in labeling and identifying simple probe atom-defect pairs. Based on these results, new experiments were performed on defect structures in cold-worked samples, on the dynamics of He-defect interactions and on the growth of vacancy clusters. Recent results on interactions between the acceptor atom In and different donor atoms or intrinsic defects in Si indicate that PAC may provide atomistic information also in the case of semiconductor materials.