Positron annihilation spectroscopy studies of irradiated Fe-based alloys using different radioisotope sources

Positron annihilation spectroscopy as a unique tool for the characterization of radiation-induced defects often faces difficulties with the reproducibility of the experimental data obtained from different irradiation experiments. A particularly challenging problem is the correlation of near-surface and bulk irradiations as well as the correlation of the experiments performed with different types of positron sources. This work addresses some practical issues on the application of radioisotope sources Na-22 and Ti-44 to the investigation of Fe-based samples. Conventional Kapton-encapsulated Na-22 positron sources external to the sample and a Ti-44 source internal to the sample are discussed in terms of ion-implanted samples and samples irradiated in spallation neutron source, respectively. While the first type of experiment suffers from the distinct ion and positron stopping profiles, the latter is sensitive to the geometry of the measured sample and the emission of positrons out of the studied volume. The first part of the present work concentrates on theoretical and experimental investigation of the positron stopping in Fe-based materials with particular attention to the near-surface region. In the second part, Doppler broadening data obtained on FeCr samples irradiated in a spallation neutron source are discussed concerning the contribution of out-of-the-sample annihilation due to the use of internal and external positron sources.