Bulk materials analysis using high-energy positron beams

Applied Surface Science, 2002

This review will summarize current positron research at Lawrence Livermore National Laboratory(LLNL) using high-energy positron beams. We are combining positron lifetime and orbital electron momentum spectroscopic methods with theoretical simulations to provide a better understanding of positron annihilation behavior in materials. Topics covered include correlation of positron annihilation characteristics with structural and mechanical properties of bulk metallic glass and compositional studies of embrittling features in reactor pressure vessel steels. #

Bulletin of Materials Science, 1994

Positron annihilation spectroscopy (PAS) is a method dedicated to detection of openvolume type of defects. Nowadays, this technique is of a great interest due to the practical character of obtained results. New devices using monoenergetic positron beams are built. The paper presents the basics of PAS, a description of common experimental techniques and two examples of applications.

Physics of Atomic Nuclei, 2011

Steels used in nuclear industry have been experimentally studied by positron annihilation spectroscopy. Analysis of the experimental results and their comparison with the existing data make it possible to reveal vacancy defects, in particular, those caused by neutron radiation and to determine their size.

Journal De Physique Iv, 1995

Positron Annihilation Spectroscopy is a valuable microscopic method to study radiation damage in nuclear reactor pressure vessel steels. At SCKoCEN, a new positron lifetime setup, suited to measure positron lifetimes in irradiated steel, is being constructed. The particularity of this setup is that the prompt count rate originating from the induced 60Co-activity in the steel is highly reduced. Some preliminary

physica status solidi (c), 2007

Positron lifetime (LT) measurements have revealed that in porous media the annihilation pattern is probably dominated by a near-saturation level trapping both for the positron and the positronium as well, which masks the contribution from the "free annihilation", i.e., annihilation from delocalised positrons. In this work we compare positron lifetime spectra of crystalline zeolites with bulk ceramic materials fabricated by sintering from the same constituents as the zeolites and analyse common features and differences between them. For the dense ceramic samples the contribution from long-living components to the annihilation pattern is substantially lower than that for the zeolites. The results are analysed with the aim to extract new knowledge for the zeolites and for the sensitivity limits for free volume studies in ceramics.

Radiation Physics and Chemistry, 2003

The Idaho Accelerator Center (IAC) has developed new techniques for positron annihilation spectroscopy (PAS) by using highly penetrating g-rays to create positrons inside the material via pair production. Two sources of g-rays have been employed. Bremsstrahlung beams from small-electron linacs (6 MeV) were used to generate positrons inside the material to perform Doppler-broadening spectroscopy. A 2 MeV proton beam was used to obtain coincident g-rays from 27 Al target and enable lifetime and Doppler-broadening spectroscopy. This technique successfully measured stress/strain in thick samples, and showed promise to extend PAS into a variety of applications. r

Physical review. B, Condensed matter, 1996

A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively. ͓S0163-

HNPS Proceedings

Positron Annihilation Lifetime Spectroscopy (PALS) is a non-destructive technique, used to investigate open volume defects in materials. The usual sandwich configuration for Positron Annihilation Lifetime Spectroscopy measurements includes two similar pieces of material. In this study, the proposed configuration consisted of two different materials with one of them being of known lifetime. Specifically, the lifetime of a lead (Pb) specimen, measured together with a tungsten (W) specimen of known positron lifetime, was determined using this methodology. This methodology can be useful in the cases that two pieces of the same material are not available.

Acta Materialia, 2004

The potential of positron annihilation spectroscopy (PAS) in the study of light alloys is illustrated with special regards to agehardening, severe plastic deformation, fatigue and fracture in aluminium-and magnesium-based alloys. First, the physical grounds of PAS sensitivity to open-volume defects are explained. Then the main conventional variants of PAS, lifetime spectroscopy and Doppler-broadening spectroscopy, are introduced. State-of-the-art equipment, based on intense positron sources and energy-controlled beams, is also described, in view of applications where microscopic spatial resolution and sub-nanosecond time resolution are combined. Various examples of PAS studies in the field of light alloys, mainly based on the latest experience of the authors, are presented. It is shown how PAS detects structural changes in age-hardenable alloys, helps to describe the solute aggregation kinetics and gives information on vacancy-solute interactions. PAS characterisation of internal surfaces (misfit interfaces and grain boundaries) in terms of local structure (degree of disorder, chemistry) is also discussed. Lastly, recent advances in the study of fatigue by positron microscopy are reported.

NDT International, 1979

Positron annihilation linewidth measurements have been used to study defect behaviour in titanium samples which were first cold-worked and then annealed at temperatures ranging up to 850°C. The line width for a highly deformed high purity (0.9998) sample showed a gradual increase with increasing annealing temperature up to the recrystallization temperature, above which there was a rapid increase. In contrast, a commercially pure (0.9984) sample deformed by 20% displayed a smooth increase in linewidth over the entire annealing range. Hardness measurements correlate reasonably well with the overall trend of the positron lineshape parameter, but the positron annihilation measurements reveal defect changes that occur at temperatures too low to give any observed changes in hardness.