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A slow-positron source has been installed in the therminal of an electrostatic 6.5 MeV accelerator and provides a monoenergetic positron beam in the few-MeV range. It will be used to operate a “fast” positron lifetime spectrometer based onß+? coincidences. The properties of the beam, the expected performance of the spectrometer, its advantages over conventional?? lifetime measurements, a number of intended applications, as well as recent positron-electron scattering experiments and plans for positron channelling and channelling-radiation studies are outlined.

From a compilation of known literature, the substitutional fraction of boron in silicon is depicted in a diagram as a function of implantation dose and annealing temperature. Samples implanted at 1 1016 B/cm2 and annealed at 900, 1000 and 1100°C for 1 hour each are examined by the (n, α) method and compared to the RBS signal of the host atoms in various directions. Our results indicate a gradual transition from completely random positions after 900°C anneal to substitutional sites after 1100°C anneal.The observation of random positions for B in Si, annealed at 900°C, disagrees with earlier experiments after application of (p, α) reactions. We ascribe the differences to proton irradiation induced boron diffusion in those former accelerator experiments by other authors.Our results for the samples annealed above 900°C indicate lattice positions for the interstitial boron fraction which were not yet discussed in the literature.After 1100°C anneal, nearly all B atoms are found on positions slightly (approx. 7%) displaced from substitutional sites. This may result from the formation of small B2 or B3 clusters during the cooling period of the thermal annealing procedure.

The angular distribution of 2.8 MeV helium ions backscattered from silicon crystals containing self-interstitials has been calculated by channeling computer simulations for a double alignment channeling configuration. The structures of the self-interstitial defects are assumed to be the split <110> interstitial and the split <100> di-interstitial with various inter-atomic distances. The obtained profiles of the backscattering yield are found to show shapes characteristic for each structure which can be used for differentiating between these structures. When comparing these profiles with experimental profiles obtained with the same double alignment channeling configuration in a previous study one finds reasonable agreement for the split <110> interstitial defect with an inter-atomic distance of 1.76 Å.

Vacancies in high-purity gold Single crystals have been investigated after quenching from temperatures between 900 and 960°C and after Several steps of an isochronal annealing programme by measurements of the diffuse X-ray scattering intensity close to Bragg reflections, of the lattice parameter change and of the electrical resistivity. From the combined results the volume relaxation of a vacancy was determined to be ΔVrel = - 0·15±0·06 atomic volumes at 5 K. The difference from published values for ΔVrel at high temperatures, that are between - 0·35 and - 0·5 atomio volumes suggests a temperature dependence for Vrel in Au. Vacancy clustering is observed at annealing temperatures as low as 0°C, and cluster growth continues right through annealing stage III. The amount of vacancies that agglomerate in clusters (stacking-fault tetrahedra) and the observed decrease in the resistivity per vacancy in the large agglomerates are in reasonable agreement with published results from electron microscope studies.

In a double alignment backscattering channeling experiment the angular dependence (for detector rotation) of helium ions-backscattered from a radiation damaged silicon crystal at room temperature-has been studied. The 1.5 MeV He+-beam was incident on the specimen parallel to <110>, the backscattered ions were registered on plastic films in the blocking directions [110], [010] and [111] and in small angular ranges around these blocking directions. The position of silicon interstitials is discussed by comparing the backscattering yield profiles-obtained as a function of emission angle from the spatial distribution of ion tracks on the plastic films-with calculated yield profiles on the computer simulations.

Delayed elastic phenomenon during high-temperature creep of polycrystalline materials is correlated with strain due to grain boundary sliding. This correlation has been used to develop a phenomenological viscoelastic model that includes the grain-size effect. With ice as a reference material, it is shown that the contribution of the grain-boundary sliding strain to the total strain, and its dependence on stress, time, temperature and grain diameter can be systematically analysed by the proposed model. The results appear to agree with the observed trends in other materials.