Helsinki University of Technology

About: Helsinki University of Technology is a based out in . It is known for research contribution in the topics: Thin film & Vortex. The organization has 8962 authors who have published 20136 publications receiving 723787 citations. The organization is also known as: TKK & Teknillinen korkeakoulu.

Vacancies and carbon impurities in α - iron: Electron irradiation

Abstract: Positron-lifetime measurements are reported on electron-irradiated high-purity $\ensuremath{\alpha}$-iron and on iron doped with controlled amounts of carbon impurities. We show that in pure iron monovacancies are mobile at stage III around 220 K causing vacancy clustering into small three-dimensional agglomerates. These clusters anneal out between 500 and 600 K. In carbon-doped iron a competing mechanism is the formation of a highly asymmetric carbon-vacancy pair at 200 K. These pairs dissociate around 490 K, resulting in another release of free vacancies in the lattice. Controversial aspects on vacancy properties in earlier investigations are discussed.

Doppler factors, Lorentz factors and viewing angles for quasars, BL Lacertae objects and radio galaxies

Abstract: Aims. We have calculated variability Doppler boosting factors, Lorentz factors, and viewing angles for a large sample of sources by using total flux density observations at 22 and 37 GHz and VLBI data. Methods. We decomposed the flux curves into exponential flares and determined the variability brightness temperatures of the fastest flares. By assuming the same intrinsic brightness temperature for each source, we calculated the Doppler boosting factors for 87 sources. In addition we used new apparent jet speed data to calculate the Lorentz factors and viewing angles for 67 sources. Results. We find that all quasars in our sample are Doppler-boosted and that the Doppler boosting factors of BL Lacertae objects are lower than of quasars. The new Lorentz factors are about twice as high as in earlier studies, which is mainly due to higher apparent speeds in our analyses. The jets of BL Lacertae objects are slower than of quasars. There are some extreme sources with very high derived Lorentz factors of the order of a hundred. These high Lorentz factors could be real. It is also possible that the sources exhibit such rapid flares that the fast variations have remained undetected in monitoring programmes, or else the sources have a complicated jet structure that is not amenable to our simple analysis. Almost all the sources are seen in a small viewing angle of less than 20 degrees. Our results follow the predictions of basic unification schemes for AGN.