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Diffusional phase transformations on the atomic scale: Experiment and modeling

Description: In the early stages of phase transformations, microstructures are generated with the dimensions of only a few atomic spacings. Investigation of structures on such a fine scale poses severe difficulties, not only for experimental studies, but also for conventional theories based on continuum models. In this paper the authors report on a combination of atomic-scale microanalysis using the position-sensitive atom probe and atomistic simulations using the dynamic Ising model. Three phase transformations have been studied: spinodal decomposition in Fe-Cr alloys, nucleation and growth in dilute Cu-Co alloys and finally a conditional spinodal reaction in Ti-Al. The model provided a good quantitative match to the kinetics of spinodal decomposition observed in Fe-45%Cr, and nucleation and growth in Cu-1%Co. It also predicts the development of coupled ordering and phase separation in Ti-15%Al.
Date: December 31, 1995
Creator: Hyde, J.M.; Cerezo, A.; Setna, R.P.; Smith, G.D.W. & Miller, M.K.
Partner: UNT Libraries Government Documents Department

Progress towards antihydrogen production by the reaction of cold antiprotons with positronium atoms

Description: An experiment aimed at producing antihydrogen atoms by the reaction of cold antiprotons stored in a Penning trap with injected ground state positronium atoms is described. The apparatus developed in an attempt to observe the charge conjugate reaction using proton projectiles is discussed. Technically feasible upgrades to this apparatus are identified which may allow, in conjunction with the PS200 trap, antihydrogen production at LEAR.
Date: March 1, 1995
Creator: Charlton, M.; Laricchia, G. & Deutch, B.I.
Partner: UNT Libraries Government Documents Department

Muonium quantum diffusion and localization in cryocrystals

Description: The authors review their recent study of atomic muonium ({mu}{sup +}e{sup {minus}} or Mu, a light isotope of the hydrogen atom) diffusion in the simplest solids--Van der Walls cryocrystals. They give experimental evidence of the quantum-mechanical nature of the Mu diffusion in these solids. The results are compared with the current theories of quantum diffusion in insulators. The predicted T{sup {+-}7} power-law temperature dependence of the Mu hop rate is observed directly for the first time in solid nitrogen ({delta}-N{sub 2}) and is taken as confirmation of a two-phonon scattering mechanism. In solid xenon and krypton, by contrast, the one-phonon interaction is dominant in the whole temperature range under investigation due to the extremely low values of the Debye temperatures in those solids. Particular attention is devoted to processes of inhomogeneous quantum diffusion and Mu localization. It is shown that at low temperatures static crystal disorder results in an inhomogeneity of the Mu quantum diffusion which turns out to be inconsistent with diffusion models using a single correlation time {tau}{sub c}. Conventional trapping mechanisms are shown to be ineffective at low temperatures in insulators. Muonium localization effects are studied in detail in solid Kr. In all the cryocrystals studied, muonium atoms turn out to be localized at the lowest temperatures.
Date: August 1, 1995
Creator: Storchak, V.; Brewer, J.H. & Morris, G.D.
Partner: UNT Libraries Government Documents Department