Muonium formation and diffusion in cryocrystals

• Main Author: Morris, Gerald D.
• Format: Others
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8614

Energetic positive muons stopping in insulators often form the hydrogen-like neutral atom muonium by capturing an electron from the stopping medium. In this thesis it is shown that some of this muonium is formed by free electrons, produced along the muon's radiolysis track, diffusing to the muon, and subsequently forming muonium. Electron transport properties of the lattice play a role in delayed muonium formation in these solids. Application of an electric field along the initial muon momentum reveals a strong anisotropy of the spatial distribution of electrons in the vicinity of the muon, implying that the muon's direction of motion during thermalization is not completely lost by multiple scattering. Estimates of the initial electron-muon separation and muonium formation time are given. Diffusion of muonium in cryocrystals has been studied with both transverse and longitudinal field muon spin relaxation techniques. Experimental results are compared to the theory of quantum tunnelling diffusion. In solid nitrogen at temperatures much smaller than the Debye temperature of the lattice, the data and theory are in good agreement, with a temperature dependence approaching the T⁷ law predicted by the theory of twophonon quantum diffusion. At higher temperatures the agreement is qualitative only, but does show a key feature of two-phonon quantum tunnelling diffusion - a rapid increase in hop rate as temperature decreases due to the reduction of the phonon scattering rate. Muonium in solid Xe is an extreme case of a light interstitial atom in a heavy lattice. This system was chosen to provide an example of tunnelling diffusion at relatively high temperatures where lattice dynamics could be expected to play a role in determining the muonium hop rate. The hop rate of muonium atoms in solid Xe was measured over a === Science, Faculty of === Physics and Astronomy, Department of === Graduate