MUONS AS A PROBE IN SOLID-STATE AND SURFACE PHYSICS AND IN MATERIALS RESEARCH

Facilities providing muons are coming up more and more. Correspondingly, muons, positive (mu+) as well as negative (mu-), are increasingly being used as tools in pure and applied research. mu+ are needed for muon spin rotation (mu-sr) where one measures mainly internal magnetic fields, but also for channeling; in both cases the site where the muon or pion decays is of interest. In chemistry the mu+ acts as a marked light " proton". mu-sr with mu- is also of some importance. Muonic X-rays, emitted when the mu- cascades down the atomic ladder, provide the unique opportunity to investigate quantitatively the elemental composition of a sample in a fully nondestructive way. Imaging with mu+ and mu- using the incoming pion or muon track and the outgoing decay electron track is possible and has been demonstrated. Slow muons (mu+ and mu-) are stopped in the surface layer (down to monomolecular layers) of a sample and are, hence, excellent thin-film and surface probes. Muonic hydrogen transfers the mu- to heavier atoms; this transfer can be used to study surfaces and the spatial structure of gaseous molecules. Present techniques and techniques envisaged for the future are reviewed; emphasis will be put on phase space compression of mu-beams.