“His Majesty the Spin and μSR, the Youngest Princess,”—this was the title of the keynote address of Anatole Abragam, professor in the College de France, who opened the 4th International Conference, μSR, held in Uppsala, Sweden June 23-27, 1986. With more than 100 papers and almost 150 participants, the topics discussed covered the range from the most recent level-crossing resonance (LCR) studies and the quantum diffusion of muons in metals through muons in spin glasses, semiconductors, alkali halides and oxides, to muonium-radical chemistry and reaction pion dynamics as well as channeling and other new techniques.

Although μSR studies began in 1957, the first real advances in the field were not made until the mid 1970s when muon trapping sites in solids were first unambiguously identified. These last ten years have seen materials scientists join with nuclear and particle physicists at the meson facilities: LAMPF at Los Alamos, TRIUMF at Vancouver, CERN at Geneva, SIN at Zurich, and KEK at Tokyo. U.S. federal support of visiting materials science user groups at these facilities has recently been examined by a National Academy of Science Panel.

For the materials scientist, the muon offers a new, light, hydrogen-like probe of great sensitivity with which to investigate internal magnetic fields. This report will examine just a few current problems in order that workers outside the field might catch a glimpse of the present excitement of the subject and obtain a brief introduction to current techniques.

μSR is based on the following sequence of events: pions produced by high-intensity proton accelerators decay to muons, which are highly polarised along their momenta, and to neutrinos.

Neutrons have played an important role in materials research for the last 30 years. Recently a new method of producing neutrons, with proton accelerators and specifically designed targets and moderators, has been developed. Many of the techniques developed with these powerful spallation sources open new opportunities for materials research, some of which will be covered in this article. An effort will be made to explain the types of science that can be done, rather than details of the techniques. Spallation sources are being operated as user facilities so the inexperienced can count on expert assistance.