History of µSR

Introduction to µSR --> here

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Origins of µSR

Muons were first discovered in the 1930's and their true nature was learned in the 1940's, when they also found their first use as probes of magnetism in matter by Rasetti. However, the story of µSR really begins with a subtle revolution in theoretical physics. In 1956 and 1957, T.D. Lee and C.N. Yang predicted that any process governed by the weak nuclear interaction might not have a corresponding "mirror image"' process of equal probability. Before they proposed this resolution of the "tau-theta puzzle" (involving the decay modes of electrically neutral strange particles now known as "kaons"), [Fitch and Cronin, 1981] it was firmly believed by the physics community that if a reaction were viewed in a mirror, the mirror image was a priori just as likely to occur as the original process -- a principle known as parity (P) symmetry.

Although P "violation" was first observed in kaon decay, credit for its experimental discovery is usually given to C.S. Wu et al., [Wu et al., 1957] who confirmed its existence in the beta-decay of ⁶⁰Co.

At almost the same time, however, experiments were performed at the Nevis cyclotron by R.L. Garwin, L.M. Lederman and M. Weinrich [Garwin et al., 1957] and at the Chicago cyclotron by J.I. Friedman and V.L. Telegdi [Friedman and Telegdi, 1957] which showed a dramatic effect in the decay of pions to muons and the subsequent decay of muons to electrons and neutrinos. The Nevis experiment was the precursor of modern µSR.

Of these famous measurements confirming the hypothesis of Lee and Yang, [Lee and Yang, 1957], one also suggested that P-nonconservation in <math>\pi \to \mu \to e</math> decay might furnish a sensitive general-purpose probe of matter. The history of µSR began with that experiment (Garwin et al., 1957), which used an experimental method similar to the most common and familiar of modern µSR techniques: transverse field (TF)-µSR.

1957-1973: The Formative Years of µSR

Were it not for the enthusiasm with which muons were used in history's most rigourous test of QED (quantum electrodynamics), µSR would probably not exist today. These Herculean feats (especially the CERN "g-2" experiment, which was recently repeated by Vernon Hughes et al. at even higher precision!) not only produced the basic experimental apparatus and techniques needed to perform the first µSR experiments, but generated the original interest in doing them -- in order to explain the subtle environmental effects confounding the fundamental measurements that were the primary objective.

Many people pursued these "sidelines" to discover that those annoying complications were in fact interesting areas of study in their own right, often ones for which the muon was either the best or the only available probe.

These early experimental programs were pursued at a variety of (by today's standards) low intensity accelerators such as the Nevis and Chicago cyclotrons, the 184 inch Synchrocyclotron at Berkeley, the similar machines at JINR (Dubna) and Gattchina in Russia, the CERN SC in Geneva and the SREL cyclotron in Virginia.

The Meson Factories

In the early 1970's new high-intensity, intermediate-energy accelerators were built at laboratories in Villigen (just outside Zurich), Switzerland, Los Alamos, NM, USA and Vancouver, BC, Canada. These new "meson factories" produced virtually no mesons heavier than the pion, but they produced pions (and therefore muons) in unprecedented numbers -- several orders of magnitude more than previous sources -- and, in doing so, ushered in a new era of exponential growth in the techniques and applications of µSR. These three laboratories were known respectively as SIN (Schweizerisches Institut fur Nuklearforschung, since renamed the Paul Scherrer Institut or PSI), LAMPF (the Los Alamos Meson Physics Facility, now no longer involved in µSR) and TRIUMF (the TRI-University Meson Facility, a joint venture of various CAnadian Universitites).

Pulsed Muons

The three continuous-beam (CW or DC) meson factories were joined in the 1980s and 1990s by two important new pulsed accelerators: the Meson Science Lab of the BOOM (BOOster Muon) facility at KEK in Tsukuba, Japan, and the µSR Facility of ISIS at RAL (the Rutherford Appleton Laboratory, also known as CCLRC) in the UK. The main difference between CW and pulsed µSR is that in the latter case a large number of muons arrive at (approximately) the same time, whereas in CW µSR there is literally only one muon in the sample at a time. (If a second muon arrives before the first one has decayed, it is difficult to know which one produces any subsequent positron, and any such event must be rejected.) In principle, this gives pulsed facilities an enormous advantage in rate, although actually counting a large number of positrons all at once requires expensive subdivision of detectors. Meanwhile, the muons arrive only approximately simultaneously -- the beam pulse is typically a few dozen ns wide -- which gives CW muon beams a significant advantage in time resolution. More will be said about this later.

A new pulsed muon facility will be included in the J-PARC (Japan Proton Accelerator Research Complex) laboratory now under construction in Tokai, Japan, where first beam is anticipated in 2007 or 2008.