A Brief History of µSR
Canadian Inst. for Advanced Research
Dept. of Physics & Astronomy,
Univ. of British Columbia, Vancouver, BC, Canada V6T 1Z1
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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
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 [Lee and Yang, 1956]
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, 1981; 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 () symmetry.
``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 -nonconservation in
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.
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 is now being 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.
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.