About µSR : Muon Properties
muons (and their negative anti-particles) are leptons with spin 1/2 carrying
one elementary electric charge. From a particle-physics point of view they
are "heavy electrons". The properties of muons relevant for their use as
probes in solid state physics and chemistry are listed below.
muon rest mass is about 207 times the rest mass of the electron or 1/9th
of the proton rest mass. From a solid-state-physics or chemistry point
of view positive muons are thus to be considered "light
protons" rather than heavy electrons.
||The muon magnetic moment is about three times larger than
that of the proton. Consequently, muons are very
sensitive magnetic probes. (In a magnetic field, the muon spin precesses
at a frequency of 13.5 kHz/Gauss).
condensed matter, positive muons are repelled
by the nuclei. Thus, they probe magnetic fields in the interstitial regions
between the atoms.
Negative muons are attracted by the nuclei,
form muonic atoms (i.e. atoms in which one electron is replaced by the negative
muon) and eventually get captured by the nucleus.
result from the decay of positive or negative pions into a muon and a neutrino.
In the rest frame of the pion the muon magnetic moment is parallel or antiparallel
to the muon momentum for negative or positive muons, respectively, allowing
to produce muon beams with a very high degree of spin polarisation (nearly
100% when the muons are collected from pions decaying at rest).
(negative) muons decay into a positron (electron) and two neutrinos. Free
muons have a mean lifetime of 2.2 µs. (The lifetime of negative muons
in matter is reduced by nuclear capture and depends on the atomic number
Z of the capturing nucleus).
The positrons (electrons) are emitted preferentially in the direction of the
muon spin, allowing to measure the time evolution of the muon polarisation
by detecting the decay positrons (electrons).
insulators, semiconductors, and in organic materials positive muons may
capture an electron, thus forming the hydrogen-like quasi-atom muonium
(Mu). Due to the hyperfine interaction between
muon and electron spin, muonium is an even more sensitive magnetic probe
than the bare muon.
Muonium can be used as a substitute for hydrogen in organic molecules or radicals,
giving information on the structure, dynamics and reactions of these species.