About µSR : Muon Properties

Mass:

The 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.

Magnetic Moment:

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).

Charge:

In 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.

Spin Polarisation:

Muons 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).

Decay:

Positive (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).

Muonium Formation:

In 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.