What is a muon?

Design drawing of the new muon instrument Hifi

Design drawing of the new muon instrument Hifi
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In addition to producing neutrons, ISIS is also the world's most intense source of pulsed muons for studies in materials science. 

Muons are produced when the proton beam passes through a thin carbon target upstream of the neutron target. Muon spectroscopy provides an alternative and often complementary technique to neutron scattering. Spin polarised muons can be implanted into virtually any material, their spin polarisation is monitored to enable their position in crystal lattices or molecules to be determined, giving information about the local atomic structure and dynamics.  Muons are very sensitive to their local environment and can sense weak magnetic fields or mimic hydrogen atoms.

Free muons have a mean lifetime of 2.2 µs, decaying into a positron and two neutrinos, with the positron emitted preferentially in the direction of the muon spin, allowing the time evolution of the muon polarisation to be measured by detecting the position of the decay positrons.

The muon

Resulting from the decay of positive or negative pions into a muon and a neutrino, muons have spin 1/2, carry one elementary electric charge, and have a mass about 207 times the rest mass of the electron or 1/9th of the proton rest mass. Thus, from a particle-physics point of view they are "heavy electrons", whereas from a solid-state-physics or chemistry point of view they are "light protons".

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Why use muons for condensed matter studies?
In contrast to the neutron, the muon is usually a perturbative probe since it represents a defect carrying a unit positive charge. The defect interactions are essentially identical to those of the proton, allowing, for example, studies of the isolated hydrogen defect centres in semiconductors via their muonium analogues.
In insulators, semiconductors, and in organic materials positive muons may capture an electron, to form hydrogen-like quasi-atoms known as 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.

Muons have approximately one-ninth of the proton mass, resulting in large isotope effects. This favours the observation of quantum effects, notably the influence of zero point energy in chemical bonds and quantum tunnelling.

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