Muon spin spectroscopy
From Freepedia
Muon spin spectroscopy is an experimental technique based on the implantation of spin polarized muons in matter and on the detection of the influence of the atomic, molecular or crystalline surroundings on their spin motion. The motion of the muon spin is due to the magnetic field experienced by the particle and may provides information on its local environment in a very similar way to other magnetic resonance techniques, such as Electron spin resonance (ESR or EPR) and, more closely, Nuclear magnetic resonance (NMR).
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Acronym
In analogy with the acronyms for these previously established spectroscopies, the muon spin spectroscopy is also known as μSR, which stands for Muon Spin Rotation, or Relaxation, or Resonance, depending respectively on whether the muon spin motion is predominantly a rotation (more precisely a precession around a still magnetic field), or a relaxation towards an equilibrium direction, or, again, a more complex dynamics dictated by the addition of short radio frequency pulses.
How it works
The time scale on which the spin motion may be exploited is that of the muon decay, i.e. few mean lifetimes, each roughly 2.2 μs (2.2 millionths of a second). Both the production of muon beams with nearly perfect alignment of the spin to the beam direction (what was referred to above as spin polarization), and the ability to detect the muon spin direction at the instant of the muon decay rely on the violation of parity, taking place whenever weak nuclear forces are at play.
In short this means that certain elementary events happen only when including clockwise (or only when including counterclockwise) rotations, For instance, the positive muon decays into a positron plus two neutrinos and the positron is preferentially emitted in the direction of the muon spin. therefore it would most often see the spin as a counterclockwise rotation while flying away from the decay point.
Applications and Facilities
Muon Spin Rotation and Relaxation are mostly performed with positive muons. They are well suited to study magnetic fields at the atomic scale inside matter, such as those provided by the very different kinds of magnetic order and of superconducting states that are either encountered in some natural compounds or artificially produced by modern material science.
Another important field of application of μSR exploits the fact that positive muons behave chemically as light isotopes of the Hydrogen ion. This allows the investigation of the early stages of the formation of radicals in organic chemicals and in semiconductors.
μSR requires a particle accelerator for the production of a muon beam. This is presently achieved at few international large scale facilities in the world: The CMMS continuous source at TRIUMF, Vancouver, Canada; The LMU continuous source at the Paul Scherrer Institut, Villigen, Switzerland; The ISIS and the Riken-RAL pulsed sources at the Rutherford Appleton Laboratory, Chilton, United Kingdom; the J-Park facility, in Tokai, Japan, is completing a new pulsed source to replace that at KEK.
