A positive muon μ+ is a fundamental particle which has a positive charge and is about 200 times heavier than an electron. It has a magnetic moment, and therefore muons can be considered as little magnetic gyroscopes which precess in magnetic fields. The muon has a finite life time of 2.2 μs and decays into a positron e+ and two neutrinos. Due to parity violation the positron is emitted preferentially along the direction of the magnetic moment of the muon. From the measurement of the number of decay positrons as a function of time after a muon has stopped in a sample, the local magnetic field experienced by the muon can be extracted. This technique is known as muon-spin rotation (μSR) spectroscopy.
|
 |
 |
Why using muons ?
Implanted into a material, muons can probe the internal, local magnetic field, and with low-energy muons even on a nanometer scale. Therefore the technique is very useful for studying magnetic compounds and superconductors, as shown in the scientific article of this Newsletter.
In other experiments, muons can be used to provide information on the properties of hydrogen in materials or to probe structural defects in crystals.
How to produce muons ?
A muon beam can be produced by hitting a graphite target with a proton beam accelerated to 80% of the speed of light. Muons are obtained as decay products of pions produced in such collisions.
Such beams are available in Switzerland at the Paul Scherrer Institut (PSI) in Villigen.
Unique muon facilities at the Paul Scherrer Institute ?
The muon facilities at PSI allow a wide spectrum of experiments attracting solid-state physicists, chemists and materials scientists from all over the world.
Muons are universally applicable microscopic magnetic probes that can be implanted into a large variety of materials and under many different external conditions. The measurable magnetic fields using muons range from only one tenth to a hundred thousand times the magnetic field of the earth with characteristic times from nano- to milliseconds. This turns then into a powerful tool to investigate fundamental and technologically relevant aspects of structural, magnetic and electronic phenomena in magnetic compounds, such as superconduc-tors, semiconductors or insulators. These materials further range from pure elements to complex alloys like organic compounds or molecular systems with manifold structures (crystalline, amorphous, liquid, etc.). The experimental versatility is extended by the possibility to perform muon studies at variable temperature, pressure, or electromagnetic fields.
 Six different instruments are available for research using the muon beams at PSI. Among them, two very special facilities that are worldwide unique:
- A low energy muon beam allowing to implant muons at very small and controllable depths (down to a few nanometers) below the surface of a sample, thus making a wealth of new applications available by extending the studies to very thin samples, multilayered structures and surface regions, and by measuring material properties as a function of the implantation depth on a nanometer scale (read the scientific article published in MaNEP newsletter no. 2).
- The extraction of one muon at a time from a continuous beam – Muons on Request, “MORE” – provides unique sensitivity to small magnetic field differences and extends the measurable characteristic times into the milliseconds range at the full timing resolution.
For more information on muon spectroscopy:
|
