What are muons?
Muons are subatomic elementary particles, similar to electrons, but around 200 times heavier. Muons have a spin of a half and can be either positively or negatively charged (the antiparticle).
The muons used in research are formed into beams and implanted into materials. After an average lifetime of around 2.2μs, muons decay into three particles: one electron and two neutrinos. This decay releases information about the environment they were embedded in.
Positive and negative muons can interact with their environment and can thermalise into the following states:
Bare (diamagnetic muon)
The polarised subatomic particle which has not interacted with its environment yet. It is either positively or negatively charged.
A complex in which an electron orbits a positive muon. Muonium generally results from muons implanting themselves into an insulating sample. Hence, muonium is never seen in metallic samples.
Muonium is considered as a light isotope of hydrogen. Their similar structures mean they have qualitatively similar chemical behaviour. However, muonium has a mass approximatively 9 times smaller than hydrogen, which results in quantitative differences. Muonium is frequently used to learn about hydrogen behaviour in a material.
Muons are naturally generated in the Earth’s upper atmosphere when cosmic rays (high-energy protons) collide with the atomic nuclei of molecules in the air.
Muons can also be produced in a two-step process at large research facilities:
1) High-energy protons (>500 MeV), generated by a particle accelerator, are collided into a carbon or beryllium target. The high-energy interaction between the incoming protons and the target nuclei produces particles known as pions (π). Positive and negative pions are unstable and almost immediately decay into positive and negative muons and neutrinos.
2) The muons are selectively channelled into beamlines that transport the muons towards a sample.
Two types of muon beam can be formed:
1) Surface muon beams, which are formed from pions decaying at the surface of the target. This beam consists of positive muons only, as the negative muons are captured.
2) High-momentum beams, consisting of positive and negative muons, where the pion decays in flight.
The muon technique involves implanting spin-polarised positive muons into a material, where they undergo decay and release positrons. The positrons are analysed to understand the muon’s behaviour inside the material – particularly how the muon polarisation changed within the sample. In turn, this provides information about the atomic-level properties of the material and allows the system to be probed on a unique timescale.
The knowledge obtained from this analysis is opening up new areas of science and potential solutions to key global challenges.
Source: La Physique Autrement YouTube channel
Research with muons
Muons have a wide variety of applications, including studies of superconductors, molecular systems and chemical reactions, novel battery materials and a variety of organic systems.
Notably, muons are very sensitive probes of magnetic systems and can often detect effects that are too weak to be detected by other methods.
In some studies, the positive muon is treated as a light proton (muons have a mass of one-ninth of the proton mass). Implanted muons sometimes pick up an electron, forming a light isotope of hydrogen called muonium (Mu). As such, muons can be used to study proton and hydrogen inside a material. This is important in semiconducting materials, proton conductors and hydrogen storage materials.
Muon sitting inside a Carbon 60 molecule
Muon spectroscopy is used in research that impacts many areas of modern-day life, including: