Negative muons can be considered as 'heavy electrons' and, on implantation, replace an electron in the outer shell of an atom, then travel closer to the nucleus through the modified energy states of the atom. Each transition on this path produces an X-ray specific to the atom which absorbed the muon, hence the atomic species is uniquely characterised by the spectrum produced. The sensitivity of this technique is such that even light atoms can be detected (such an experiment is sensitive to all elements with atomic masses greater than Li). Furthermore, this technique can be used as a depth analysis tool, since by varying the momentum of the incident muon beam it is possible to change the depth of implantation for the negative muon.
Currently, elemental analysis commonly uses X-ray and electron beams, which accurately measure surfaces, however a significant advantage of muonic X-rays over those of electronic X-rays is their higher energy (0.01-6 MeV) due to the mass of the muon. These high energy muonic X-rays are emitted from the bulk of the samples without significant photon self-absorption. The penetration depth of the muons can be varied by controlling the muon momentum, providing data from a thin slice of sample at a given depth. This can be over a centimetre in iron, silver and gold or over 4 cm in less dense materials such as carbon. The X-rays that are emitted can be simply detected by a semiconductor detector. In addition, this technique is completely non-destructive.
Negative muons as an elemental analysis tool has been used in a wide-range of materials in the science areas such as: cultural heritage, environmental studies, bio-materials, condensed matter, renewable energy and storage, advanced manufacturing, engineering applications and electronic pad layouts.
MuX has HPGe detectors and Si(Li) for measuring muonic X-rays and is sensitive to all elements from Li upwards. The variable momentum allows for depth-dependence compositional analysis to be investigated.