Muonium

muonium [myü′ō·nē·əm]

(particle physics)

An atom consisting of an electron bound to a positively charged muon by their mutual Coulomb attraction, just as an electron is bound to a proton in the hydrogen atom.

---

Muonium

An exotic atom, Mu or (μ⁺e^-), formed when a positively charged muon (μ⁺) and an electron are bound by their mutual electrical attraction. It is a light, unstable isotope of hydrogen, with a muon replacing the proton. Muonium has a mass 0.11 times that of a hydrogen atom due to the lighter mass of the muon, and a mean lifetime of 2.2 microseconds, determined by the spontaneous decay of the muon (μ⁺ → e⁺ν_e). Muonium is formed when beams of μ⁺ produced in particle accelerators are stopped in certain nonmetallic targets.

Since muonium is a system consisting only of leptons, it serves as a testing ground for the theory of quantum electrodynamics (QED), which describes the electromagnetic interaction between particles. Muonium chemistry and muonium spin rotation (MSR) are two developing subfields which seek to understand the chemical and physical behavior of a light hydrogen isotope in matter and to probe the structure of materials. See Positronium, Quantum electrodynamics

---

Muonium 

a particle that consists of a positive muon (μ⁺) and an electron (e^-). It is designated as μ⁺e^- or Mu. The hypothesis of the existence of the muonium was advanced simultaneously in 1957 by L. D. Landau and A. Salam. The structure of muonium is similar to that of the hydrogen atom, from which it differs in that a μ ⁺ is substituted for the proton. Muonium is formed by the deceleration of a μ ⁺ particle in matter. In the process the μ⁺ attaches to itself an electron from the shell of an atom, and the atom becomes a positive ion. For example, μ⁺+ Xe → μ⁺e^- + Xe⁺. The lifetime τ of a muonium is 2.2 × 10~⁶ sec; it is determined by the lifetime of μ⁺.

Since μ⁺ and e^- have intrinsic magnetic moments (spins), in muonium their spins may be directed either parallel or antiparallel to one another. The energies of these two states differ by about 2 × 10^-5 electron volt, and quantum transitions between the states with the radiation of electromagnetic waves at a frequency of 4,463.16 megahertz are possible. Observation of these transitions and comparison of the measured frequency of the radiation with the theoretically predicted frequency is one of the most accurate methods of verifying the equations of quantum electrodynamics.

Three-quarters of the atoms of muonium are formed in the state with the spins of μ ⁺ and e^- parallel. The magnetic moment of these muonium atoms is about 200 times greater than that of the μ ⁺ meson, and the frequency of precession of this system in a magnetic field is 100 times greater than that of a free μ⁺. The direction of emission of the positrons that are formed in the decay of the μ⁺ in muonium (μ⁺ → e⁺ + v_e 4- ν_μ) changes with the same frequency. This phenomenon is used to observe muonium and to investigate various chemical reactions involving hydrogen. Since muonium may be considered a light isotope of hydrogen, in such studies it plays the role of a “tagged” hydrogen atom, whose motion can be followed by observing the precession of its spin in a magnetic field. If muonium, like the hydrogen atom, enters into a chemical reaction, the bond between the spins of the muon μ ⁺ and the electron e^- is “broken,” and the rate of precession of the free μ⁺ is observed instead of the rate of precession of muonium. The rates of many chemical reactions of atomic hydrogen with various substances have been measured in this manner.

REFERENCES

Hughes, V. “Miuonii.” Uspekhi fizicheskikh nauk, 1968, vol. 95, fasc. 3. Gol’danskii, V. I., and V. G. Firsov. “Khimiia novykh atomov.” Uspekhi khimii, 1971, vol. 40, fasc. 8.

L. I. PONOMAREV