µ⁺ Charge Exchange, Muonium Formation and Depolarizaton in Gases

Abstract

The muon, like the electron, can be found in two charge states, µ⁺ and µ⁻, and can be produced at ‘MeV’ energies and 100% (longitudinally) polarized from π → µv decay.¹ Most of this initial energy is lost in ionization processes as the muon slows down, during which no loss in polarization occurs. At ‘keV’ energies, when the muon velocity is comparable to orbital electron velocities in the moderator, additional mechanisms begin to dominate the energy loss process. Negative muons are captured into highly excited “mesic” orbits and because of this are often referred to as “heavy electrons” (m_µ = 206 m_e). They are not considered further here. Positive muons suffer an entirely different fate. At ‘keV’ energies, the µ⁺ undergoes charge exchange with the moderator, producing the muonium atom (Mu = µ⁺e⁻) and returning the free muon in a series of charge exchange cycles, which can be thought of in complete analogy with those for protons. Subsequent thermalization at ‘eV’ energies is accomplished by elastic and inelastic scattering processes. The same features underlie the Ore concept of positronium (Ps) formation in gases¹, but the µ⁺ in matter acts much more like a light proton (m_µ = 1/9 m_p) than a heavy positron, a theme which is emphasized throughout this paper.