Delayed Muonium Formation in Sapphire

In sapphire (crystalline alumina, Al₂O₃) muonium (Mu) is formed when the stopped µ⁺ captures a radiolysis electron created in its ionization track. At low temperatures, this takes place over a long enough time (~300ns) to be observed directly in the decay of the diamagnetic muon precession signal. The rate of Mu formation (presumably governed primarily by the effective mobility of the electronic polaron as it is "sucked in" by the muon's electic field) has an Arrhenius-like temperature dependence with an activation temperature of 140(1)K, suggesting classical "over-barrier" hopping as a diffusion mechanism.

A slowly-relaxing diamagnetic signal is also present at low temperature, presumably representing those muons which stop further than one Onsager radius from the last radiolysis electron created in their path.

At high temperature, muonium is formed within a few ns and the precursor diamagnetic signal is barely detectable. All that remains is a slowly-relaxing diamagnetic component that can be affected by an applied electric field E in the usual way: for positive E (along the original muon momentum direction) the formation of muonium (Mu) at early times is strongly inhibited, increasing the long-lived diamagnetic signal amplitude; whereas negative E (in the opposite direction) is significantly less effective at overcoming the Coulomb attraction between the µ⁺ and e^- polaron. This anisotropy indicates that the radiolysis electrons produced as the muon comes to rest are on average "behind" the µ⁺ but their spatial distribution overlaps the muon's stopping position.

Author: JHB. Figure created ~1996. See TRIUMF Expt 693.

Prepared by Jess H. Brewer

Last modified: Mon Dec 1 13:58:43 EST