experiments in which an external electric field had
a measurable effect was performed by Krasnoperov et al.
in superfluid and normal helium.
[28,26,27]
In the superfluid, in zero electric field, about 90%
of the muons formed muonium atoms.
The effect of an external electric field was asymmetric:
a field along the initial muon direction sharply decreased the
muonium amplitude. In the opposite direction the amplitude first
reached a maximum (at 50V/cm) then decreased more gradually
as the electric field increased.
In normal liquid He the mobilities of both
positively and negatively charged particles are much lower
and consequently the formation times longer.
A small muonium signal was detected in a very weak magnetic
field of 0.4 G.
With an electric field pushing the charged particles together,
the formation time was reduced so that muonium formed over a shorter
time with less dephasing, resulting in a larger muonium amplitude.
For both of these samples the results were interpreted in terms
of the motion of a positively charged helium ``snowball"
and a negatively charged ``bubble" that form around the muon
and electron respectively. It was concluded that the electrons were
distributed asymmetrically with respect to the thermal muon at the end
of the muon's track. It was estimated that the muons came to rest
some 300-400 nm further downstream than the electrons.
In the present work this technique has been extended to other samples, demonstrating muonium formation via the convergence of electrons and muons in solids for the first time. The principal result is definitive data showing that the muon is not isolated from its own radiolysis track products, and that electrons from the track do reach the muon in insulating solids. We now have strong evidence that muonium is often formed as a result of recombination with these electrons and some estimates of the distances and timescales involved.