4.5 Implications of Delayed Muonium Formation for $\mu{\cal SR}$

Until now it has always been assumed that the muon could be considered an extremely dilute impurity, inserted one at a time into a sample without much affecting the bulk of the sample, and certainly not beyond the immediate vicinity of the muon. We now know that the electrons generated along the muon track do interact with the muon on time scales important to $\mu{\cal SR}$.It is probable that in all insulators, including semiconductors, at least some of the muonium results from the delayed formation process.

There are two principal concerns related to this. First, there is the question of how much the muon and muonium signals in various (insulating) materials are affected by direct interaction with the stray electrons. For both muons and muonium, the most obvious affect would be a contribution to the relaxation rate of the polarization function. Second, the bulk properties of the sample in the vicinity of these stray electrons may also be affected, and have an indirect influence on the muon or muonium signals. In both situations it is possible that a misleading result could be obtained in the measurement of other unrelated phenomena. At the present time neither of these potential problems can be ruled out. More electric field experiments on a variety of ``conventional" samples will be necessary to determine the circumstances under which such effects may safely be ignored, but until such time that this is fully understood, practitioners of $\mu{\cal SR}$ in insulators should bear this in mind.