Preamble

From Swanson's (1958) initial measurements of the magnitude of the muon asymmetry (AD, here) it was clear that the chemical fate of the muon varied dramatically with the composition of the medium. For instance, he found all the incident muons to be observable in diamagnetic environments in CCl4, 62% of them in water and only 15% of them in benzene. Why this diversity, and what is the nature of the remaining muons? These questions occupy a good fraction of this chapter. When Hughes, McColm, Ziock & Prepost (1960) first observed free muonium atoms by direct μSR methods in argon gas, it became apparent that Mu atoms form and, at least in some systems, survive into the observation time-period of > 10−7s. Then when Mu-radicals were observed (Roduner et al., 1978) the third type of identifiable muon magnetic state was finally established to exist with the necessary lifetime. But the sum of these three yields still does not account for the total number of incident muons in most systems, so there is also a missing fraction. Conceivably, this could be due to their existence in another type of magnetic state precessing in transverse fields at a frequency not yet seen; but, much more likely, the missing yield represents those muons which were spin-depolarized, or dephased, by fast interactions prior to the observation time of 10−7s.

The intention here is to provide the basic information on these muon yields, and to report on the rationalizations that have been offered.