1937 was marked by the birth of the author and the discovery of the muon, although another 10 years were required for positive proof of the latter. But it was not until 1957 when chemists should have shown any interest, with the discovery of muonium, a bound state between a positive muon, μ~+, and an electron, which is commonly referred to as a pseudo-isotope (m_μ = 1/9 m_H) of the hydrogen atom. Actually the negatively charged muon also exists but its analogy to a heavy electron (ca. 200m_e) has not been put to use by chemists. The unexpected rise to fame in chemistry of this unexpected fundamental particle must be associated with the easy formation of muonium and its subsequent reactions. But this is too glib; really its success arises from two facts: firstly the muon polarisation at production is preserved and secondly this polarisation direction at decay is the preferred direction of the ejected positron.
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