Pion decay, positive muons

The techniques of u.SR and p-LCR are based on the fact that parity is violated in weak interactions. Consequently, when a positive muon is created from stationary pion decay its spin is directed opposite to its momentum. This makes it possible to form a beam of low energy (4 MeV) positive muons with nearly 100% spin polarization at high intensity particle accelerators such as TRIUMF in Canada, the PSI in Switzerland, LAMPF and BNL in the USA, KEK in Japan, and RAL in England. Furthermore the direction of position emission from muon decay is positively correlated with the muon spin polarization direction at the time of decay. This allows the time evolution of the muon spin polarization vector in a sample to be monitored with a sensitivity unparalleled in conventional magnetic resonance. For example, only about 101 7 muon decay events are necessary to obtain a reasonable signal. Another important point is that //.SR is conventionally done such that only one muon is in the sample at a time, and for p,LCR, even with the highest available incident muon rates, the 2.2 fis mean lifetime of the muon implies that only a few muons are present at a given time. Consequently, muonium centers are inherently isolated from one another. 

Muonium is formed when a positive muon thermalizes in a target and picks up an electron from the stopping medium into a bound state. Muons, both high energy (28 MeV) and low energy (4.1 MeV), are the product of positive pion decay, in flight or at rest, respectively. 

The positive pion decays with a mean lifetime of 26 ns to give a muon and a neutrino ... 

Muons take part in numerous elementary particle reactions. For p,SR applications, the decay of pions is the source used. That means for positive muons... 

Elementary Particles ), in decay (O 28.1) of positive pions, the outgoing muons are polarized against the direction of their momentum and in the muon decay reaction. 

Two types of positive muon beams are possible (1) surface muons, arising from pions decaying at rest close to the surface of the production target, with a momentum of 28 MeV/c and a stopping range of 140 mg cm 2 (corresponding to a water layer 1.4 mm... 

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