Positron from muon decay

Fig. 1. Schematic for /zSR and fiLCR experiments. For pSR the muon spin polarization vector starts off in the x direction (open arrow). It then precesses about an effective field (the vector sum of the external field and the internal hyperfine field), which is normally approximately the z direction. The muons are detected in the M counter, and positrons from muon decay are detected in the L or R counters. For pLCR, the muon spin polarization is initially along the external field or t axis (solid arrow). The positron rates in the F and B counters are measured as a function of external field. A sharp decrease in the asymmetry of the F and B counting rates signifies a level crossing.

Fig. 3. Polar diagram of the angular distribution of positrons from muon decay. The pattern with Oq 1 results if only positrons near are counted the pattern Oq = when aU positron energies are sampled with equal probability. The distributions are rotationally symmetric around the muon spin direction (z-axis).

The steep decline up to 2000 keV, the normal spectrometry range, is due to Compton events, backscatter and bremsstrahlung resulting from the decay of muons into high-energy electrons and positrons. On this are superimposed the 511 keV annihilation radiation and all the gamma-ray peaks from the background nuclides and the peaks from activations described below. 

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