Channeling of positrons from μ+ decay

The first attempt to observe the steering or channeling effect of a host crystal lattice on the trajectories of decay positrons from interstitial positive muons is described. An enhanced (flux peaking) or diminished (blocking) positron counting rate for emission along a low index crystalline axis would be evidence of such an effect and would help to determine the lattice location of the emitting muon. The expected angular widths of these features is approximately 0.20. A 29.8 MeV/c surface μ+ beam was stopped in a high quality silicon crystal wafer which was elastically bent to a good approximation to a spherical cap. This brought the [100] axes, which were initially normal to the wafer surface, to a focus at the radius of curvature R = 110 cm. The normalized e+ rate was measured as a function of position with a small two-counter scintillation telescope which was moved through the focus. We found no evidence for channeling at the 17% level, suggesting that the μ+ in Si either (1) makes large vibratory excursions, (2) occupies a site of low symmetry, or (3) occupies one of several possible inequivalent stopping sites