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Muons which are initially longitudinally polarized can be rotated into the transverse plane by a burst of RF power at their resonant frequency. To do this at a continuous beam facility like TRIUMF required asynchronously triggered phase-locked RF amplification (every muon gets its own pulse). |
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At still higher RF power one can reverse the spins by 180o after they have lost all phase coherence due to random local fields, so that the original polarization returns (the ``direct echo'') after an equal time interval. In a simple spin system like the FµF complex, one can also use an RF pulse at the resonant frequency of the other spins (in this case 19F) to ``refocus'' the muon polarization (the ``indirect echo''). |