2.2 Hardware for $\mu{\cal SR}$ Spectroscopy

An experimental area used for $\mu{\cal SR}$ is shown in Fig. 2.3, this one being the M20 channel at the TRIUMF laboratory. The $\mu{\cal SR}$ experimental apparatus is positioned at the end of the secondary beamline, downstream from the final focusing quadrupole magnets.

  

Figure 2.3: A view of the M20 secondary channel and experimental area of the TRIUMF laboratory, configured with a conventional $\mu{\cal SR}$ spectrometer capable of transverse and longitudinal field measurements. (G.D.Morris)

The spectrometer consists of a set of Helmholtz magnet coils to produce a externally applied magnetic field, a cryostat (or for some experiments an oven) for controlling sample temperature, and a set of fast scintillation detectors that signal the passage of incoming muons and outgoing positrons. Most beamlines used for $\mu{\cal SR}$ incorporate spin rotators into the positron separators that allow one to precess the muons spins in flight by $\pi/2$ with respect to their momenta before they arrive at the experiment. This spin-rotated mode allows the experimenter more flexibility in orienting the initial spin polarization with respect to the applied field. This is especially useful when the experiment to be performed requires a strong magnetic field transverse to the muon spin, which would otherwise steer the beam off the sample.