The experiment reported in this thesis used the M20 beamline at TRIUMF
in conjunction with the Helios SR spectrometer to study the
LuNi2B2C sample described in Section 3.2. The
M20 beamline delivers muons of mean momentum
so they stop throughout the bulk of the sample. Figure 4.1
illustrates the path of the muon beam through the experimental apparatus. The collimator restricts the diameter d of the muon beam to before the beam exits the vacuum of the beamline through a thin plastic window. Each muon then triggers the muon counter (M), starting the time to digital converter (TDC). The muon then travels through two more thin plastic windows, and the intervening vacuum of a helium gas flow cryostat. This vacuum thermally insulates the sample space from the warm bore of the Helios magnet. The muon finally comes to rest in the sample, which is attached with a little Apiezon N grease to a thin Mylar film stretched over the end of an aluminium sample holder tube. The crystal axis lies parallel to the applied field H. An average of later the muon decays, emitting a positron preferentially along the muon spin direction. When this positron is detected, the TDC is stopped. The TDC therefore records the elapsed time between the arrival of a muon and the detection of the subsequent decay positron. Sometimes a muon misses the sample, and instead it or its decay positron arrives at the veto (V) counter. Electronic logic modules reject such decay events, as well as ambiguous ones where the detectors register more than one muon or more than one positron within specified time periods of about .
Histograms are constructed for the number Ni of positrons
detected in the ith positron counter during each time interval
after the TDC starts, from which the muon spin polarisation
computed. Figure 4.2 depicts the arrangement of the positron
Feedback control systems stabilise the applied field H and temperature T. Current carrying copper coils on the outside of the Helios superconducting magnet compensate for field drifts detected with a Hall probe situated near the sample. This keeps external field Hfluctuations below . Helium streaming past the sample from the nearby diffuser supplies cooling power, while a Lakeshore 330autotuning temperature controller heats the diffuser. The heating varies in such a way as to maintain the temperature of the diffuser at the set value. One calibrated GaAlAs diode monitors the diffuser temperature, and two others provide independent readings of the sample temperature. The analysis of the collected data is explained in the next chapter.