3.1 Cryostats and Techniques

The sample environment suitable for typical muon spin relaxation experiments must allow a beam of muons into the sample, allow the decay positrons to escape and allow one to control the experimental variables, usually temperature and applied magnetic field at the sample. Meeting these requirements imposes several conditions on the experimental apparatus; most important of these (in the sense that is it the easiest requirement to violate unintentionally) is that the total mass of material in the path of the low momentum surface muon beam must be small enough that the muons reach the sample without appreciable attenuation or scattering. The range of these low energy (4.2 MeV) muons is about 140 mg/cm² of low-Z material. This places a practical upper limit of about 70 mg/cm² on the total mass of material present in beamline windows, air gaps, muon detectors, cryostat windows, coolants and heat shields if we are to avoid stopping a significant number of muons before they reach the bulk of the sample. Normally, this is not a difficult technical problem for the majority of samples that are room-temperature solids. In the case of Van der Waals solids that condensed from room-temperature gases, this is made much more difficult by the need for the sample to be held within a miniature pressure vessel, which is in turn surrounded by vacuum in order to provide thermal isolation. Crystals of these solids are grown from the liquid, under a pressure slightly greater than their equilibrium vapour pressure at the melting point, typically 100 - 700 mbar depending on the material. Even in the solid phase, the equilibrium vapour pressure can be a considerable fraction of an atmosphere. Therefore, the cryostat must simultaneously provide the contradictory conditions of thin beam windows and considerable mechanical strength. To make matters more difficult it must also be able to withstand changing pressure at cryogenic temperatures due to thermal cycling. In addition to this, the cryostat must provide a way for the sample gas to be piped into the sample space from room temperature outside the cryostat.

The cryostat used for the majority of the results presented here was a commercial Janis Supertran continuous-flow coldfinger cryostat with modifications enabling the condensation of room-temperature gases to liquids that were subsequently frozen to solid samples of a size suitable for $\mu{\cal SR}$.Samples filled the volume of a cell measuring 22 x 22 x 6 mm machined from a solid block of copper, with the open square faces covered with 0.012 cm thick transparent Mylar windows. (Figures 3.8 and 3.9 show the coldfinger cryostat and the sample cell in detail.) These windows were epoxied to the copper cell, providing a gas-tight seal, and backed up with a copper frame to provide strength against peeling under the strain due to the pressure differential. Sample gases passed from outside the cryostat to the cell via a stainless-steel tube that entered the cell on its top edge. Condensation of sample gases within the tube was prevented by a heater wound about the tube over its entire length inside the cryostat. Cooling of the cell was provided through its attachment to the cold finger on the cell's bottom edge. A heater mounted on the top edge of the cell served to ensure that the top of the sample, near the exit tube, was the warmest part and to allow the establishment of a temperature gradient down the sample during sample preparation. Sample temperature was measured with either carbon glass resistive thermometers or GaAlAs diode thermometers set into the edge of the copper cell. Temperatures down to about 4 K were attainable with this system.