2.2 Sample Handling

The alkali fullerides are generally air-sensitive (although it has been reported that some of the A₁ materials are air-stable[85]). Consequently precautions must be taken to keep them in an inert environment. In addition, the samples are usually finely powered and difficult to press into pellet form, so thermal contact at cryogenic temperatures is maintained by the surrounding thermal exchange gas. Thus, in order to access a wide range of temperature, some fraction of the exchange gas must be Helium. Furthermore, the rather small range of surface $\mu^+$necessitates thin window(s) in any kind of sample vessel. Practically for cryogenic temperatures, thin (2 or 3 thousandths of an inch thick) windows made of the polyamide film known as Kapton (from duPont) is used. For high temperatures, thin metal foil windows can be used, but the stopping density of such windows is usually considerably higher than for the Kapton. For example the 1 thou. Ag foil window used in the high temperature cell up to 450K had a stopping density of $\sim$ 26 mg/cm²; whereas 2 thou. Kapton has only 7.2 mg/cm².

For the benefit of anyone planning to use $\mu {\cal SR}$ on similar air-sensitive materials and to make a complete description of sample handling, mounting and storage employed for the experiments on which this thesis is based, a detailed description of the various sample cells is given below.

Initial sample cells employed the low background technique using a cup shaped veto counter behind the sample[84]. This was necessary in order to obtain an acceptable signal to noise ratio from the small quantities of material that were available at the time. In such a cell, the sample, a lightly pressed pellet wrapped in thin Al foil, was stuck to a thin aluminized mylar (0.5 thou. thick) sheet with silicone vacuum grease. The aluminized mylar sheet was held taught (like a drumskin) in the cylindrical cell cavity by a thin Al ring. The cell (shop aluminum) was equipped with Kapton windows in front and behind the sample, so that $\mu^+$ entering the cell and not stopping in the sample would exit throught the back window and trigger the veto cup. The window seals were made by crushing an indium o-ring in a rectangular groove of cross-sectional area slightly smaller than the In wire. The actual seal was made in the area on either side of the groove. Because the sealing area was too large, this groove design lead to loss of integrity of the cell. Subsequent cells did not employ this low background technique because material was available in sufficient quantity that the entire muon beam could be stopped in the sample. In the new cell, the groove design was improved in the following way: a triangular groove (of cross-sectional area appropriate to the In o-ring thickness) was used, and the groove was flanked on either side by a releived area (10 thou. higher than the surrounding surface) about 1mm wide. The releived area provided the sealing area, where the In would be pressed very thin, the crushing force of the screws being concentrated on this area. Excess In would flow beyond the releived region and not interfere. Note that it is imperative that the edges of such cells that come into contact with the Kapton window(s) be smooth (usually ``radiused'' at about 1mm). This is because the window flexes against these edges as the pressure gradient across the window varies during the course of an experiment, and any roughness may puncture the Kapton.

Nearly all the cells employed standard #4-40 hex socket head 18-8 stainless steel screws to provide the sealing force. Concerns about magnetism at low temperature in the screws led us to commission the manufacture of Al-bronze non-magnetic socket head screws. The socket head is the preferred type for use in inert atmosphere gloveboxes where thick gloves make handling of other screw types difficult (and dangerous for the glovebox integrity). Furthermore, it is easier to get a large torque on such screws. It should be noted that with such small screws it was found (even for the stainless steel) that it is easy to apply enough torque with a standard Allan key to shear the socket heads off. The screw itself, and not the threads in the Al cell, is the weakest point in this case. Concern over the relative weakness of the bronze screws led us to use stainless screws to crush the o-ring, and subsequently replace the screws one-by-one with the bronze screws. Significant improvements to this process could be made by using a small press to crush the In o-ring. With a jig appropriate to the cell, one could, while maintaining pressure in the press, insert the bronze screws.

To avoid temperature dependent background signals from muons stopping in Al, whenever any of the non-low background cells were used, a 0.25 mm thick high purity silver mask (99.9985% Ag, Goodfellow) was attached to the front of the Al vessel. Furthermore, before the sample was loaded, a similar silver backing was installed in the back of the sample cavity. In all cases the samples were contained in Al foil sufficiently thin to stop very few muons.

We note that nearly all the experiments described in this thesis were conducted in a He gas flow cryostat. The sample thermometry (e.g. Lakeshore calibrated carbon glass resistor or diode thermometers) in some cases was about 2cm from the sample, but in others was placed in blank holes in the back of the sample vessel. Thermal gradients in the gas flow cryostat were not significant as the difference between the sample and He diffuser thermometry indicated. The cells for two exceptional experiments not conducted in such a cryostat are described below.

For the high temperature cell, the considerable expertise of the TRIUMF machine shop was employed in manufacturing a cell from 99.9% (Johnson-Matthey) Ag with a welded 1 thou. Ag foil window. The sample was loaded into the cell cavity via an annealed oxygen-free Cu tube, Ag-soldered into the rear of the cell. After the cell was filled, the tube was sealed by crimping the tube. It was found that the tube (1/8'' outer diameter) was inconveniently small for loading and unloading the sample in a glovebox. The cell was also equipped with a blank hole in the rear where a Pt resistor thermometer was placed during the experiment.

Spatial constraints in the Oxford Instruments Model 400 Top Loading Dilution Refridgerator (DR) are such that sealing a window with an In o-ring is impractical. Thus a cell using an epoxied Kapton window was designed (see Fig. 2.9). The (99.9% Ag) cell was sealed with an In o-ring seal and a plug held in place with four #2-56 screws (again using stainless first, then replacing them with bronze). A c-clamp and jig was also used to provide crushing force on the thin (typically 20 thou. diameter) In o-ring. The powdered samples were introduced into the cell cavity in thin (0.3 thou.) Al foil packets.

Gluing such a window presented many difficulties. The best method found is as follows: The epoxied surfaces (Ag and Kapton) are sandblasted (the Kapton widow can be masked with scotch tape) and cleaned with methanol. A cryogenic epoxy (e.g. Industial Formulators of Canada Ltd. G-R, nominally a woodworking epoxy) is mixed and bubbles removed by pumping on it in a bell jar. Both surfaces are lightly epoxied with a fine brush. Excess epoxy may leak into the cavity and harden into a sharp point capable of puncturing the Kapton. Furthermore, excess epoxy may stop muons and contribute a background signal. The window is attached at one end to the back of the cell and is wrapped around, being held taught by hand and tape. The cell is then introduced into a channel shaped Al form lined with a thin sheet of neoprene rubber and clamped with a small c-clamp. The window is then left for the hardening time of the epoxy (12 hours for G-R). The success of the gluing process can be observed through the transparent Kapton, but other tests are recommended to ensure that the window is vacuum tight. For instance, the cell can be filled with Ar or He gas and sealed using a thin rubber o-ring, and then the leak-rate measured by placing the cell in an evacuable dessicator and using a residual gas analyzer. We note that Kapton is permeable to He but not to Ar, so some calibration of the expected leak-rates is required. An epoxied window of dimensions roughly the same as shown in Fig. 2.9 was tested to high (internal) pressure, and was found to burst at about 8 atm, with the Kapton and not the epoxy failing.

The low thermal conductivity of all materials below 1K[86] makes both thermal contact and thermometry serious concerns for any experiment at DR temperatures. The use of an exchange gas alleviates the problem of thermal contact, but the only gas with sufficient vapour pressure below 77K is He. The practical low temperature limit for any exchange gas is set by the temperature at which the saturated vapour pressure drops drastically, for ⁴He this is about 1K and for ³He it is about 0.3K.

For a powder (or even pressed powder), thermal contact via an exchange gas (as opposed to conduction) is clearly preferable. However, one must always be concerned when placing a sealed vessel into a coldfinger DR cryostat, such as this, that the vacuum seal of the sample cell may have been lost during loading or cooling and the exchange gas may have leaked away. There are several ways to test this. One method involves use of a paramagnetic thermometer such as Cerium Magnesium Nitrate (CMN). Such a method has not yet been tried, but one would simply load the CMN in the same way as a sample, and use the temperature dependence of the muon frequency[87] to reliably measure the temperature in the cell. Loss of exchange gas would manifest itself in the ``bottoming out'' of the CMN temperature above the minimum exchange gas temperature. This method relies on the reproducibility of the sample cell seal, but has the advantage of allowing calibration of the DR resistive thermometers with the actual temperature of an insulating sample in the exchange gas. Another method is to compare the thermal gradients, as measured by an array of resistive thermometers, (typically RuO₂ film thermometers) along the length of the DR coldfinger between runs with the cell empty, filled both with and without the exchange gas. If the exchange gas has escaped or completely liquefied, the main thermal contact to the sample will have been broken, and the sample will provide a radiative/conductive heatload on the coldfinger. Observation of a difference in the temperature gradient caused by this heatload at some temperature would set a lower limit on the sample temperature that could be obtained reliably.

For the early measurements, mounting the samples in vessels was carried out at the Laboratory for Research on the Structure of Matter at the University of Pennsylvania. More recently, though, an inert atmosphere handling facility at the University of British Columbia has been refurbished and was used for sample mounting. This facility consists of a Vacuum Atmospheres Dri-Lab glovebox and associated equipment. The glovebox atmosphere is typically Ar with a fraction of He added prior to sample mounting.