Cryostats

For the $\mu$SR measurements of this thesis, two types of cryostats were employed: a ⁴He gas flow cryostat and a dilution refrigerator. In the ⁴He gas flow cryostat, the samples were suspended close to the end of a sample chamber, which is a tube ($\phi\sim 7.5\,{\rm cm}\times L\sim 50\,{\rm cm}$)with a sealed beam window at the sample position. The chamber has a ⁴He-diffuser close to the sample position, which is connected to a ⁴He dewar through a capillary with a needle valve. The diffuser provided gaseous ⁴He to the sample chamber. The cooling power was roughly controlled with the needle valve. The other end of the sample chamber is connected to a rotary vacuum pump. By reducing the pressure in the sample chamber, the cryostat can reach $\sim$1.8 K. There are two heaters equipped in this cryostat: one at the diffuser, and the other at the sample position. The diffuser heater finely tuned the cooling power, and determined the temperature of the ⁴He gas flowing to the sample chamber. The sample heater was connected to a temperature controller (Lakeshore DRC-92C) and stabilized the temperature at the sample position. For temperature reading and control, a carbon glass register and a platinum thermometer (or a GaAlAs diode for the full temperature range) were used at the sample position and at the diffuser.

The dilution refrigerator we used (Oxford 400) is a conventional closed-cycle refrigerator, circulating the ³He/⁴He mixture. The sample was attached to a sample holder, which was screwed onto the mixing chamber. Since the samples were in vacuum, the cooling power for the sample depends on the thermal conduction. To maximize the thermal conduction, Apiezon grease or GE varnish was applied between the sample and the sample holder. The cooling power was controlled by the circulation rate of the ³He/⁴He mixture, while the temperature was controlled using a heater on the mixing chamber. The mixing chamber was equipped with a carbon resistor for temperature reading; the base temperature of the refrigerator was typically 20 mK. The dilution refrigerator shares a ⁴He dewar with a superconducting Helmholtz coil which supplied a field parallel to the beam axis at the sample position.

Table 2 summarizes the features of the three spectrometers currently available at TRIUMF. Most of the data presented in this thesis have been obtained using the OMNI and the DR spectrometers, often in combination.

 

Name Cryostat Longitudinal

$$H_{\rm max}$$ (temperature range)

DR Dilution Refrigerator Superconducting

$$\sim$$ Helmholtz (60 kG)

OMNI ⁴He flow cryostat Normal Helmholtz

$$\sim$$ coil (3.5 kG)

HELIOS ⁴He flow cryostat Superconducting

$$\sim$$ Solenoid (70 kG)