A new enclosure for 54 standard cells with 25-μK stability

Ten enclosures, each capable of containing 54 standard cells have been designed and constructed. Each enclosure consists of three concentric aluminum cubes separated by foamed polystyrene insulation. The second cube only is controlled at a temperature of 22°C by a precision thermistor and an ac transformer-ratio bridge followed by a phase-sensitive detector, a dc amplifier and a heater. The drift in temperature from the initial settings over a period of 10 months has been less than 300 μK whilst the gradient across the innermost cube has not exceeded 20 μK.