Helium dilution refrigerator

At milli-kelvin temperatures, the problem of contact resistance between helium and solids becomes more complex. Thermal transfer phenomena take place involving spins and thermal resistance of sintered materials. The understanding of the thermal transport at very low temperature is of the utmost importance, also from a technical point of view, since helium is the working substance in dilution refrigerators (see Chapter 6). 

The gas is also used to fill balloons, in gas discharge lamps, and as an additive in the breathing gases of astronauts and scuba divers. The rarer stable isotope of helium (3He) is produced by the decay of radioactive tritium, and is used in resonance imaging and in the attainment of very low temperatures, about 0.010 kelvin, via a process known as dilution refrigeration. see also Noble Gases Nuclear Fusion. 

Figure 2. Pictorial diagram of the hydrogen trap. The magnets are immersed in liquid helium the inner region of the trap is connected to a dilution refrigerator.

Figure 5.1 Alignment of optical windows and sample in the cryostat. The sample is placed inside the mixing chamber of the dilution refrigerator. The shields are connected to the distillation chamber, the helium tank, and the nitrogen tank, respectively. The liquid N2 windows possess an infrared reflective coating.

Mixtures of helium-3 and helium-4 are not completely miscible at very low temperatures, as shown in Fig. 2.11. In fact, as absolute zero is approached, helium-3 appears to be completely insoluble in helium-4. At other temperatures, a phase separation occurs in any mixture whose average concentration, at the given temperature, lies beneath this curve. The concentrations of the separated phases are given by the two intersections of this curve with the (horizontal) line of constant temperature. This separation into two liquid phases and difference in vapor pressures forms the basis for the helium-3/helium-4 dilution refrigerator used to obtain temperatures close to absolute zero (see Chapter 4 for further details). 

DISPOSAL AND STORAGE METHODS absorb in dry earth, sand or vermiculite and place in a secured sanitary landfill store in a cool, dry location with adequate ventilation isolate from air, moisture, halogens, alkali metals, aluminum and rust outside storage is preferred normally refrigerated usually shipped in pressurized cylinders diluted with hydrogen, argon, nitrogen, or helium. 

Question by H. B. Sherlock, McDonnell Aircraft What does your experience show in the percent dilution of the slush with helium during the refrigeration cycle ... 

Answer by Author Because the refrigeration cycle of Fig. 2 involves only hydrogen, there is no dilution with helium during the refrigeration cycle. 

---