Helium-3 Cryostats

Abstract

The temperature range accessible with a liquid 4He bath is typically 4.2÷1.3 K, but this temperature range can be extended to about 0.3 K if the rare isotope 3He is used instead of the common isotope 4He. The main reason is that 3He has a substantially larger vapour pressure than 4He at the same temperature; the ratio P3/P4 is 74 at 1 K but about 104 at 0.5 K (Figs. 2.6,7). A further advantage of using liquid 3He instead of liquid 4He at temperatures below their normal boiling point is due to the fact that the specific heat of liquid 3He varies much less between, for example, 2 and 0.5 K than the specific heat of liquid 4He does (Fig.2.8). One therefore has to evaporate only about 20% of 3He to cool this liquid from 1.5 to 0.3 K by using its own heat of evaporation. Furthermore, the specific heat of liquid 3He is larger than the specific heat of liquid 4He below 1.5 K, resulting in a larger heat reservoir in this temperature range Finally, liquid 3He is not superfluid in the temperature range of concern in this chapter. One therefore does not have the heat transfer problems sometimes arising from the superfluid film flow of liquid 4He (Sect.2.3.5).