An Apparatus for Preparing Isotopically Pure He

The helium extracted from gas wells consists predominantly of He but it also contains traces (a few parts in 10) of the other stable isotope, He. For many purposes, the presence of the He may be neglected. Commercial He, widely used for a variety of cryogenic and other technological applications, usually consists of the two isotopes in their natural relative abundances. There are certain other circumstances, however, where the presence of even this very small proportion of isotopic impurity can be of great significance, and it then becomes essential for the He to be purified. Examples include: studies of phonon scattering in solid helium; measurements of the surface properties of the liquid at very low temperatures, investigations of the ideal breakdown of superfluidity using negative ions at millikelvin temperatures; the use of He gas as a coolant in high temperature nuclear reactors (HTRs); and the recently proposed construction of a superthermal source of ultra-cold neutron (UCN) gas, using superfluid He as the internal medium. The isotopic purity required for the latter application is perhaps the most exacting of all: in order that the average neutron lifetime inside the containing bottle should be limited by β-decay, rather than through absorption by He atoms, it is necessary that the He/He ratio (= R43) should be substantially in excess of 2× 10, i.e. about four orders of magnitude greater than that of natural helium. The purity criterion for the millikelvin ion experiments is almost as stringent, it being necessary for a negative ion to be able to travel about 10 cm through superfluid He without encountering any He atoms: the latter have a tendency to